Table of contents

Random walks with a general, nonlinear barrier have found recent applications ranging from reionization topology to refinements in the excursion set theory of halos. Here we derive the first-crossing distribution of random walks with a moving barrier of arbitrary shape. Such a distribution is shown to satisfy an integral equation that can be solved by a simple matrix inversion, without the need for Monte Carlo simulations, making this useful for exploring a large parameter space. We discuss examples in which common analytic approximations fail, a failure that can be remedied using the method described here.

We present a comprehensive series of dissipationless N-body simulations to investigate the evolution of density distribution in equal-mass mergers between dark matter (DM) halos and multicomponent galaxies. The DM halo models are constructed with various asymptotic power-law indices ranging from steep cusps to corelike profiles and the structural properties of the galaxy models are motivated by the ΛCDM paradigm of structure formation. The adopted force resolution allows robust density profile estimates in the inner ~1% of the virial radii of the simulated systems. We demonstrate that the central slopes and overall shapes of the remnant density profiles are virtually identical to those of the initial systems, suggesting that the remnants retain a remarkable memory of the density structure of their progenitors, despite the relaxation that accompanies merger activity. We also find that halo concentrations remain approximately constant through hierarchical merging involving identical systems and show that remnants contain significant fractions of their bound mass well beyond their formal virial radii. These conclusions hold for a wide variety of initial asymptotic density slopes, orbital energies, and encounter configurations, including sequences of consecutive merger events, simultaneous mergers of several systems, and mergers of halos with embedded cold baryonic components in the form of disks, spheroids, or both. As an immediate consequence, the net effect of gas cooling, which contracts and steepens the inner density profiles of DM halos, should be preserved through a period of dissipationless major merging. Our results imply that the characteristic universal shape of DM density profiles may be set early in the evolution of halos.

We propose a method for CMB component separation based on standard Bayesian parameter estimation techniques. We assume a parametric spectral model for each signal component and fit the corresponding parameters pixel by pixel in a two-stage process. First we fit for the full parameter set (e.g., component amplitudes and spectral indices) in low-resolution and high signal-to-noise ratio maps using MCMC, obtaining both best-fit values for each parameter and the associated uncertainty. The goodness of fit is approximated by a χ² statistic. Then we fix all nonlinear parameters at their low-resolution best-fit values and solve analytically for high-resolution component amplitude maps. This likelihood approach has many advantages: the fitted model may be chosen freely, and the method is therefore completely general; all assumptions are transparent; no restrictions on spatial variations of foreground properties are imposed; the results may be monitored by goodness-of-fit tests; and, most importantly, we obtain reliable error estimates on all estimated quantities. We apply the method to simulated Planck satellite and 6 year WMAP data based on realistic models and show that separation at the microkelvin level is indeed possible in these cases. We also outline how the foreground uncertainties may be rigorously propagated through to the CMB power spectrum and cosmological parameters using a Gibbs sampling technique.

We examine the black hole mass-galaxy bulge relationship in high-redshift QSOs. Black hole masses are derived from broad emission lines, and the host galaxy stellar velocity dispersion σ_* is estimated from the widths of the radio CO emission lines. At redshifts z > 3, the CO line widths are narrower than expected for the black hole mass, indicating that these giant black holes reside in undersized bulges by an order of magnitude or more. The largest black holes (M_BH > 10⁹M_☉) evidently grow rapidly in the early universe without commensurate growth of their host galaxies. CO line widths offer a unique opportunity to study AGN host galaxy dynamics at high redshift.

We present four improved empirical relationships useful for estimating the central black hole mass in nearby AGNs and distant luminous quasars alike using either optical or UV single-epoch spectroscopy. These mass scaling relationships between line widths and luminosity are based on recently improved empirical relationships between the broad-line region size and luminosities in various energy bands and are calibrated to the improved mass measurements of nearby AGNs based on emission-line reverberation mapping. The mass scaling relationship based on the Hβ line luminosity allows mass estimates for low-redshift sources with strong contamination of the optical continuum luminosity by stellar or nonthermal emission, while that based on the C IV λ1549 line dispersion allows mass estimates in cases where only the line dispersion (as opposed to the FWHM) can be reliably determined. We estimate that the absolute uncertainties in masses given by these mass scaling relationships are typically around a factor of 4. We include in an appendix mass estimates for all of the Bright Quasar Survey (PG) quasars for which direct reverberation-based mass measurements are not available.

The Very Long Baseline Array (VLBA) polarimetric observations of extragalactic compact radio sources and quasars show rotation measure (RM) values that are much smaller than the enormous values predicted by the theory. This is under the assumption that the narrow-line region (NLR) and the broad-line region (BLR) are the Faraday medium in which the observed RM is produced. It is expected that the polarized emission in the parsec scale of these sources displays rest-frame RM values ranging from 10⁵ to 10⁸ rad m^-2, whereas, the observed RMs of quasars and active galactic nuclei typically range from 10² to 10⁴ rad m^-2 and, for most extragalactic sources, from 1 to 100 rad m^-2. Recently, a dispersion model for highly relativistic jets has been proposed. The dependence of the observed radiation on the amount of relativistic motion, as well as on the geometry and intrinsic properties of the jets, has been analyzed. We employ several results of this dispersion model in order to discuss a possible solution to the RM problem. The RM and the electric vector of polarization angle (EVPA) in a relativistically moving plasma have been defined. We propose taking into account the relativistic motion of the medium in which the radiation is propagating, in order to avoid the large depolarization and also to permit the existence of high rest-frame rotation measure (RM') values.

We present deep (70-80 ks) Chandra and multicolor HST ACS images of two jets hosted by the powerful quasars 1136-135 and 1150+497, together with new radio observations. The sources have an FR II morphology and were selected from our previous X-ray and optical jet survey for detailed follow-up aimed at obtaining better constraints on the jet multiwavelength morphology and X-ray and optical spectra of individual knots and to test emission models to derive physical parameters more accurately. All the X-ray and optical knots detected in our previous short exposures are confirmed, together with a few new faint features. The overlaid maps and the emissivity profiles along the jet show good correspondence between emission regions at the various wavelengths; a few show offsets between the knot peaks of <1''. In 1150+497 the X-ray, optical, and radio profiles decrease in similar ways with distance from the core up to ~7'', after which the radio emission increases more than does the X-ray one. No X-ray spectral variations are observed in 1150+497. In 1136-135 an interesting behavior is observed, whereby, downstream of the most prominent knot at ~65 from the core, the X-ray emission fades, while the radio emission brightens. The X-ray spectrum also varies, with the X-ray photon index flattening from Γ_X ~ 2 in the inner part to Γ_X ~ 1.7 to the end of the jet. We interpret the jet behavior in 1136-135 in a scenario in which the relativistic flow suffers systematic deceleration along the jet, and we briefly discuss the major consequences of this scenario. The latter is discussed in more detail in our companion paper (Tavecchio et al.).

By modeling the multiwavelength emission of successive regions in the jet of the quasar PKS 1136-135, we find indications that the jet suffers deceleration near its end on a (deprojected) scale of ~400 kpc. We adopt a continuous flow approximation, and we discuss the possibility that the inferred deceleration from a Lorentz factor of Γ = 6.5 to 2.5 is induced by entrainment of external gas. Some consequences of this scenario are discussed.

We report on a multiwavelength campaign on the TeV γ-ray blazar Mrk 421 performed during 2002 December and 2003 January. These target of opportunity observations were initiated by the detection of X-ray and TeV γ-ray flares with the All Sky Monitor (ASM) on board the Rossi X-Ray Timing Explorer (RXTE) and the 10 m Whipple γ-ray telescope. The campaign included observational coverage in the radio (University of Michigan Radio Astronomy Observatory), optical (Boltwood, La Palma KVA 0.6 m; WIYN 0.9 m), X-ray (RXTE pointed telescopes), and TeV γ-ray (Whipple and HEGRA) bands. At TeV energies, the observations revealed several flares at intermediate flux levels, peaking between 1 and 1.5 times the flux from the Crab Nebula. While the time-averaged spectrum can be fitted with a single power law of photon index Γ = 2.8 from dN_γ/dE ∝ E^-Γ, we find some evidence for spectral variability. Confirming earlier results, the campaign reveals a rather loose correlation between the X-ray and TeV γ-ray fluxes. In one case, a very strong X-ray flare is not accompanied by a comparable TeV γ-ray flare. Although the source flux was variable in the optical and radio bands, the sparse sampling of the optical and radio light curves does not allow us to study the correlation properties in detail. We present a simple analysis of the data with a synchrotron self-Compton model, emphasizing that models with very high Doppler factors and low magnetic fields can describe the data.

We present a spatially resolved analysis of the temperature and gas density profiles in six relaxed galaxy clusters at z = 0.4-0.54 using long-exposure Chandra observations. We derive the total cluster masses within the radius r₅₀₀, assuming hydrostatic equilibrium but without assuming isothermality of the intracluster gas. Together with a similar study based on the XMM-Newton observations (Kotov & Vikhlinin), we obtained the mass and temperature measurements for 13 galaxy clusters at 0.4 < z < 0.7 spanning a temperature interval of 3 keV < T < 14 keV. The observed evolution of the M-T relation, relative to the low-redshift references from the Chandra sample of Vikhlinin et al., follows M₅₀₀/T^3/2 ∝ E(z)^-α, where we measure α = 1.02 ± 0.20 and 1.33 ± 0.20 for the spectroscopic and gas mass-weighted temperatures, respectively. Both values are in agreement with the expected self-similar evolution, α = 1. Assuming that the cluster mass for a given temperature indeed evolves self-similarly, the derived slopes, γ, of the high-redshift M-T relation, E(z)M₅₀₀ ∝ T^γ, are γ = 1.55 ± 0.14 for T_spec and γ = 1.65 ± 0.15 for T_mg. Our results show that both the shape and evolution of the cluster M-T relation at z ≃ 0.5 are close to predictions of the self-similar theory.

