ABSTRACT
We present photometric and spectroscopic observations of SN 2000ch, an unusual and extremely luminous variable star located in the galaxy NGC 3432. The object was discovered on 2000 May 3.2 during the course of the Lick Observatory Supernova Search, at an unfiltered magnitude of about 17.4. Prediscovery images obtained in 1997, 1998, and 2000 April show the object with R = 19.2–19.5 mag. Optical spectra obtained beginning on 2000 May 6 show a smooth, flat continuum and strong, broad hydrogen Balmer emission lines at wavelengths consistent with the cataloged redshift of NGC 3432, strengthening the association of the variable with the galaxy. Photometric monitoring reveals a complex and erratic light curve over a time span of ∼10 days. Subsequent optical spectra over the next ∼3 months continued to show strong Balmer emission lines with a mean full width at half‐maximum intensity ∼1550 km s−1 and a distinct red asymmetry. A spectrum obtained 9 months after the outburst is similar to the previous spectra, but the integrated flux in Hα is nearly half that observed during the outburst. The object's photometric behavior, spectrum, and luminosity suggest that it is a very massive and luminous variable star and might be related to some luminous blue variable stars such as η Carinae and SN 1997bs in NGC 3627. The brightest apparent magnitude implies an absolute magnitude of MV ≈ - 12.7 at the distance of NGC 3432, a value that is comparable to η Car during its outburst in the mid‐19th century.
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1. INTRODUCTION
Recent systematic searches for supernovae (SNe) in external galaxies, such as the Lick Observatory Supernova Search (LOSS; Li et al. 2000; Filippenko et al. 2001), have revealed not only a plethora of new SNe, but many other luminous variable objects as well. Some of these include a class of peculiar Type II supernovae (SNe II), designated SNe IIn, that exhibit hydrogen emission lines consisting of a broad complex base underlying a stronger, sometimes slightly blueshifted, narrow emission component with no trace of the broad P Cyg profiles that are characteristic of normal SNe II (Schlegel 1990; Filippenko 1991).1 SNe IIn probably originate from the explosion of massive stars in which the high‐speed ejecta and hard radiation field interact with the dense circumstellar envelope, created previously by extreme mass‐loss from the progenitor.
It has become increasingly clear that some objects cataloged previously as peculiar SNe II are actually SNe IIn, while others are not genuine SNe, defined to be the final demise of a star at the end of its life. The latter objects are now believed to result from outbursts of luminous, massive stars such as the class of luminous blue variables (LBVs). Historical examples include SN 1954J in NGC 2403 (Humphreys & Davidson 1994; Smith, Humphreys, & Gehrz 2001), SN 1961V in NGC 1058 (Goodrich et al. 1989; Filippenko et al. 1995; Van Dyk et al. 2002), and SN 1997bs in NGC 3627 (Van Dyk et al. 2000). An excellent review of the observational properties and challenges posed by LBVs is given by Humphreys & Davidson (1994).
LBVs are evolved, massive, and unstable hot stars that undergo sporadic, sometimes violent, mass‐loss events and whose nature is not well understood. The group is not entirely homogeneous, since there are stars that are luminous, blue, and slightly variable that are not LBVs. In general, well‐studied LBVs have bolometric absolute magnitudes brighter than −9.5, suggesting masses in excess of 50 M⊙. These stars exhibit relatively large mass‐loss rates of 10−5 to 10−4 M⊙ yr−1, which can be observed as an infrared excess from circumstellar ejecta. An exceptional circumstellar envelope is the "homunculus" surrounding η Car. LBVs exhibit eruptions of 1–2 mag on timescales of 10–40 yr and may have superoutbursts of more than 2 mag on timescales of hundreds to thousands of years. Their optical spectra typically show emission lines of H and He i, in addition to both permitted and forbidden lines of Fe ii. These lines are often accompanied by P Cyg profiles with typical expansion velocities of 100–200 km s−1. At minimum light, the spectra are typical of hot supergiants with Teff ≈ 12,000–30,000 K, while at maximum the spectra resemble cooler supergiants and are characteristic of an optically thick extended photosphere with Teff ≈ 7000–8000 K. Examples in the Galaxy include P Cyg, η Car, S Dor, and AG Car. Their eruptions can be dramatic: η Car underwent an outburst between 1830 and 1860 in which it briefly reached Mbol ≈ - 14 mag.