We report on our serendipitous discovery with the XMM-Newton Observatory of a luminous X-ray-emitting cluster of galaxies that is located behind the Andromeda galaxy (M31). X-ray emission from the cluster was detected previously by ROSAT and cataloged as RX J0046.4+4204, but it was not recognized as a galaxy cluster. The much greater sensitivity of our XMM-Newton observations revealed diffuse X-ray emission that extends at least 5' and has a surface brightness profile that is well fit by the α-β model with β = 0.70 ± 0.08, a core radius r_c = 56'' ± 16'', and α = 1.54 ± 0.25. A joint global spectral fit of the EPIC MOS1, MOS2, and pn observations with the Mewe-Kaastra-Liedahl plasma emission model gives a cluster temperature of 5.5 ± 0.5 keV. The observed spectra also show high significance iron emission lines that yield a measured cluster redshift of z = 0.290 with 2% accuracy. For a cosmological model with H₀ = 71 km s^-1 Mpc^-1, Ω_M = 0.3, and Ω_Λ = 0.7, we derive a bolometric luminosity of L_X = (8.4 ± 0.5) × 10⁴⁴ ergs s^-1. This discovery of a cluster behind M31 demonstrates the utility of X-ray surveys for finding rich clusters of galaxies, even in directions of heavy optical extinction.

We present new optical observations of young massive star clusters in Arp 220, the nearest ultraluminous infrared galaxy, taken in UBVI with the Hubble Space Telescope ACS HRC camera. We find a total of 206 probable clusters whose spatial distribution is centrally concentrated toward the nucleus of Arp 220. We use model star cluster tracks to determine ages, luminosities, and masses for 14 clusters with complete UBVI indices or previously published near-infrared data. We estimate rough masses for 24 additional clusters with I < 24 mag from BVI indices alone. The clusters with useful ages fall into two distinct groups: a "young" population (<10 Myr) and an intermediate-age population (≃300 Myr). There are many clusters with masses clearly above 10⁶M_☉ and possibly even above 10⁷M_☉ in the most extreme instances. These masses are high enough that the clusters being formed in the Arp 220 starburst can be considered to be genuine young globular clusters. In addition, this study allows us to extend the observed correlation between global star formation rate and maximum cluster luminosity by more than 1 order of magnitude in star formation rate.

The apparent shapes of spiral galaxies in the Two Micron All Sky Survey Large Galaxy Atlas are used to constrain the intrinsic shapes of their disks. When the distribution of apparent axis ratios is estimated using a nonparametric kernel method, the shape distribution is inconsistent with axisymmetry at the 90% confidence level in the B band and at the 99% confidence level in the K_s band. If spirals are subdivided by Hubble type, the late-type spirals (Sc and later) are consistent with axisymmetry, while the earlier spirals are strongly inconsistent with axisymmetry. The distribution of disk ellipticity can be fitted adequately with either a Gaussian or a lognormal distribution. The best fits for the late spirals imply a median ellipticity of ≈ 0.07 in the B band and ≈ 0.02 in the K_s band. For the earlier spirals, the best fits imply a median ellipticity of ≈ 0.18 in the B band and ≈ 0.30 in the K_s band. The observed scatter in the Tully-Fisher relation, for both late and early spirals, is consistent with the disk ellipticity measured in the B band. This indicates that excluding spirals of Hubble type earlier than Sc will minimize the intrinsic scatter in the Tully-Fisher relation used as a distance indicator.

We study the chemical and kinematic properties of the first galaxies that formed at high redshift, using high-resolution cosmological numerical simulations, and compare them with the recent observational results for the Sculptor dwarf spheroidal galaxy of Tolstoy et al., who found two distinct stellar populations: the lower metallicity stars are more spatially extended and possess a higher velocity dispersion than the higher metallicity stars. Our calculations reproduce these observations as the result of a steep metallicity gradient within a single population, induced by dissipative collapse of the gas component. We also predict strong [N/O] enhancements in the lowest metallicity stars in dwarf spheroidals, due to the preferential retention of ejected gas from intermediate-mass stars, compared to Type II supernovae.

Archival observations of 18 starburst galaxies that span a wide range in metallicity reveal for the first time a correlation between the ratio of emission-line fluxes of [Fe II] at 26 μm and [Ne II] at 12.8 μm and the 7.7 μm PAH strength, with the [Fe II]/[Ne II] flux ratio decreasing with increasing PAH strength. We also find a strong correlation between the [Fe II]/[Ne II] flux ratio and the host galaxy metallicity, with the flux ratio decreasing with increasing metallicity. Since [Fe II] emission has been linked primarily to supernova shocks, we attribute the high [Fe II]/[Ne II] ratios in low-metallicity galaxies to enhanced supernova activity. We consider this to be a dominant mechanism for PAH destruction, rather than grain destruction in photoionized regions surrounding young massive stars. We also consider whether the extreme youth of the low-metallicity galaxies is responsible for the lack of PAH emission.

INTEGRAL and RXTE performed three simultaneous observations of the nearby radio galaxy Centaurus A in 2003 March, 2004 January, and 2004 February with the goals of investigating the geometry and emission processes via the spectral/temporal variability of the X-ray/low-energy gamma-ray flux, and intercalibration of the INTEGRAL instruments with respect to those on RXTE. Cen A was detected by both sets of instruments from 3 to 240 keV. When combined with earlier archival RXTE results, we find the power-law continuum flux and the line-of-sight column depth varied independently by 60% between 2000 January and 2003 March. Including the three archival RXTE observations, the iron-line flux was essentially unchanging, and from this we conclude that the iron-line-emitting material is distant from the site of the continuum emission, and that the origin of the iron-line flux is still an open question. Taking X-ray spectral measurements from satellite missions since 1970 into account, we discover a variability in the column depth between 1.0 × 10²³ and 1.5 × 10²³ cm^-2 separated by approximately 20 yr, and suggest that variations in the edge of a warped accretion disk viewed nearly edge-on might be the cause. The INTEGRAL OSA 4.2 calibration of JEM-X, ISGRI, and SPI yields power-law indices consistent with the RXTE PCA and HEXTE values, but the indices derived from ISGRI alone are about 0.2 greater. Significant systematics are the limiting factor for INTEGRAL spectral parameter determination.

The stellar content of the spiral galaxy NGC 247 is investigated using deep visible and near-infrared images. The main-sequence turnoff (MSTO) in the inner 12 kpc of the disk corresponds to an age of ~6 Myr. A mean star formation rate (SFR) of 0.1 M_☉ yr^-1 during the past 16 Myr is computed from star counts. The color of the red supergiant plume does not change with radius, suggesting that the mean metallicity of young stars does not vary by more than ~0.1 dex. The number of bright main-sequence stars per local stellar mass density climbs toward larger radii out to a distance of 12 kpc; the scale lengths that characterize the radial distributions of young and old stars in the disk thus differ. The density of bright main-sequence stars with respect to projected H I mass gradually drops with increasing radius. The population of very young stars disappears in the outer disk; the MSTO at galactocentric radii between 12 and 15 kpc corresponds to ~16 Myr, while between 15 and 18 kpc the age is ≥40 Myr. Red giant branch (RGB) stars are resolved at a projected minor-axis galactocentric distance of ~12 kpc. There is a broad spread in metallicity among the RGB stars, with a mean [M/H] ~ -1.2. The RGB tip occurs at i' = 24.5 ± 0.1, indicating that the distance modulus is 27.9 ± 0.1. Luminous AGB stars with an age ~3 Gyr are also seen in this field.

We present V- and I-equivalent HST WFPC2 stellar photometry of an area in the Large Magellanic Cloud (LMC), located to the west of the bar of the galaxy, which accounts for the general background field of its inner disk. The WFPC2 observations reach magnitudes as faint as V = 25 mag, and the large sample of more than 80,000 stars allows us to determine in detail the present-day mass function (PDMF) of the detected main-sequence stars, which is identical to the initial mass function (IMF) for masses M ≲ 1 M_☉. The low-mass main-sequence mass function of the LMC field is found not to have a uniform slope throughout the observed mass range; i.e., the slope does not follow a single power law. This slope changes at about 1 M_☉ to become more shallow for stars with smaller masses down to the lowest observed mass of ~0.7 M_☉, giving clear indications of flattening for even smaller masses. We verified statistically that for stars with M ≲ 1 M_☉ the IMF has a slope Γ around -2, with an indicative slope Γ ≃ -1.4 for 0.7 ≲ M/M_☉ ≲ 0.9, while for more massive stars the main-sequence mass function becomes much steeper with Γ ≃ -5. The main-sequence luminosity function (LF) of the observed field is in very good agreement with the Galactic LF as it was previously found. Taking into account several assumptions concerning evolutionary effects, which should have changed through time the stellar content of the observed field, we reconstruct qualitatively its IMF for the whole observed mass range (0.7 ≲ M/M_☉ ≲ 2.3), and we find that the number of observed evolved stars is not large enough to have affected significantly the form of the IMF, which thus is found almost identical to the observed PDMF.

We construct orbit-based axisymmetric dynamical models for the globular cluster M15 that fit ground-based line-of-sight velocities and Hubble Space Telescope line-of-sight velocities and proper motions. This allows us to constrain the variation of the mass-to-light ratio M/L as a function of radius in the cluster and to measure the distance and inclination of the cluster. We obtain a best-fitting inclination of 60° ± 15°, a dynamical distance of 10.3 ± 0.4 kpc, and an M/L profile with a central peak. The inferred mass in the central 0.05 pc is 3400 M_☉, implying a central density of at least 7.4 × 10⁶M_☉ pc^-3. We cannot distinguish the nature of the central mass concentration. It could be an intermediate mass black hole, or it could be a large number of compact objects, or it could be a combination. The central 4'' of M15 appears to contain a rapidly spinning core, and we speculate on its origin.

We carried out global three-dimensional resistive magnetohydrodynamic simulations of galactic gaseous disks to investigate how the galactic magnetic fields are amplified and maintained. We adopt a steady axisymmetric gravitational potential given by Miyamoto & Nagai and Miyamoto et al. As the initial condition, we assume a warm (T ~ 10⁵ K) rotating gas torus centered at ϖ = 10 kpc threaded by weak azimuthal magnetic fields. Numerical results indicate that in differentially rotating galactic gaseous disks, magnetic fields are amplified due to magnetorotational instability and magnetic turbulence develops. After the amplification of magnetic energy saturates, the disk stays in a quasi-steady state. The mean azimuthal magnetic field increases with time and shows reversals with a period of 1 Gyr (2 Gyr for a full cycle). The amplitude of B_φ near the equatorial plane is B_φ ~ 1.5 μG at ϖ = 5 kpc. The magnetic fields show large fluctuations whose standard deviation is comparable to the mean field. The mean azimuthal magnetic field in the disk corona has a direction opposite to the mean magnetic field inside the disk. The mass accretion rate driven by the Maxwell stress is ~10^-3M_☉ yr^-1 at ϖ = 2.5 kpc when the mass of the initial torus is ~5 × 10⁸M_☉. When we adopt an absorbing boundary condition at r = 0.8 kpc, the rotation curve obtained by numerical simulations almost coincides with the rotation curve of the stars and the dark matter. Thus, even when magnetic fields are not negligible for gas dynamics of a spiral galaxy, the galactic gravitational potential can be derived from observations of the rotation curve using the gas component of the disk.

We use a high-resolution grid-based hydrodynamics method to simulate the multiphase interstellar medium (ISM) in a quiescent Milky Way-sized disk galaxy. The models are global and three-dimensional, and they include a treatment of star formation and feedback. We examine the formation of gravitational instabilities and show that a form of the Toomre instability criterion can successfully predict where star formation will occur. Two common prescriptions for star formation are investigated. The first is based on cosmological simulations and has a relatively low threshold for star formation but also enforces a comparatively low efficiency. The second only permits star formation above a number density of 10³ cm^-3 but adopts a high efficiency. We show that both methods can reproduce the observed slope of the relationship between star formation and gas surface density (although at too high a rate for our adopted parameters). A run that includes feedback from Type II supernovae is successful at driving gas out of the plane, most of which falls back onto the disk. This feedback also substantially reduces the star formation rate. Finally, we examine the density and pressure distribution of the ISM and show that there is a rough pressure equilibrium in the disk, but with a wide range of pressures at a given location (and even wider for the case including feedback).

We present high-resolution near-infrared spectra, obtained with the NIRSPEC spectrograph on the W. M. Keck II Telescope, of a collection of hot, massive stars within the central 25'' of the Galactic center. We have identified a total of 21 emission-line stars, seven of which are new radial velocity detections, with five of those being classified as He I emission-line stars for the first time. These stars fall into two categories based on their spectral properties: (1) those with narrow 2.112, 2.113 μm He I doublet absorption lines, and (2) those with broad 2.058 μm He I emission lines. These data have the highest spectral resolution ever obtained for these sources and, as a result, both components of the absorption doublet are separately resolved for the first time. We use these spectral features to measure radial velocities. The majority of the measured radial velocities have relative errors of 20 km s^-1, smaller than those previously obtained with proper-motion or radial velocity measurements for similar stellar samples in the Galactic center. The radial velocities estimated from the He I absorption doublet are more robust than those previously estimated from the 2.058 μm emission line, since they do not suffer from confusion due to emission from the surrounding ISM. Using this velocity information, we agree with previous stellar velocity studies that the stars are orbiting in a somewhat coherent manner but are not as defined into a disk or disks as previously thought. Finally, multiepoch radial velocity measurements for IRS 16NE show a change in its velocity, presumably due to an unseen stellar companion.

The evolution of supernova remnants (SNRs) is studied, with particular attention to the effect of magnetic fields with axisymmetric two-dimensional magnetohydrodynamic simulations. We study the interaction of SNRs with a quiescent, magnetized ISM having uniform density, temperature, and magnetic field. The evolution of magnetic SNRs is the same as that of nonmagnetic ones in the adiabatic Sedov stage. After a thin shell is formed, the shell is driven by the pressure of the hot interior gas (bubble). Evolution in the pressure-driven snowplow phase is much affected by the magnetic field. The shell sweeping the magnetic field lines thickens owing to the magnetic pressure force. After 5 × 10⁵ to 2 × 10⁶ yr, the inner boundary of the thick shell begins to contract. This compresses the hot bubble radially and maintains its thermal pressure. Thus, the bubble forms a prolate spheroidal shape and becomes thinner and thinner, since it expands in a direction parallel to the magnetic field for B₀ ≳ 3 μG. Finally, the bubble contracts. The porosity of the hot low-density gas in the ISM is reduced, taking the effect of the magnetic field into account.

We present new optical emission-line images of the young SNR 1E 0102-7219 in the SMC obtained with the ACS on HST. This object is a member of the oxygen-rich class of SNRs showing strong oxygen, neon, and other metal-line emissions in its optical and X-ray spectra, and an absence of hydrogen and helium. The progenitor of 1E 0102-7219 may have been a Wolf-Rayet star that underwent considerable mass loss prior to exploding as a Type Ib/c or IIL/b supernova. The ejecta in this SNR are generally fast-moving (V > 1000 km s^-1) and emit as they are compressed and heated in the reverse shock. In 2003 we obtained optical [O III], Hα, and continuum images with the ACS Wide Field Camera. The [O III] image through the F475W filter captures the full velocity range of the ejecta and shows considerable high-velocity emission projected in the middle of the SNR that was Doppler-shifted out of the narrow F502N bandpass of a previous WFPC2 image from 1995. Using these two epochs separated by ~8.5 yr, we measure the transverse expansion of the ejecta around the outer rim in this SNR for the first time at visible wavelengths. From proper-motion measurements of 12 ejecta filaments, we estimate a mean expansion velocity for the bright ejecta of ~2000 km s^-1 and an inferred kinematic age for the SNR of ~2050 ± 600 yr. The age we derive from HST data is about twice that inferred by Hughes et al. from X-ray data, although our 1 σ error bars overlap. Our proper-motion age is consistent with an independent optical kinematic age derived by Eriksen et al. in 2003 using spatially resolved [O III] radial-velocity data. We derive an expansion center that lies very close to conspicuous X-ray and radio hot spots, which could indicate the presence of a compact remnant (neutron star or black hole).

X-ray absorption spectroscopy provides a powerful tool in determining the metal abundances in various phases of the interstellar medium (ISM). We present a case study of the sight line toward 4U 1820-303, based on Chandra grating observations. The detection of O I, O II, O III, O VII, O VIII, and Ne IX Kα absorption lines allows us to measure the atomic column densities of the neutral, warm ionized, and hot phases of the ISM through much of the Galactic disk. By comparing these measurements with the 21 cm hydrogen emission and with the pulsar dispersion measure, we estimate the mean oxygen abundances in the neutral and total ionized phases as 0.3(0.2, 0.6) and 2.2(1.1, 3.5) in units of Anders & Greversse's solar value (90% confidence intervals). This significant oxygen abundance difference is apparently a result of molecule/dust grain destruction and recent metal enrichment in the warm ionized and hot phases. We also measure the column density of neon from its absorption edge and obtain a solar value of the Ne/O ratio accounting for the expected oxygen contained in molecules and dust grains. From a joint analysis of the O VII, O VIII, and Ne IX lines, we obtain the Ne/O abundance ratio of the hot phase as 1.4(0.9, 2.1) solar, which is not sensitive to the exact hot gas temperature distribution assumed. These comparable ISM Ne/O ratios for the different phases are thus considerably less than the value recently inferred from corona emission of solar-like stars.

The rotation curve for the fourth Galactic quadrant within the solar circle is derived from the Columbia University-Universidad de Chile CO (J = 1 → 0) survey of molecular gas. A new sampling, 4 times denser in longitude than in our previous analysis, is used to compute kinematical parameters that require derivatives with respect to galactocentric radius, the angular velocity Ω(R), the epicyclic frequency κ(R), and the parameters A(R) and B(R) describing, respectively, gas shear and vorticity. The face-on surface density of molecular gas is computed from the CO data in galactocentric radial bins for the subcentral vicinity, the same spectral region used to derive the rotation curve, where the twofold ambiguity in kinematical distances is minimum. The rate of massive star formation per unit area is derived for the same radial bins from the luminosity of IRAS pointlike sources with FIR colors of UC H II regions detected in the CS (J = 2 → 1) line. Massive star formation occurs preferentially in three regions of high molecular gas density, coincident with lines of sight tangent to spiral arms. The molecular gas motion in these arms resembles that of a solid body, characterized by constant angular velocity and by low shear and vorticity. The formation of massive stars in the arms follows the Schmidt law, Σ_MSFR ∝ [Σ_gas]ⁿ, with an index of n = 1.2 ± 0.2. Our results suggest that the large-scale kinematics, through shear, regulate global star formation in the Galactic disk.

Star formation is usually accompanied by outflow phenomena. There is strong evidence that these outflows and jets are launched from protostellar disks by magnetorotational processes. Here we report on our three-dimensional, adaptive mesh, magnetohydrodynamic simulations of collapsing, rotating, magnetized Bonnor-Ebert spheres, whose properties are taken directly from observations. In contrast to the pure hydro case, in which no outflows are seen, our present simulations show an outflow from the protodisk surface at ~130 AU and a jet at ~0.07 AU after a strong toroidal magnetic field buildup. The large-scale outflow, which extends up to ~600 AU at the end of our simulation, is driven by toroidal magnetic pressure (spring), whereas the jet is powered by magnetocentrifugal force (fling). At the final stage of our simulation these winds are still confined within two respective shock fronts. Furthermore, we find that the jet-wind and the disk-anchored magnetic field extract a considerable amount of angular momentum from the protostellar disk. The initial spin of our cloud core was chosen high enough to produce a binary system. We indeed find a close binary system (separation ~3 R_☉), which results from the fragmentation of an earlier formed ring structure. The magnetic field strength in these protostars reaches ~3 kG and becomes about 3 G at 1 AU from the center, in agreement with recent observational results.