In this paper we describe the discovery and evolution of a new luminous variable star designated SN 2000ch (Papenkova & Li 2000; Wagner et al. 2000a, 2000b), which might be related to LBVs. The object exhibits erratic photometric variations on relatively fast timescales compared to classical LBVs, but it has similar optical spectra when compared to some LBVs in outburst. During the outburst, its luminosity and size appear comparable to that of η Car, while in quiescence it remains a luminous object and sufficiently bright for detailed study.
2. DISCOVERY AND IDENTIFICATION
Papenkova & Li (2000), representing the LOSS team, discovered a new variable star in the field of the galaxy NGC 3432 on 2000 May 3.2 with the 0.76 m Katzman Automatic Imaging Telescope (KAIT). The presence of the object was confirmed on unfiltered images obtained on May 4.2 (Fig. 1). Photometry of the discovery images gives unfiltered magnitudes of 17.4 and 18.2 on May 3 and 4, respectively. The position of the object is α = 10h52m41
40, δ = +36°40'08
5 (J2000.0; Papenkova & Li 2000), which is 123 east and 180 north of the diffuse nucleus of NGC 3432. An image obtained with KAIT on April 29.2 showed nothing at the position of the new object brighter than a limiting magnitude of about 19. An identification chart for SN 2000ch is shown in Figure 2.
Fig. 1.— Unfiltered discovery images of the luminous variable star (SN 2000ch) in the field of NGC 3432, obtained with KAIT on 2000 May 3.2 (left) and May 4.2 (right). The variable is indicated by the tick marks. Each frame measures 6
1 × 66 on a side. North is to the top, and east is to the left. Photometry of the variable gives 17.4 and 18.2 mag, respectively.
Fig. 2.— Identification chart for the variable star in NGC 3432, obtained with the USNO 1.0 m telescope. Lettered field stars correspond to those listed in Table 1. North is to the top, and east is to the left. The image covers 6' × 6' on a side.
NGC 3432 is a diffuse and possibly interacting galaxy. Schwarzkopf & Dettmar (2000a, 2000b) recently studied NGC 3432 as part of a sample of high‐inclination galaxies that exhibit perturbations in their vertical disk structure as the result of tidal interactions and mergers. Its cataloged heliocentric velocity from the NASA/IPAC Extragalactic Database is 616 km s−1, which corresponds to a distance of 10.5 Mpc, corrected for Virgocentric flow and assuming H0 = 75 km s−1 Mpc−1 (Schwarzkopf & Dettmar 2000b). Its revised Hubble type is 9.0, based on the NASA/IPAC Extragalactic Database, placing it between the Sc‐Irr and Irr‐I galaxies of the original Hubble (1926) classification scheme.
3. OBSERVATIONS
3.1. Photometry
Photometry of SN 2000ch was obtained with the 1.0 m Ritchey‐Chrétien reflector of the United States Naval Observatory Flagstaff Station (USNOFS) between 2000 May 7 and July 4. The 1 K × 1 K CCD camera was used in combination with UBVRI filters to image a 113 × 113 region of the sky at a scale of 067 pixel−1. The seeing during the run typically averaged ∼2. Bias and twilight sky flat‐field frames were also obtained to facilitate the data reduction, which was performed using the IRAF2 package.
In addition, infrared (IR) images of NGC 3432 were obtained between 2000 May 13 and May 22 with the USNO 1.55 m Kaj Strand astrometric reflector and the 256 × 256 pixel Rockwell HgCdTe (NICMOS) infrared camera (IRCAM). JHK images were obtained covering a field of view of 23 × 23 at a scale of 054 pixel−1.
BVR images of NGC 3432 and various standard stars under photometric conditions were used to accurately calibrate the magnitudes and colors of eight stars in the field of the galaxy and surrounding SN 2000ch. Astrometry and photometry for these stars are presented in Table 1, and their identification with respect to NGC 3432 and SN 2000ch is shown in Figure 2.