We explore low angular momentum accretion flows onto black holes formed after the collapse of massive stellar cores. In particular, we consider the state of the gas falling quasi-spherically onto stellar mass black holes in the hypercritical regime, where the accretion rates are in the range 10^-3M_☉ s^-1 ≲ ≲ 0.5 M_☉ s^-1 and neutrinos dominate the cooling. Previous studies have assumed that in order to have a black hole switch to a luminous state, the condition l ≫ r_gc, where l is the specific orbital angular momentum of the infalling gas and r_g is the Schwarszchild radius, needs to be fulfilled. We argue that flows in hyperaccreting, stellar mass disks around black holes are likely to transition to a highly radiative state when their angular momentum is just above the threshold for disk formation, l ~ 2r_gc. In a range r_gc < l < 2r_gc, a dwarf disk forms in which gas spirals rapidly into the black hole due to general relativistic effects, without any help from horizontal viscous stresses. For high rotation rates l ≥ 2r_gc, the luminosity is supplied by large, hot equatorial bubbles around the black hole. The highest neutrino luminosities are obtained for l ≈ 2r_gc, and this value of angular momentum also produces the most energetic neutrinos and thus also the highest energy deposition rates. Given the range of l explored in this work, we argue that, as long as l ≥ 2r_gc, low angular momentum cores may in fact be better suited for producing neutrino-driven explosions following core collapse in supernovae and γ-ray bursts.

We study the early afterglows of gamma-ray bursts produced by geometrically thick fireballs, following the development of the external shock as energy is continually supplied to the shocked material. We study the dependence of the early afterglow slope on the luminosity history of the central engine. The resulting light curves are modeled with power-law functions, and the importance of a correct choice of the reference time t₀ is investigated. We find that deviations from a simple power law are observed only if a large majority of the energy is released at late times. The light curve in this case can be described as a simple power law if the reference time is set to be close to the end of the burst. We applied our analysis to the cases of GRB 050219a and GRB 050315. We show that the early steep decay of the afterglow cannot result from the interaction of the fireball with the ambient medium. We conclude that the early X-ray afterglow emission is associated with the prompt phase, and we derive limits on the radius at which the prompt radiation is produced.

Models for the synchrotron emission of gamma-ray burst afterglows suggest that the magnetic field is generated in the shock wave that forms as relativistic ejecta plow through the circumburst medium. Transverse Weibel instability efficiently generates magnetic fields near equipartition with the postshock energy density. The detailed saturated state of the instability, as seen in particle-in-cell simulations, consists of magnetically self-pinched current filaments. The filaments are parallel to the direction of propagation of the shock and are about a plasma skin depth in radius, forming a quasi-two-dimensional structure. We argue that the Weibel filaments are susceptible to pressure-driven instabilities and use a rudimentary analytical model to illustrate the development of a particular, kinklike unstable mode. The instabilities destroy the quasi-two-dimensional structure of the Weibel filaments. For wavelengths longer than the skin depth, the kinklike mode grows at the rate equal to the speed of light divided by the wavelength. We calculate the transport of collisionless test particles in the filaments experiencing the instability and show that the particles diffuse in energy. This diffusion marks the beginning of thermalization in the shock transition layer and causes initial magnetic field decay as particles escape from the filaments. We discuss the implications of these results for the structure of the shock and the polarization of the afterglow.

First-order Fermi acceleration processes at ultrarelativistic (γ ~ 5-30) shock waves are studied with Monte Carlo simulations. The accelerated particle spectra are derived by integrating the exact particle trajectories in a turbulent magnetic field near the shock. The magnetic field model upstream of the shock assumes finite-amplitude perturbations within a wide wavevector range and with a predefined wave power spectrum, imposed on the mean field component inclined at some angle to the shock normal. The downstream field structure is obtained as the compressed upstream field. We show that the main acceleration process at superluminal shocks is the particle compression at the shock. Formation of energetic spectral tails is possible in a limited energy range only for highly perturbed magnetic fields. Cutoffs in the spectra occur at low energies within the resonance energy range considered. These spectral features result from the anisotropic character of particle transport in the magnetic field downstream of the shock, where field compression produces effectively two-dimensional perturbations. We also present results for parallel shocks. Because of the turbulent field compression at the shock, the acceleration process becomes inefficient for larger turbulence amplitudes, and features observed in oblique shocks are recovered in this case. For small-amplitude perturbations, particle spectra are formed in the wide energy range, and modifications of the acceleration process due to the existence of long-wave perturbations are observed, as reported previously for mildly relativistic shocks. The critical turbulence amplitude required for efficient acceleration at parallel shocks decreases with increasing shock Lorentz factor γ. In both subluminal and superluminal shocks, an increase of γ leads to steeper spectra with lower cutoff energies. The spectra obtained for the "realistic" background conditions assumed in our simulations do not converge to the "universal" spectral index claimed in the literature. Thus, the role of the first-order Fermi acceleration in astrophysical sources hosting relativistic shocks requires serious reanalysis.

We present the results of a systematic analysis of gamma-ray burst afterglow spectral energy distributions (SEDs) in the optical/near-infrared bands. Our input list includes the entire world sample of afterglows observed in the pre-Swift era by the end of 2004 that have sufficient publicly available data. We apply various dust extinction models to fit the observed SEDs (Milky Way, Large Magellanic Cloud, and Small Magellanic Cloud) and derive the corresponding intrinsic extinction in the GRB host galaxies and the intrinsic spectral slopes of the afterglows. We then use these results to explore the parameter space of the power-law index of the electron distribution function and to derive the absolute magnitudes of the unextinguished afterglows.

Until recently, X-ray flares during the afterglow of gamma-ray bursts (GRBs) were a rarely detected phenomenon; thus, their nature is unclear. During the afterglow of GRB 050502B, the largest X-ray flare ever recorded rose rapidly above the afterglow light curve detected by the Swift X-Ray Telescope. The peak flux of the flare was >500 times that of the underlying afterglow, and it occurred >12 minutes after the nominal prompt burst emission. The fluence of this X-ray flare, (1.0 ± 0.05) × 10^-6 ergs cm^-2 in the 0.2-10.0 keV energy band, exceeded the fluence of the nominal prompt burst. The spectra during the flare were significantly harder than those measured before and after the flare. Later in time, there were additional flux increases detected above the underlying afterglow, as well as a break in the afterglow light curve. All evidence presented below, including spectral and, particularly, timing information during and around the giant flare, suggests that this giant flare was the result of internal dissipation of energy due to late central engine activity, rather than an afterglow-related effect. We also find that the data are consistent with a second central engine activity episode, in which the ejecta is moving slower than that of the initial episode, causing the giant flare and then proceeding to overtake and refresh the afterglow shock, thus causing additional activity at even later times in the light curve.

We have numerically studied the instability of the spherically symmetric standing accretion shock wave against nonspherical perturbations. We have in mind the application to collapse-driven supernovae in the postbounce phase, where the prompt shock wave generated by core bounce is commonly stalled. We take an experimental standpoint in this paper. Using spherically symmetric, completely steady, shocked accretion flows as unperturbed states, we have clearly observed both the linear growth and the subsequent nonlinear saturation of the instability. In so doing, we have employed a realistic equation of state, together with heating and cooling via neutrino reactions with nucleons. We have performed a mode analysis based on the spherical harmonics decomposition and found that the modes with l = 1,2 are dominant not only in the linear regime but also after nonlinear couplings generate various modes and saturation occurs. By varying the neutrino luminosity, we have constructed unperturbed states both with and without a negative entropy gradient. We have found that in both cases the growth of the instability is similar, suggesting that convection does not play a dominant role, which also appears to be supported by the recent linear analysis of the convection in accretion flows by Foglizzo et al. The oscillation period of the unstable l = 1 mode is found to fit better with the advection time rather than with the sound crossing time. Whatever the cause may be, the instability favors a shock revival.

Type IIP (plateau) supernovae are thought to come from stars with initial mass ~8-25 M_☉ that end their lives as red supergiants. The expected stellar endpoints can be found from evolutionary calculations, and the corresponding mass-loss properties at these points can be estimated from typical values for Galactic stars. The mass-loss densities of observed supernovae can be estimated from observations of the thermal X-ray and radio synchrotron emission that result from the interaction of the supernova with the surrounding wind. Type IIP supernovae are expected to have energy-conserving interaction during typical times of observation. Because Type IIP supernovae have an extended period of high optical luminosity, Compton cooling could affect the radio-emitting electrons, giving rise to a relatively flat radio light curve in the optically thin regime. Alternatively, a high efficiency of magnetic field production results in synchrotron cooling of the radio-emitting electrons. Both the X-ray and radio luminosities are sensitive to the mass loss and initial masses of the progenitor stars, although the turn-on of radio emission is probably the best estimator of circumstellar density. Both the mass-loss density and the variation of density with stellar mass are consistent with expectations for the progenitor stars deduced from direct observations of recent supernovae. Current observations are consistent with mass being the only parameter; observations of supernovae in metal-poor regions could show how the mass loss depends on metallicity.

We present u'g'r'i'BV photometry and optical spectroscopy of the Type Ib/Ic SN 2005bf covering the first ~100 days following discovery. The u'g'BV light curves displayed an unprecedented morphology among Type Ib/Ic supernovae, with an initial maximum some 2 weeks after discovery and a second, main maximum about 25 days after that. The bolometric light curve indicates that SN 2005bf was a remarkably luminous event, radiating at least 6.3 × 10⁴² ergs s^-1 at maximum light and a total of 2.1 × 10⁴⁹ ergs during the first 75 days after the explosion. Spectroscopically, SN 2005bf underwent a unique transformation from a Type Ic-like event at early times to a typical Type Ib supernova at later phases. The initial maximum in u'g'BV was accompanied by the presence in the spectrum of high-velocity (>14,000 km s^-1) absorption lines of Fe II, Ca II, and H I. The photospheric velocity derived from spectra at early epochs was below 10,000 km s^-1, which is unusually low compared with ordinary Type Ib supernovae. We describe one-dimensional computer simulations that attempt to account for these remarkable properties. The most favored model is that of a very energetic (2 × 10⁵¹ ergs), asymmetric explosion of a massive (8.3 M_☉) Wolf-Rayet WN star that had lost most of its hydrogen envelope. We speculate that an unobserved relativistic jet was launched producing a two-component explosion consisting of (1) a polar explosion containing a small fraction of the total mass and moving at high velocity and (2) the explosion of the rest of the star. At first, only the polar explosion is observed, producing the initial maximum and the high-velocity absorption-line spectrum resembling a Type Ic event. At late times, this fast-moving component becomes optically thin, revealing the more slowly moving explosion of the rest of the star and transforming the observed spectrum to that of a typical Type Ib supernova. If this scenario is correct, then SN 2005bf is the best example to date of a transition object between normal Type Ib/Ic supernovae and γ-ray bursts.

The Type Ib/c supernova SN 2001em was observed to have strong radio, X-ray, and Hα emission at an age of ~2.5 yr. Although the radio and X-ray emission have been attributed to an off-axis gamma-ray burst, we model the emission as the interaction of normal SN Ib/c ejecta with a dense, massive (~3 M_☉) circumstellar shell at a distance of ~7 × 10¹⁶ cm. We investigate two models, in which the circumstellar shell has or has not been overtaken by the forward shock at the time of the X-ray observation. The circumstellar shell was presumably formed by vigorous mass loss with a rate of ~(2-10) × 10^-3 M_☉ yr^-1 at ~(1-2) × 10³ yr prior to the supernova explosion. The hydrogen envelope was completely lost and subsequently was swept up and accelerated by the fast wind of the presupernova star up to a velocity of 30-50 km s^-1. Although interaction with the shell can explain most of the late emission properties of SN 2001em, we need to invoke clumping of the gas to explain the low absorption at X-ray and radio wavelengths.

The stars that end their lives as supernovae (SNe) have been directly observed in only a handful of cases, mainly because of the extreme difficulty of identifying them in images obtained prior to the SN explosions. Here we report the identification of the progenitor for the recent Type II-plateau (core collapse) SN 2005cs in pre-explosion archival images of the Whirlpool Galaxy (M51) obtained with the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS). From high-quality ground-based images of the SN obtained with the Canada-France-Hawaii Telescope, we precisely determine the position of the SN and are able to isolate the SN progenitor to within 004 in the HST ACS optical images. We further pinpoint the SN location to within 0005 from HST ACS ultraviolet images of the SN, confirming our progenitor identification. From photometry of the SN progenitor obtained with the pre-SN ACS images, and also from limits to its brightness in pre-SN HST NICMOS images, we infer that the progenitor is a red supergiant star of spectral type K3-M4 with initial mass 10 ± 3 M_☉. We also discuss the implications of the SN 2005cs progenitor identification and its mass estimate. There is an emerging trend that the most common Type II-plateau SNe originate from low-mass supergiants (8-20 M_☉).

The details of ignition of Type Ia supernovae remain fuzzy, despite the importance of this input for any large-scale model of the final explosion. Here, we begin a process of understanding the ignition of these hot spots by examining the burning of one zone of material, and then we investigate the ignition of a detonation due to rapid heating at single point. We numerically measure the ignition delay time for onset of burning in mixtures of degenerate material and provide fitting formulae for conditions of relevance in the Type Ia problem. Using the neon abundance as a proxy for the white dwarf progenitor's metallicity, we then find that ignition times can decrease by ~20% with the addition of even 5% of neon by mass. When temperature fluctuations that successfully kindle a region are very rare, such a reduction in ignition time can increase the probability of ignition by orders of magnitude. If the neon comes largely at the expense of carbon, a similar decrease in the ignition time can occur. We then consider the ignition of a detonation by an explosive energy input in one localized zone, for example, a Sedov blast wave leading to a shock-ignited detonation. Building on previous work on curved detonations, we confirm that surprisingly large inputs of energy are required to successfully launch a detonation, leading to required match heads of ≈4500 detonation thicknesses—tens of centimeters to hundreds of meters—which is orders of magnitude larger than naive considerations might suggest. This is a very difficult constraint to meet for some pictures of a deflagration-to-detonation transition, such as a Zel'dovich gradient mechanism ignition in the distributed burning regime.

Stellar models that incorporate simple diffusion or shear-induced mixing are used to describe canonical extra mixing in low-mass red giants of low and solar metallicity. These models are able to simultaneously explain the observed Li and CN abundance changes along the upper red giant branch (RGB) in field low-metallicity stars and match photometry, rotation, and ¹²C/¹³C ratios for stars in the old open cluster M67. The shear mixing model requires that main-sequence (MS) progenitors of upper RGB stars possessed rapidly rotating radiative cores and that specific angular momentum was conserved in each of their mass shells during their evolution. We surmise that solar-type stars will not experience canonical extra mixing on the RGB because their more efficient MS spin-down resulted in solid-body rotation, as revealed by helioseismological data for the Sun. Thus, RGB stars in the old, high-metallicity cluster NGC 6791 should show no evidence for mixing in their ¹²C/¹³C ratios. We develop the idea that canonical extra mixing in a giant component of a binary system may be switched to its enhanced mode with much faster and somewhat deeper mixing as a result of the giant's tidal spin-up. This scenario can explain photometric and composition peculiarities of RS CVn binaries. The tidally enforced enhanced extra mixing might contribute to the star-to-star abundance variations of O, Na, and Al in globular clusters. This idea may be tested with observations of ¹²C/¹³C ratios and CN abundances in RS CVn binaries.

A Monte Carlo simulation exploring uncertainties in standard stellar evolution theory on the red giant branch of metal-poor globular clusters has been conducted. Confidence limits are derived on the absolute V-band magnitude of the bump in the red giant branch luminosity function (M_V,b) and the excess number of stars in the bump, R_b. The analysis takes into account uncertainties in the primordial helium abundance, abundance of α-capture elements, radiative and conductive opacities, nuclear reaction rates, neutrino energy losses, the treatments of diffusion and convection, the surface boundary conditions, and color transformations. The uncertainty in theoretical values for the red giant bump magnitude varies with metallicity between +0.13 and -0.12 mag at [Fe/H] = -2.4 and between +0.23 and -0.21 mag at [Fe/H] = -1.0. The dominant sources of uncertainty are the abundance of the α-capture elements , the mixing length, and the low-temperature opacities. The theoretical values of M_V,b are in good agreement with observations. The uncertainty in the theoretical value of R_b is ±0.01 at all metallicities studied. The dominant sources of uncertainty are the abundance of the α-capture elements, the mixing length, and the high-temperature opacities. The median value of R_b varies from 0.44 at [Fe/H] = -2.4 to 0.50 at [Fe/H] = -1.0. These theoretical values for R_b are in agreement with observations.

We present the results of proper-motion measurements of the dust shell structure in the Egg Nebula (AFGL 2688, CRL 2688, V1610 Cyg), based on the archived two-epoch data at 2 μm taken with the Hubble Space Telescope. We measured the amount of motion of local structures in the nebula by determining their relative shifts over an interval of 5.5 yr. The dynamical age of the nebula is found to be roughly 350 yr based on the overall Hubble-law-esque motion of the nebula. By adopting the deprojected velocity of 45 km s^-1 at the tips of the bipolar lobes, our proper-motion measurements indicate that the distance to the Egg Nebula is about 420 pc and that the lobes are inclined at 77 with respect to the plane of the sky. The refined distance estimate yields a luminosity of the central star of 3.3 × 10³L_☉, a total shell mass of 1.2 M_☉, and a mass-loss rate (the upper limit) of 3.6 × 10^-3M_☉ yr^-1. Assuming a 0.6 M_☉ central post-AGB stellar mass, the initial mass of the Egg is 1.8 M_☉. Upon analysis, we also discovered that (1) the central star of the Egg Nebula has a proper motion of its own at a rate of 17 mas yr^-1, (2) the tips of the lobes increased their velocity due to shock acceleration, and (3) the apparent bipolar lobes consist of multiple outflows at distinct inclination angles projected onto each other.

The study of Be in stars of differing metal content can elucidate the formation mechanisms and the Galactic chemical evolution of the light element, Be. We have obtained high-resolution, high signal-to-noise ratio (S/N) spectra of the resonance lines of Be II in eight stars with the High Dispersion Spectrograph (HDS) on the 8.2 m Subaru Telescope on Mauna Kea. Abundances of Be have been determined through spectrum synthesis. The stars with [Fe/H] values greater than -1.1 conform to the published general trend of Be versus Fe. We have confirmed the high Be abundance in HD 94028 and have found a similarly high Be abundance in another star, HD 132475, at the same metallicity: [Fe/H] = -1.5. These two stars are 0.5-0.6 dex higher in Be than the Be-Fe trend. While that general trend contains the evidence for a Galaxy-wide enrichment in Be and Fe, the higher than predicted Be abundances in those two stars shows that there are also local Be enrichments. Possible enrichment mechanisms include hypernovae and multiple supernova explosions contained in a superbubble. One of our stars, G64-37, has a very low metallicity of [Fe/H] = -3.2; we have determined its Be abundance to look for evidence of a Be plateau. Its Be abundance appears to extend the Be-Fe trend to lower Fe abundances without any evidence for a plateau, as had been indicated by a high Be abundance in another very metal-poor star, G64-12. Although these two stars have similar Be abundances within the errors, it could be that their different Be values indicate a Be dispersion even at the lowest metallicities.