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The brightness of SN 2000ch was measured by digital aperture photometry of the CCD images using the APPHOT package in IRAF and compared differentially with stars from the standard calibration of the surrounding field described above.
Additional photometric observations of SN 2000ch were obtained with the 0.57 m f/5.2 reflector of the Kleť Observatory, which is equipped with a SBIG ST‐8 CCD camera (Tichá, Tichý, & Moravec 2000). This CCD camera is equipped with a Kodak KAF1600 detector with quantum efficiency of about 40% for a wavelength interval of 5000–8500 Å. The field of view is 160 × 107. The limiting magnitude is mV = 19.5 for a 120 s exposure under standard conditions. Under very good sky conditions and no moonlight, the limit is mV = 20.2 for a 120 s exposure, and mV = 20.4 for a 180 s exposure.
The photometric measurements of SN 2000ch are summarized in Table 2.
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3.2. Spectroscopy
Spectra of SN 2000ch were obtained at several observatories; a journal of observations is given in Table 3. Spectral reductions were performed in the standard way using the IRAF package; these generally included bias subtraction, flat‐fielding, wavelength calibration, flux calibration, and removal of telluric bands (see, e.g., Matheson et al. 2000). Mean extinction coefficients were assumed at each observatory. The spectra were placed on an absolute flux scale by matching the V‐band magnitude of SN 2000ch inferred from the interpolated light curve shown in Figure 3.
Fig. 3.— R‐band light curve of the variable star in NGC 3432 beginning on 2000 April 10, just prior to the discovery by KAIT on 2000 May 3, and continuing through observations obtained on July 4. The light curve consists of a rapid brightening to R = 17.4 mag, followed by an equally rapid fading to R = 20.8 mag on May 10. Subsequently, the object brightened to R = 18.6 mag on May 14, after which it varied between R = 18.6 and 19.4 mag. The KAIT and Kleť "open" observations are unfiltered but correspond roughly to the R band.
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The initial spectrum was obtained on 2000 May 6, the day following the discovery announcement, using the CCD spectropolarimeter (Schmidt, Stockman, & Smith 1992) attached to the Steward Observatory 2.3 m Bok telescope at Kitt Peak National Observatory (KPNO). Standard‐star observations were not obtained with the adopted grating; hence, secondary calibration to absolute units was made with respect to white dwarfs observed for another program on both that night and the previous nights. Removal of the atmospheric "B" band was not possible.
Another spectrum was obtained on 2000 June 5.15 using the Ohio State University Boller and Chivens CCD spectrograph on the Hiltner 2.4 m telescope of the MDM Observatory on Kitt Peak.
SN 2000ch was observed twice with the Kast spectrograph (Miller & Stone 1993) on the Shane 3 m reflector at Lick Observatory, on 2000 May 31.3 and June 27.2. The air mass was 1.9–2.1 during the latter observation, and the slit position angle (68°, through the star that is east‐northeast of the SN; see Fig. 2) was very close to the parallactic angle to avoid loss of light at the slit (Filippenko 1982).
On 2001 January 19, a final spectrum of SN 2000ch was obtained with the Boller and Chivens CCD spectrograph at the 2.3 m telescope at KPNO. The absolute intensity of the spectrum was scaled to match the average V magnitude of SN 2000ch in quiescence.
4. RESULTS
4.1. Archival Photometric Behavior
Both POSS‐I and POSS‐II epoch frames were examined for additional outbursts or other activity related to SN 2000ch. NGC 3432 appears on POSS‐I 103aO and 103aE plates obtained on 1953 May 8. We find that at the location of SN 2000ch, B>20.1 mag and R>20.0 mag. NGC 3432 was photographed again on four epochs as part of the POSS‐II survey. We have examined these scanned images and find for SN 2000ch: on 1996 March 19, B>20; 1998 April 17, I = 19.8 ± 0.5; 1998 May 16, R = 19.5 ± 0.3; and 1998 May 17, R = 20.0 ± 0.3 mag. These magnitudes were determined by transforming the USNO‐A2.0 instrumental magnitudes to the standard system utilizing our photometric calibration of the surrounding field as described above. Our results are in good agreement with an examination of the POSS images by Yamaoka (2000). Thus, no major or extended outbursts were detected in the rather limited sky survey plate material.