We analyze the stability of the dust layer in protoplanetary disks to understand the effect of relative motion between gas and dust. Previous analyses not including the effect of the relative motion between gas and dust show that shear-induced turbulence may prevent the dust grains from settling sufficiently to be gravitationally unstable. We determine the growth rate of the Kelvin-Helmholtz instability in a wide range of parameter space and propose a possible path toward planetesimal formation through gravitational instability. We expect the density of the dust layer to become ρ_d/ρ_g ~ 100 if the dust grains can grow up to 10 m.

Gas giant planets have been discovered in binary or triple star systems with a range of semimajor axes. We present a new suite of three-dimensional radiative gravitational hydrodynamics models suggesting that binary stars may be quite capable of forming planetary systems similar to our own. One difference between the new and previous calculations is the inclusion of artificial viscosity in the previous work, leading to significant conversion of disk kinetic energy into thermal energy in shock fronts and elsewhere. New models are presented showing how vigorous artificial viscosity can help to suppress clump formation. The new models with binary companions do not employ any explicit artificial viscosity and also include the third (vertical) dimension in the hydrodynamic calculations, allowing for transient phases of convective cooling. The new calculations of the evolution of initially marginally gravitationally stable disks show that the presence of a binary star companion may actually help to trigger the formation of dense clumps that could become giant planets. Earth-like planets would form much later in the inner disk regions by the traditional collisional accumulation of progressively larger, solid bodies. We also show that in models without binary companions, which begin their evolution as gravitationally stable disks, the disks evolve to form dense rings, which then break up into self-gravitating clumps. The latter models suggest that the evolution of any self-gravitating disk with sufficient mass to form gas giant planets is likely to lead to a period of disk instability, even in the absence of a trigger such as a binary star companion.

We determine the disk mass distribution around 336 stars in the young (~1 Myr) Orion Nebula cluster by imaging a 25 × 25 region in 3 mm continuum emission with the Owens Valley Millimeter Array. For this sample of 336 stars, we observe 3 mm emission above the 3 σ noise level toward 10 sources, six of which have also been detected optically in silhouette against the bright nebular background. In addition, we detect 20 objects in 3 mm continuum emission that do not correspond to known near-IR cluster members. Comparisons of our measured fluxes with longer wavelength observations enable rough separation of dust emission from thermal free-free emission, and we find substantial dust emission toward most objects. For the sample of 10 objects detected at both 3 mm and near-IR wavelengths, eight exhibit substantial dust emission. Excluding the two high-mass stars (θ¹ Ori A and the BN object) and assuming a gas-to-dust ratio of 100, we estimate circumstellar masses ranging from 0.13 to 0.39 M_☉. For the cluster members not detected at 3 mm, images of individual objects are stacked to constrain the mean 3 mm flux of the ensemble. The average flux is detected at the 3 σ confidence level and implies an average disk mass of 0.005 M_☉, comparable to the minimum-mass solar nebula. The percentage of stars in Orion surrounded by disks more massive than ~0.1 M_☉ is consistent with the disk mass distribution in Taurus, and we argue that massive disks in Orion do not appear to be truncated through close encounters with high-mass stars. Comparison of the average disk mass and number of massive dusty structures in Orion with similar surveys of the NGC 2024 and IC 348 clusters is used to constrain the evolutionary timescales of massive circumstellar disks in clustered environments.

High-resolution K-band imaging polarimetry of the β Pic dust disk has been conducted with adaptive optics and a coronagraph using the Subaru 8.2 m telescope. Polarization of ~10% is detected out to r ~ 120 AU with a centrosymmetric vector pattern around the central star, confirming that the disk is seen as an infrared reflection nebula. We have modeled our near-infrared and previous optical polarization results in terms of dust scattering in the disk and have found that both the degrees of polarization and the radial intensity profiles are well reproduced. We argue that the observed characteristics of the disk dust are consistent with the presence of ice-filled fluffy aggregates consisting of submicron grains in the β Pic system. There is a gap around 100 AU in both the intensity and polarization profiles, which suggests a paucity of planetesimals in this region. The radial intensity profile also shows ripple-like structures, which are indicative of the presence of multiple planetesimal belts, as in the case of the M-type Vega-like star AU Mic.

We present an analysis of the HD 82943 planetary system based on a radial velocity data set that combines new measurements obtained with the Keck telescope and the CORALIE measurements published in graphical form. We examine simultaneously the goodness of fit and the dynamical properties of the best-fit double-Keplerian model as a function of the poorly constrained eccentricity and argument of periapse of the outer planet's orbit. The fit with the minimum χ is dynamically unstable if the orbits are assumed to be coplanar. However, the minimum is relatively shallow, and there is a wide range of fits outside the minimum with reasonable χ. For an assumed coplanar inclination i = 30^° (sin i = 0.5), only good fits with both of the lowest order, eccentricity-type mean-motion resonance variables at the 2 : 1 commensurability, θ₁ and θ₂, librating about 0° are stable. For sin i = 1, there are also some good fits with only θ₁ (involving the inner planet's periapse longitude) librating that are stable for at least 10⁸ yr. The libration semiamplitudes are about 6° for θ₁ and 10° for θ₂ for the stable good fit with the smallest libration amplitudes of both θ₁ and θ₂. We do not find any good fits that are nonresonant and stable. Thus, the two planets in the HD 82943 system are almost certainly in 2 : 1 mean-motion resonance, with at least θ₁ librating, and the observations may even be consistent with small-amplitude librations of both θ₁ and θ₂.

We outline a method for quantifying the performance of extrapolation methods for magnetic fields. We extrapolate the field for two model cases, using a linear force-free approach and a nonlinear approach. Each case contains a different topological feature of the field that may be of interest in solar energetic events. We are able to determine quantitatively whether either method is capable of reproducing the topology of the field. In one of our examples, a subjective evaluation of the performance of the extrapolation suggests that it has performed quite well, while our quantitative score shows that this is not the case, indicating the importance of being able to quantify the performance. Our method may be useful in determining which extrapolation techniques are best able to reproduce a force-free field and which topological features can be recovered.

We exploit a rare joint set of high-resolution, very high cadence TRACE UV images and high-resolution magnetograms from SOHO MDI to investigate the dynamical properties of flare ribbons in a GOES M1 class flare from NOAA active region 9236 on 2000 November 23 at 23:28 UT. Assuming that flare ribbons locate the chromospheric footpoints of magnetic field lines reconnecting in the corona and that magnetic flux is conserved, we measure the magnetic reconnection rate (in maxwells per second) by overlaying the ribbons on co-registered magnetograms and using intensity-based binary masks to track the magnetic flux swept over by the evolving ribbons, and by assumption swept up in the reconnection. In the event observed, the ribbons did not separate with time but remained stationary while they brightened, lengthened, and faded in place. Thus, the ribbons may be akin to hard X-ray flare kernels moving antiparallel to each other, which others interpret as caused by strong photospheric shear. The derived reconnection rate is noisy, with little correlation between adjacent 1.4 s samples; the peak rate for pixels summed over the ribbon is ~5 × 10¹⁸ Mx s^-1; the average rise-phase rate is 10 times lower. The "local" rates for adjacent pixels added to the ribbon at adjacent times show correlations with 1600 Å band intensities, supporting the reconnection interpretation. For simple assumptions about geometry, the reconnection appears fast (V_in ≥ 0.01V_A). The peak reconnection rates, along with estimates of the current-sheet length scale suggested by measured quantities, imply peak electric fields of order 40 V cm^-1. We discuss caveats to these results.

We observed two X-class white-light flares (WLFs) on 2003 October 29 (~20:40 UT) and November 2 (~17:16 UT) using the Dunn Solar Telescope (DST) and its High-Order Adaptive Optics (HOAO) system in several wavelengths. The spatial resolution was close to the diffraction limit of DST's 76 cm aperture, and the cadence was as high as 2 s. This is the first time that WLFs have been observed in the near-infrared (NIR) wavelength region. We present a detailed study in this paper comparing photospheric continuum observations during the two events with corresponding line-of-sight magnetograms from the Solar and Heliospheric Observatory (SOHO) Michelson Doppler Imager (MDI) and hard X-ray (HXR) data from the Ramaty High-Energy Solar Spectroscopic Imager (RHESSI). We also discuss several models that provide possible mechanisms to explain these continuum enhancements, especially in the NIR.

We present observations of the 2002 September 30 white-light flare, in which the optical continuum emission near the Hα line is enhanced by ~10%. The continuum emission exhibits a close temporal and spatial coincidence with the hard X-ray (HXR) footpoint observed by RHESSI. We find a systematic motion of the flare footpoint seen in the continuum emission; the motion history follows roughly that of the HXR source. This gives strong evidence that this white-light flare is powered by heating of nonthermal electrons. We note that the HXR spectrum in 10-50 keV is quite soft with γ ≈ 7 and there is no HXR emission above 50 keV. The magnetic configuration of the flaring region implies magnetic reconnection taking place at a relatively low altitude during the flare. Despite a very soft spectrum of the electron beam, its energy content is still sufficient to produce the heating in the lower atmosphere, where the continuum emission originates. This white-light flare highlights the importance of radiative back-warming to transport the energy below when direct heating by beam electrons is obviously impossible.

We present the results of numerical simulations of solar energetic particles (SEPs) moving in a model heliospheric magnetic field. We find that the onset time of a given SEP event can be sooner than estimated by assuming that particles move along the usual Parker spiral, which is generally believed to be the earliest time. This is because for a random magnetic field, some magnetic field lines are shorter than the nominal (Parker spiral) length. This is important with regard to the physics of injection and/or acceleration at the origin of the particle event.