In their study of irregular galaxies, Schwarzkopf & Dettmar (2000a) imaged NGC 3432 in the R band on 1997 May 31, using the 1.07 m Hall telescope of the Lowell Observatory on Anderson Mesa, and the TI 800 × 800 pixel CCD camera. The image scale was 036 per pixel. The transparency was excellent and the seeing was 11. SN 2000ch was easily detected in their image. Photometry of this image gives R = 19.2 ± 0.1 mag, which is comparable to the other quiescent magnitude determinations.
In addition, NGC 3432 was observed 32 times between 1998 December 11 and 2000 April 29 as part of the LOSS program. SN 2000ch was typically fainter than R = 19 mag for nearly 90% of these observations, and fainter than R = 18.4 mag for the remainder. Taken together, examination of archival plate and CCD image material indicates that no previous outbursts comparable to that discovered in 2000 May have been observed and that the data are consistent with a roughly constant pre‐outburst brightness of R = 19.4 ± 0.4 mag. However, previous outbursts could certainly have been missed if their timescale was similar to that of this recent fast event.
4.2. Light Curve
An R‐band light curve of SN 2000ch based on observations obtained primarily at the USNO and KAIT, with smaller contributions from other observatories, is shown in Figure 3. The unfiltered KAIT magnitudes have been transformed to approximate R magnitudes based on the observed colors of the variable (see Riess 1999 for a discussion). Archival observations obtained with KAIT about 20 days before the May 3 outburst show SN 2000ch at R = 19.3 mag and are consistent with its quiescent brightness. The object brightened rapidly to R = 17.4 mag on May 3.2, after which it faded quickly to R = 20.8 mag on May 10. The object then recovered and brightened to R = 18.6 mag on May 14, after which it varied between R = 18.6 and 19.4 mag until the conclusion of our observations on 2000 July 4. The deep minimum may be indicative of a brief dust formation phase, occultation, or eclipse, or more likely results from the evolution of an optically thick, cooling, ejected envelope. If the fluctuations observed after the deep minimum correspond to the multiple ejection and evolution of optically thick clouds or envelopes, then the evolutionary timescale for such events is ∼5–7 days.
The variability exhibited by the broadband colors compared with the V‐band magnitude is shown in Figure 4 between TJD 51,679–51,706 and immediately after the outburst. The V‐band magnitude monitors changes in the overall brightness of the optical continuum, while B−V is largely sensitive to changes in the slope of the continuum. U−B is sensitive to changes in the strength of the Balmer continuum, while V−R and R−I together are sensitive to changes in Hα emission‐line flux. Little variability is apparent in the U−B and B−V light curves, except for one outlying point near TJD 51,684, suggesting little change in the slope of the optical continuum or the strength of the Balmer continuum. Larger fluctuations are, however, present in both V−R and R−I. There is marginal evidence that the variations are anticorrelated, which suggests variability of Hα emission on nightly timescales.
Fig. 4.— Variation of the broadband colors of SN 2000ch compared with the overall V‐band light curve immediately after the outburst. The largest variations occur in V−R and R−I, with some evidence they are anticorrelated, suggesting variability of Hα emission.
4.3. Spectroscopy
Spectra of SN 2000ch obtained on 2000 May 6, May 31, June 5, and June 27 are shown in Figure 5. They exhibit prominent emission lines of Hα, Hβ, and Hγ superposed on a flat continuum. Weak He i λ5876 and He ii λ4686 emission are also visible in spectrum (b) of Figure 5. The mean full width at half‐maximum (FWHM) intensity of Hα emission is 1550 km s−1 after correction for instrumental resolution. This gives a mean expansion velocity of ∼800 km s−1 for the envelope. The measured FWHM is comparable to other LBV and η Car variables. A red wing is sometimes visible in the Hα profile. No forbidden lines are present in any of the spectra, indicating that the electron density is in excess of ∼108 cm−3; the absence of [O i] emission suggests that ne≳109–1010 cm−3. Our spectra lack the presence of Fe ii and [Fe ii] emission lines that are typically present in the spectra of LBVs in both outburst and quiescence. In addition, our spectra lack emission lines with accompanying P Cyg profiles often seen in LBVs at visual maximum when the optically thick expanded atmosphere resembles a much cooler A or F supergiant (Humphreys & Davidson 1994).