We provide improved atomic calculation of wavelengths, oscillator strengths, and autoionization rates relevant to the 2 → 3 inner-shell transitions of Fe VI-XVI, the so-called Fe M-shell unresolved transition array (UTA). A second-order many-body perturbation theory is employed to obtain accurate transition wavelengths, which are systematically larger than previous theoretical results by 15-45 mÅ. For a few transitions of Fe XVI and Fe XV for which laboratory measurements exist, our new wavelengths are accurate to within a few mÅ. Using these new calculations, the apparent discrepancy in the velocities between the Fe M-shell UTA and other highly ionized absorption lines in the outflow of NGC 3783 disappears. The oscillator strengths in our new calculation agree well with the previous theoretical data, while the new autoionization rates are significantly larger, especially for lower charge states. We attribute this discrepancy to the missing autoionization channels in the previous calculation. The increased autoionization rates may slightly affect the column density analysis of the Fe M-shell UTA for sources with high column density and very low turbulent broadening. The complete set of atomic data is provided as an electronic table.

Theoretical studies and current observations of the high-redshift intergalactic medium (IGM) indicate that at least two cosmic transitions occur by the time the universe reaches gas metallicities of about 10^-3Z_☉. One is the cosmological reionization of the IGM, and the second is the transition from a primordial to present-day mode of star formation. We quantify this relation through new calculations of the ionizing radiation produced in association with the elements carbon, oxygen, and silicon observed in Galactic metal-poor halo stars, which are likely second-generation objects formed in the wake of primordial supernovae. We demonstrate that sufficient ionizing photons per baryon are created by enrichment levels of [Fe/H] ~ -3 in the environment of metal-poor halo stars that provide the optical depth in the cosmic microwave background of ~0.1 detected by WMAP. We show that, on a star-by-star basis, a genuine cosmic milestone in the ionization of the IGM and in the mode of star formation occurred at metallicities of 10^-4 to 10^-3Z_☉ in these halo stars. This provides us with an important link in the chain of evidence for metal-free first stars having dominated the process of reionization by z ~ 6. We conclude that many of the Fe-poor halo stars formed close to the end of or soon after cosmological reionization, making them the ideal probe of the physical conditions under which the transition from first- to second-generation star formation happened in primordial galaxies.

We have observed 13 z ≥ 4.5 QSOs using the Multiband Imaging Photometer for Spitzer, nine of which were also observed with the Infrared Array Camera. The observations probe rest wavelengths ~0.6-4.3 μm, bracketing the local minimum in QSO spectral energy distributions (SEDs) between strong optical emission associated directly with accretion processes and thermal emission from hot dust heated by the central engine. The new Spitzer photometry combined with existing measurements at other wavelengths shows that the SEDs of high-redshift QSOs (z ≥ 4.5) do not differ significantly from typical QSOs of similar luminosity at lower redshifts (z ≲ 2). This behavior supports other indications that all the emission components and physical structures that characterize QSO activity can be established by z = 6.4. The similarity also suggests that some QSOs at high redshift will be very difficult to identify because they are viewed along dust-obscured sight lines.

The recent detection of delayed X-ray flares during the afterglow phase of gamma-ray bursts suggests an inner engine origin, at radii inside the deceleration radius characterizing the beginning of the forward-shock afterglow emission. Given the observed temporal overlap between the flares and afterglows, there must be inverse Compton (IC) emission arising from such flare photons that are scattered by forward-shock afterglow electrons. We find that this IC emission produces GeV-TeV flares, which may be detected by GLAST and ground-based TeV telescopes. We speculate that this kind of emission may already have been detected by EGRET from a very strong burst—GRB 940217. The enhanced cooling of the forward-shock electrons by the X-ray flare photons may suppress the synchrotron emission of the afterglows during the flare period. The detection of GeV-TeV flares, combined with low-energy observations, may help to constrain the poorly known magnetic field in afterglow shocks. We also consider self-IC emission in the context of internal-shock and external-shock models for X-ray flares. The emission above GeV energies from internal shocks is low, while the external-shock model can also produce GeV-TeV flares, but with a different temporal behavior from that caused by IC scattering of flare photons by afterglow electrons. This suggests a useful approach for distinguishing whether X-ray flares originate from late central-engine activity or from external shocks.

The recent localization of some short-hard gamma-ray bursts (GRBs) in galaxies with low star formation rates has lent support to the suggestion that these events result from compact object binary mergers. We discuss how new simulations in general relativity are helping to identify the central engine of short-hard GRBs. Motivated by our latest relativistic black hole-neutron star merger calculations, we discuss a scenario in which these events may trigger short-hard GRBs and compare this model to competing relativistic models involving binary neutron star mergers and the delayed collapse of hypermassive neutron stars. Distinguishing features of these models may help guide future GRB and gravitational wave observations to identify the nature of the sources.

We present new results from deep GALEX UV imaging of the cluster Cl 0024+17 at z ~ 0.4. Rest-frame far-UV emission is detected from a large fraction of so-called passive spiral galaxies—a significant population that exhibits spiral morphology with little or no spectroscopic evidence of ongoing star formation. This population is thought to represent infalling galaxies whose star formation has been somehow truncated by environmental processes, possibly in morphological transition to S0 galaxies. Compared to normal cluster spirals, we find that passive spirals are redder in FUV-optical color, while exhibiting much stronger UV emission than cluster E/S0 galaxies—as expected for recently truncated star formation. By modeling the different temporal sensitivities of UV and spectroscopic data to recent activity, we show that star formation in passive spirals decayed on timescales of less than 1 Gyr, consistent with "gas starvation"—a process where the cluster environment prevents cold gas from accreting onto the spiral disk. Intriguingly, the fraction of spirals currently observed in the passive phase is consistent with the longer period expected for the morphological transformation and the subsequent buildup of cluster S0 galaxies observed since z ≃ 0.4.

We use the number counts of X-ray-selected normal galaxies to explore their evolution by combining the most recent wide-angle shallow and pencil-beam deep samples available. The differential X-ray number counts, dN/dS, for early- and late-type normal galaxies are constructed separately and then compared with the predictions of the local X-ray luminosity function under different evolution scenarios. The dN/dS of early-type galaxies is consistent with no evolution out to z ≈ 0.5. For late-type galaxies, our analysis suggests that it is the sources with an X-ray-to-optical flux ratio log > -2 that are evolving the fastest. Including these systems in the late-type galaxy sample yields evolution of the form ≈(1 + z)^2.7 out to z ≈ 0.4. On the contrary, late-type sources with log < -2 are consistent with no evolution. This suggests that the log > -2 population comprises the most powerful and fast-evolving starbursts at moderate and high z. We argue that although residual low-luminosity AGN contamination may bias our results toward stronger evolution, this is unlikely to modify our main conclusions.

We present the detection of the reactive ion CO⁺ toward the prototypical starburst galaxy M82. This is the first secure detection of this short-lived ion in an external galaxy. Values of [CO⁺]/[HCO⁺] > 0.04 are measured across the inner 650 pc of the nuclear disk of M82. Such high values of [CO⁺]/[HCO⁺] have previously only been measured toward the atomic peak in the reflection nebula NGC 7023. This detection corroborates the scenario in which the molecular gas reservoir in the M82 disk is heavily affected by the UV radiation from recently formed stars. Comparing the column densities measured in M82 with those found in prototypical Galactic photon-dominated regions (PDRs), we need ~20 clouds along the line of sight to explain our observations. We have completed our model of the molecular gas chemistry in the M82 nucleus. Our PDR chemical model successfully explains the [CO⁺]/[HCO⁺] ratios measured in the M82 nucleus but fails by an order of magnitude to explain the large measured CO⁺ column densities [~(1-4) × 10¹³ cm^-2]. We explore possible routes to reconcile the chemical model and the observations.

New H I observations of Messier 31 (M31) obtained with the Effelsberg and Green Bank 100 m telescopes make it possible to measure the rotation curve of that galaxy out to ~35 kpc. Between 20 and 35 kpc, the rotation curve is nearly flat at a velocity of ~226 km s^-1. A model of the mass distribution shows that at the last observed velocity point, the minimum dark-to-luminous mass ratio is ~0.5 for a total mass of 3.4 × 10¹¹M_☉ at R < 35 kpc. This can be compared to the estimated Milky Way mass of 4.9 × 10¹¹M_☉ for R < 50 kpc.

From our catalog of Milky Way molecular clouds, created using a temperature thresholding algorithm on the Bell Laboratories ¹³CO survey, we have extracted two subsets: (1) clouds that are definitely larger than 10⁵M_☉, even if they are at their "near distance" (i.e., giant molecular clouds [GMCs]), and (2) clouds that are definitely smaller than 10⁵M_☉, even if they are at their "far distance." The positions and velocities of these clouds are compared to the loci of spiral arms in (l, v)-space. The radial velocity separation of each cloud from the nearest spiral arm is introduced as a "concentration statistic." Almost all of the GMCs are found near spiral arms. The density of smaller clouds is enhanced near spiral arms, but some clouds (~10% of the smaller clouds) are unassociated with any spiral arm. The median velocity separation between a GMC and the nearest spiral arm is 3.4 ± 0.6 km s^-1, whereas the median separation between smaller clouds and the nearest spiral arm is 5.5 ± 0.2 km s^-1. These separations in radial velocity are composed partly of the velocity dispersion of the cloud populations and partly of velocity differences due to spatial separations between the clouds and the spiral arms in the Galactic rotation field. A simple estimate indicates that the spatial separation component is relatively unimportant. The data are therefore consistent with the hypothesis that most molecular clouds in the Milky Way are spatially located in the spiral arms and that the velocity dispersion of the GMCs within the arms is less than that of the smaller clouds.

We report on observations of six giants in the globular cluster M15 (NGC 7078) using the Subaru Telescope to measure neutron-capture elemental abundances. Our abundance analyses, based on high-quality blue spectra, confirm the star-to-star scatter in the abundances of heavy neutron-capture elements (e.g., Eu), and we found no significant s-process contribution to them, as was found in previous studies. We have found that, for the first time, there are anticorrelations between the abundance ratios of light to heavy neutron-capture elements ([Y/Eu] and [Zr/Eu]) and the heavy neutron-capture elements (e.g., Eu). This indicates that the light neutron-capture elements in these stars cannot be explained by only a single r-process. Another process that contributed significantly to the light neutron-capture elements is required in M15. Our results suggest a complicated enrichment history for M15 and its progenitor.