Fig. 5.— Spectra of the luminous variable star (SN 2000ch) in NGC 3432, obtained with the Steward 2.3 m (a), Lick 3 m (b and d), and MDM 2.4 m (c) telescopes on 2000 May 6, May 31, June 5, and June 27, respectively. The observed FWHM of Hα emission is 1350 km s−1, giving an average expansion velocity of ∼700 km s−1. The spectra show little evolution over the interval of our observations.
A spectrum of SN 2000ch obtained 9 months after the 2000 outburst on 2001 January 19, when the variable was fainter than 19 mag, is shown in Figure 6. It is qualitatively similar to the previous spectra and exhibits strong and broad Balmer emission lines, with a red wing superposed on a slightly blue continuum. The FWHM of Hα emission is 1350 km s−1, implying an expansion velocity of ∼700 km s−1, comparable to that measured during the outburst. In general, we find that the optical spectral properties of SN 2000ch are similar during the outburst and in quiescence. In addition, no evidence is found in our long‐slit spectroscopy obtained during the outburst or in quiescence for narrow Balmer or nebular emission lines from an underlying H ii region.
Fig. 6.— Spectrum of the luminous variable star obtained with the Steward Observatory Bok 2.3 m telescope on 2001 January 19, 9 months after the 2000 outburst. This spectrum is similar to those obtained during the outburst.
The Hα to Hβ intensity ratio increases significantly from a value of 3.4 on May 6 to 5.4 on May 31, and to 6.2 on June 27. The V−R color increases from 0.21 to 0.88 mag, while B−V varies between 0.24 and 0.29 mag over approximately the same time interval, suggesting that all the variation is due to the increasing intensity of the emission lines, particularly Hα. This is readily apparent in Figure 7, where the Hα line profile observed on 2000 May 31 is ∼50% stronger than that obtained on 2001 January 19. A possible interpretation is an increasing optical thickness in the Balmer lines (case C recombination), as has been observed in some SNe II such as SN 1987A (Xu et al. 1992).
Fig. 7.— Hα line profiles obtained on 2000 May 31 (solid line) at the Lick Observatory, and on 2001 January 19 (dotted line) at the Steward Observatory. Note the large change in the strength of the line.
An estimate of the size of the line‐emitting region can be derived from the luminosity in Hα and recombination theory. Assuming a distance of 10.5 Mpc to NGC 3432 and an average integrated flux in Hα of 2 × 10-14 ergs cm−2 s−1, the luminosity of Hα is 2.5 × 1038 ergs s−1. We can use the luminosity of the emission line to estimate the size of the emission region, since
where R is the radius, NH is the hydrogen density, αB is the line emissivity (case C, ∼10−24 ergs cm3 s−1; Xu et al. 1992), and ν is the frequency corresponding to Hα. For an assumed density of 1010 cm−3, we find that R(Hα) ≈ 0.2 pc and is comparable in size to the famous "homunculus" surrounding η Car.
4.4. Spectral Energy Distribution and Maximum Luminosity
The observed spectral energy distribution (SED) of SN 2000ch is shown in Figure 8. It was constructed from optical photometry obtained on 2000 May 14 using the USNO 1 m telescope and IR photometry obtained with the 1.55 m telescope. The spectrum consists of a hot component that gives rise to the blue continuum and a marginally significant excess in the U band, possibly attributable to Balmer continuum emission, as well as a cool component that is responsible for the apparent excess in the K band. Accordingly, we have modeled the SED with two blackbodies in an iterative approach assuming AV = 0. We find that the hot component is characterized by a color temperature of T1 = 7800 ± 100 K, while the cool component is characterized by a color temperature of T2 = 1000 ± 200 K. Note, however, that the two‐temperature blackbody fit is not very good, with significant deviations at optical wavelengths; thus, the formally derived temperatures may be somewhat misleading with artificially small quoted uncertainties. The projected solid angles based on the fits imply that the ratio of their radii is R2/R1 = 35 ± 1. For a distance of 10.5 Mpc, R1 = (6.7 ± 1.5) × 1013 cm, so that the radius of the cool component is ∼ 2.5 × 1015 cm.