We argue that rich star clusters take at least several local dynamical times to form and so are quasi-equilibrium structures during their assembly. Observations supporting this conclusion include morphologies of star-forming clumps, momentum flux of protostellar outflows from forming clusters, age spreads of stars in the Orion Nebula cluster (ONC) and other clusters, and the age of a dynamical ejection event from the ONC. We show that these long formation timescales are consistent with the expected star formation rate in turbulent gas, as recently evaluated by Krumholz & McKee. Finally, we discuss the implications of these timescales for star formation efficiencies, the disruption of gas by stellar feedback, mass segregation of stars, and the longevity of turbulence in molecular clumps.

We report the discovery of a 200 mHz quasi-periodic oscillation (QPO) in the X-ray emission from a bright ultraluminous X-ray source (ULX), Holmberg IX X-1, using a long XMM-Newton observation. The QPO has a centroid at ν_QPO = 202.5 mHz, a coherence Q ≡ ν_QPO/Δν_FWHM ≈ 9.3, and an amplitude (rms) of 6% in the 0.2-10 keV band. This is only the second detection of a QPO from a ULX, after M82 X-1, and provides strong evidence against beaming. The power spectrum is well fitted by a power law with an index of ≈0.7. The total integrated power (rms) is ≈9.4% in the 0.001-1 Hz range. The X-ray spectrum shows clear evidence of a soft X-ray excess component that is well described by a multicolor disk blackbody (kT_in ~ 0.3 keV) and a high-energy curvature that can be modeled either by a cutoff power law (Γ ~ 1; E_cutoff = 9 keV) or as a strongly Comptonized continuum in an optically thick (τ ≈ 7.3) and cool (kT_e ≈ 3 keV) plasma. Both the presence of the QPO and the shape of the X-ray spectrum strongly suggest that the ULX is not in the high/soft or thermally dominated state. A truncated disk and inner optically thick corona may explain the observed X-ray spectrum and the presence of the QPO.

We have used phase-resolved high-resolution images and low-resolution spectra taken at the ESO Very Large Telescope to study the properties of the low-mass helium white dwarf companion to the millisecond pulsar PSR J1911-5958A (COM J1911-5958A), in the halo of the Galactic globular cluster NGC 6752. The radial velocity curve confirms that COM J1911-5958A is orbiting the pulsar and allows us to derive a systemic velocity of the binary system nicely in agreement with that of NGC 6752. This strongly indicates that the system is a member of the cluster, despite its very offset position (~74 core radii) with respect to the core. Constraints on the orbital inclination (≳70°) and pulsar mass (1.2-1.5 M_☉) are derived from the mass ratio M_PSR/M_COM = 7.49 ± 0.64 and photometric properties of COM J1911-5958A. The light curve in the B band shows two phases of unequal brightening (Δmag ~ 0.3 and 0.2, respectively) located close to quadratures and superimposed on an almost steady baseline emission: this feature is quite surprising and needs to be further investigated.

Despite the observational effort carried out in the last few decades, no perfect solar twin has been found to date. An important milestone was achieved a decade ago by Porto de Mello & da Silva, who showed that 18 Sco is almost a solar twin. In the present work, we use extremely high resolution (R = 10⁵), high signal-to-noise ratio Keck HIRES spectra to carry out a differential analysis of 16 solar-twin candidates. We show that HD 98618 is the second-closest solar twin and that the fundamental parameters of both HD 98618 and 18 Sco are very similar (within a few percent) to the host star of our solar system, including the likelihood of hosting a terrestrial planet within their habitable zones. We suggest that these stars should be given top priority in exoplanet and SETI surveys.

The distribution of secondary star masses in present-day post-common-envelope binaries (PCEBs) is calculated using four different models for angular momentum loss (AML) during the post-common-envelope phase: only gravitational radiation (GR), GR plus disrupted magnetic braking (DMB), GR plus reduced MB, and GR plus intermediate MB. For the DMB model, we find that the number of PCEBs decreases abruptly by 38% once MB begins to operate for non-fully convective secondaries. We do not find a similar feature in the distributions calculated using any of the other three AML models in which MB is not disrupted. This percentage decrease in the number of present-day PCEBs predicted using the DMB model is easily large enough that an observed distribution of secondary masses or even spectral types in PCEBs can provide an important test of whether magnetic braking is indeed substantially reduced in secondary stars that are fully convective. We discuss briefly the feasibility of such observations.

We present VLT/NACO SDI images of the very nearby star SCR 1845-6357 (hereafter SCR 1845). SCR 1845 is a recently discovered M8.5 star just 3.85 pc from the Sun. Using the capabilities of the unique SDI device, we discovered a substellar companion to SCR 1845 at a separation of 4.5 AU (1170 ± 0003 on the sky) and fainter by 3.57 ± 0.057 mag in the 1.575 μm SDI filter. This substellar companion has an H magnitude of 13.16 (absolute H magnitude of 15.30), making it likely the brightest mid-T dwarf known. The Simultaneous Differential Imager (SDI) consists of three narrowband filters placed around the 1.6 μm methane absorption feature characteristic of T dwarfs (T_eff < 1200 K). The flux of the substellar companion drops by a factor of 2.7 ± 0.1 between the SDI F1 (1.575 μm) and F3 (1.625 μm) filters, consistent with strong methane absorption in a substellar companion. We estimate a spectral type of T5.5 ± 1 for the companion based on the strength of this methane break. The chances that this object is a background T dwarf are vanishingly small—and there is no isolated background T dwarf in this part of the sky, according to 2MASS. Thus, it is a bound companion, hereafter SCR 1845B. For an age range of 100 Myr to 10 Gyr and spectral type range of T4.5-T6.5, we find a mass range of (9-65)M_Jup for SCR 1845B from the Baraffe et al. "COND" models. SCR 1845AB is the 24th-closest stellar system to the Sun (at 3.85 pc); the only brown dwarf system closer to the Sun is the binary brown dwarf Indi Ba-Bb (at 3.626 pc). In addition, this is the first T dwarf companion discovered around a low-mass star.

We report on observations of comet 9P/Tempel 1 carried out before, during, and after the NASA Deep Impact event (UT July 4), with the optical spectrometers UVES and HIRES mounted on the telescopes Kueyen of the ESO VLT (Chile) and Keck I on Mauna Kea (Hawaii), respectively. A total observing time of about 60 hr, distributed over 15 nights around the impact date, allowed us (1) to find a periodic variation of 1.709 ± 0.009 days in the CN and NH flux, explained by the presence of two major active regions; (2) to derive a lifetime ≳5 × 10⁴ s (at 1.5 AU) for the parent of the CN radical from a simple modeling of the CN light curve after the impact; (3) to follow the gas and dust spatial profiles' evolution during the 4 hr following the impact and derive the projected velocities (400 and 150 m s^-1, respectively); and (4) to show that the material released by the impact has the same carbon and nitrogen isotopic composition as the surface material (¹²C/ ¹³C = 95 ± 15 and ¹⁴N/¹⁵N = 145 ± 20).

In a search for the cause of the intense heating revealed by X-ray emission in filament channels, we have simulated the evolution of a twisted toroidal flux rope emerging quasi-statically into the corona. Initially, the simulated flux rope remains confined in equilibrium as the stored magnetic energy increases. With enough twist buildup, there is a sudden catastrophic loss of equilibrium and total expulsion of the flux rope. We focused on the quasi-static phase in which a current sheet forms within the flux rope cavity, along the so-called bald-patch separatrix surface (BPSS). This comprises an envelope of field lines that graze the anchoring lower boundary, enclosing the detached helical field that supports the prominence. Significant magnetic energy dissipation and heating are expected to center around such current sheets. The heating that should result provides a plausible explanation for the hot X-ray sources, although they appear to be colocated with cool material. If our physical picture is correct, then the development of X-ray "bright cores" or "sigmoids" in a filament channel suggests the presence of a BPSS separating the helical field of a twisted flux rope in stable confinement from the surrounding untwisted fields.

In order to determine whether EIT waves are generated by coronal mass ejections (CMEs) or pressure pulses in solar flares, 14 non-CME-associated energetic flares, which should possess strong pressure pulses in their loops, are studied. They are selected near solar minimum, as this favors the detection of EIT waves. It is found that none of these flares are associated with EIT waves. Particular attention is paid to AR 0720, which hosted both CME-associated and non-CME types of flares. The SOHO/EIT images convincingly indicate that EIT waves and expanding dimmings appear only when CMEs are present. Therefore, it is unlikely that pressure pulses from flares generate EIT waves.

Recent spectroscopic models of active galactic nuclei (AGNs) have indicated that the recommended electron-ion recombination rate coefficients for iron ions with partially filled M shells are incorrect in the temperature range where these ions form in photoionized plasmas. We have investigated this experimentally for Fe XIV forming Fe XIII. The recombination rate coefficient was measured by employing the electron-ion merged-beams method at the Heidelberg heavy-ion test storage ring. The measured energy range of 0-260 eV encompasses all dielectronic recombination 1s²2s²2p⁶3l3l'3l'' nl''' resonances associated with the 3p_1/2 → 3p_3/2, 3s → 3p, 3p → 3d, and 3s → 3d core excitations within the M shell of the Fe XIV (1s²2s²2p⁶3s²3p) parent ion. This range also includes the 1s²2s²2p⁶3l3l'4l'' nl''' resonances associated with 3s → 4l'' and 3p → 4l'' core excitations. We find that in the temperature range 2-14 eV, where Fe XIV is expected to form in a photoionized plasma, the Fe XIV recombination rate coefficient is orders of magnitude larger than previously calculated values.