Fig. 8.— Observed spectral energy distribution of SN 2000ch is shown based on USNO photometry obtained on 2000 May 14. The dotted line is a 7800 K blackbody, while the dashed line is a 1000 K blackbody. The composite model spectrum is shown by the heavy solid line. There is evidence for an IR excess, possibly due to heated dust or free‐free emission. The marginally significant excess in the U band might be attributable to Balmer continuum emission. The two‐temperature blackbody fit is not particularly good, but is meant to illustrate only the general trends.
Stronger evidence for the presence of Balmer continuum emission can be found by examining the position of the variable in a color‐color diagram (Fig. 9) constructed from our broadband photometry. It can be seen that the variable occupies a small region in the diagram, although its brightness varies considerably. The intrinsic colors of eight blue variables in M31 and M33, plus η Car, MWC 349, and S Dor, lie close to the blackbody line and are bluer than SN 2000ch (Humphreys 1978). It is important to note that SN 2000ch also lies far from the locus of points defining normal supergiants and above the line defining radiating blackbodies. This strongly suggests the presence of Balmer continuum emission. The tight concentration of points implies that the strength and shape of the Balmer jump does not vary much during the brightness fluctuations. These conclusions are not influenced by the presence of interstellar reddening. This interpretation is also consistent with the observations that most, but not all, known luminous blue variables have more negative (U−B)0 colors for a given (B−V)0 than main‐sequence or supergiant stars (Humphreys 1978), indicating the presence of Balmer continuum emission.
Fig. 9.— Position of SN 2000ch during its outburst in a U−B vs. B−V color‐color plot. The locus of points corresponding to main‐sequence stars (dotted line) and supergiants (dashed line) is shown. The blackbody line and a reddening vector corresponding to E(B−V) = 0.2 are also shown. The variable lies above the line defining radiative blackbodies, suggesting the presence of Balmer continuum emission.
At its brightest, SN 2000ch reached an apparent unfiltered magnitude of 17.4, corresponding to an absolute magnitude of −12.7 at the adopted distance of NGC 3432, 10.5 Mpc (μ = 30.1 mag). Its mean quiescent level of R = 19.4 mag implies a persistent absolute magnitude of −10.7. Thus, even in quiescence, SN 2000ch is an extremely luminous object. It bears many similarities to η Car during its last major eruption, between 1830 and 1860, when it reached Mbol ≈ - 14 mag and was comparable in size to SN 2000ch. If the quiescent luminosity of SN 2000ch is constrained by the Eddington limit, the mass of the progenitor must exceed 40 M⊙.
5. DISCUSSION
The spectral features observed early in the outburst of SN 2000ch are reminiscent of those observed in some M31 classical novae. This suggested to Wagner et al. (2000a) that SN 2000ch might be related to a class of superbright classical novae (Della Valle 1991), which have peak absolute magnitudes ∼1 mag brighter than typical Galactic and M31 novae (MV ≈ - 9 mag). However, a bright‐nova interpretation is inconsistent with the high luminosity (106–107 L⊙), the relatively low expansion velocity, and the fast timescale that we observed. In the study presented by Della Valle (1991), no classical nova has been observed in the Galaxy, M31, or the Virgo Cluster with a luminosity comparable to that of SN 2000ch.
Based on the early data presented here, Filippenko (2000) suggested that the variable star in NGC 3432 could be a subluminous SN IIn (Schlegel 1990; Filippenko 1991) and thus perhaps related to the class of luminous blue variables (LBVs). Nearly all SNe IIn show hydrogen emission lines consisting of a broad complex base and a strong, narrow, and slightly blueshifted component superposed on a blue continuum. Broad P Cyg absorption components are not present (Schlegel 1990). In addition, some SNe IIn have been observed in spiral galaxies in and near–H ii regions or large massive star‐forming complexes. Their absolute magnitudes at discovery are typically −17.5 to −18.0 and thus are comparable to most SNe II (Schlegel 1990). The light curves of SNe IIn, however, are not well characterized.
Recently, it has become apparent that the SN IIn class is quite heterogeneous (Filippenko 1997). It consists of objects that are certainly bona fide SNe II in which core collapse has led to the destruction of the star. But as additional objects have been discovered whose characteristics resemble SNe IIn, it appears that some are in fact not true core‐collapse SN, but are the result of superoutbursts of luminous, massive stars. Some of these stars are luminous blue variables on their way to becoming Wolf‐Rayet stars. Objects in this class include η Car (Davidson & Humphreys 1997), SN 1954J in NGC 2403 (Humphreys & Davidson 1994), SN 1961V in NGC 1058 (Goodrich et al. 1989; Filippenko et al. 1995; Van Dyk et al. 2002), SN 1997bs in NGC 3627 (Van Dyk et al. 2000), SN 1999bw in NGC 3198 (Filippenko, Li, & Modjaz 1999), and SN 2002kg in NGC 2403 (Schwartz et al.2003). All these objects are subluminous with respect to more normal SNe IIn, but otherwise exhibit similar spectral characteristics, apart from the very high velocities seen in some SNe IIn (e.g., SN 1988Z; Filippenko 1997).
SN 2000ch in NGC 3432 is clearly similar to these other subluminous objects and thus is probably related to the LBV class. Its spectrum is similar to that of SN 1997bs (Van Dyk et al. 2000), although SN 2000ch lacked the strong Fe ii emission observed in SN 1997bs. The Balmer line widths are similar in the two objects, implying comparable outflow velocities. In addition, the maximum absolute magnitude of MV ≈ - 13.8 mag for SN 1997bs compares reasonably well with the value of MV = -12.7 mag for SN 2000ch. While their basic spectral properties and maximum luminosities are similar, their host environments are quite different. SN 2000ch is located in an H ii region or large star‐forming complex at the edge of NGC 3432 (see Fig. 2), while SN 1997bs does not appear to reside in a comparable environment (Van Dyk et al. 2000).
The most striking difference is in their light curves. SN 2000ch was a very fast, erratic event characterized by a rapid rise of ∼2 mag from quiescence, a local minimum lasting ∼15 days, and finally a return back to quiescence. In contrast, the light curve of SN 1997bs reveals a gradual decline of ∼6 mag over ∼260 days. Smith et al. (2001) point out that the fast photometric variations exhibited by SN 2000ch are reminiscent of the oscillations in brightness reported for SN 1954J and η Car prior to and during their giant eruptions.
In summary, SN 2000ch appears to be related to the LBV phenomenon, but with rapid photometric variations near maximum brightness. While SN 1997bs has faded below detectability in NGC 3627, SN 2000ch is quite luminous in quiescence (R ≈ 19.4 mag) and can be studied in detail using modern instruments on large telescopes. It is brighter than the progenitor of SN 1954J (Smith et al. 2001) and thus can be photometrically monitored frequently with small telescopes or observed spectroscopically at relatively high spectral resolution. We encourage further monitoring and detailed observations of this unusual outburst in an effort to discern its nature and similarity to the class of LBVs.
The authors would like to thank Drs. Rodolfo Barba, Roberta Humphreys, and Schuyler Van Dyk for their comments and suggestions regarding the nature of SN 2000ch. We are grateful for the assistance of the support the staff at KPNO, Steward Observatory, Lick Observatory, the MDM Observatory, and the United States Naval Observatory Flagstaff Station. R. M. W. acknowledges support from NASA grants to Ohio State University. G. D. S. is grateful for NSF grant AST 97‐30792 to the University of Arizona. S. S. received partial support from NSF and NASA grants to Arizona State University. The work of A. V. F.'s group at the University of California‐Berkeley is financed by NSF grant AST 99‐87438, and also by the Sylvia and Jim Katzman Foundation. KAIT was made possible by generous donations from Sun Microsystems, Inc., the Hewlett‐Packard Company, AutoScope Corporation, Lick Observatory, the National Science Foundation, the University of California, and the Katzman Foundation.
Footnotes
- 1
Sometimes, however, a very narrow, low‐velocity absorption component is visible; see SNe 1994W and 1994ak in Fig. 14 of Filippenko (1997), for example.
- 2
IRAF is distributed by the National Optical Astronomy Observatories, which are operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation.