A Far‐Ultraviolet Spectroscopic Analysis of the Central Star of the Planetary Nebula Longmore 1¹

Longmore 1 (K1‐26, PK 255−59°1, hereafter Lo 1) was originally discovered by Longmore (1977) as a planetary nebula (PN) having a notably large angular size (∼400^''). The spectra of its central star show both hydrogen and He ii absorption features, with no evidence of a stellar wind in its UV or optical spectra (Patriarchi & Perinotto 1991; Kaler & Lutz 1985; Mèndez et al. 1985). Because of its high Galactic latitude (b≃ -60°), the reddening toward Lo 1 is thought to be minimal (Kaler & Lutz 1985). Based on its optical spectrum, Mèndez et al. (1985) termed Lo 1 an "hgO(H)" star, a high‐gravity object with very broad Balmer absorptions. Such objects can lie on the white dwarf cooling tracks, but can also be non‐post‐AGB objects. A distance of D = 800 pc (Ishida & Weinberger 1987) implies a nebular radius of ∼0.8 pc, suggesting that Lo 1 is a quite evolved central star of the planetary nebulae (CSPN; most PNs have radii ≲0.5 pc; Cahn et al. 1992).

Hot CSPNs emit most of their observable flux in the far‐UV range. We have observed the central star of Lo 1 with the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite in the 905–1187 Å range. Using this data, as well as archive International Ultraviolet Explorer (IUE) data (1150–3300 Å), we determined the parameters of the central star through stellar modeling and discuss evolutionary implications.

Parameters of Lo 1 compiled from previous literature are listed in Table 1.

Table 2 lists the spectra utilized in this paper. Lo 1 was observed as part of FUSE's Cycle 1 program P133 (Bianchi). The IUE data were retrieved from the MAST archive. The observed spectra are presented in § 3.

FUSE covers the wavelength range of 905–1187 Å at a spectral resolution of ≲30,000. The flux calibration accuracy of FUSE is ≲10% (Sahnow et al. 2000). It is described by Moos et al. (2000), and its on‐orbit performance is discussed by Sahnow et al. (2000). FUSE collects light concurrently in four different channels (LiF1, LiF2, SiC1, and SiC2). Each channel is recorded by two detectors, each divided into two segments (A and B) covering different subsets of the above range with some overlap.

The FUSE spectra were taken through the LWRS (30^'' × 30^'') aperture. These data, taken in "time tag" mode, have been calibrated using the most recent FUSE data reduction pipeline, efficiency curves and wavelength solutions (CALFUSE ver. 2.2). We combined the data from different segments, weighted by detector sensitivity, and rebinning to a uniform dispersion of 0.05 Å (which is probably close to the actual resolution, since the data were taken in the early part of the mission). Bad areas of the detectors, and those regimes affected by an instrumental artifact known as "the worm" (FUSE Data Handbook ver. 1.1), were excluded. For part of the first observation, telescope alignment problems moved the target out of the LiF2/SiC2 aperture. This also appears to have happened with the SiC2 detector halfway through the second observation. We thus omitted data taken during these target drifts for the affected detectors.

Four IUE spectra of the central star of Lo 1 are available; however, it appears that one (SWP20275) missed the central star. The only high‐resolution spectrum (SWP39146) is underexposed and was not used. The two remaining low‐resolution long‐wavelength and short‐wavelength spectra are in agreement in the region of overlap. The IUE spectra are relatively featureless and are mainly used to fit the continuum flux distribution.

Modeling of Lo 1 consisted of two parts: modeling the hot central star and modeling the sight‐line hydrogen (atomic and molecular). We describe each in turn. A virtue of its location significantly outside the Galactic plane is a low reddening, which we determine, by fitting the continuum slope, to be E_B−V<0.01 (we use a value of E_B−V = 0 throughout this paper). The data, as well as the model fits, are shown in Figures 1 and 2. We determine the radial velocity of Lo 1 to be v_rad = 100 ± 10 km s⁻¹ using stellar absorption line features in the long wavelength FUSE range (>1050 Å) such as C iv λ1169.0 and C iv λ1107.6.

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3.1. The Central Star Model

To model the white dwarf central star, we used the TLUSTY code to calculate the stellar atmosphere, and SYNSPEC to calculate the synthetic flux (Hubeny 1988; Hubeny & Lanz 1992, 1995; Hubeny et al. 1994). TLUSTY calculates the atmospheric structure assuming radiative and hydrostatic equilibrium, and a plane‐parallel geometry, in non‐LTE (NLTE) conditions. In the case of hot (T_eff≳50 kK) white dwarfs, LTE calculations are not appropriate and result in significant deviations from NLTE calculations (see, e.g., Dreizler & Werner 1996; Werner 1996; Werner et al. 1991; Napiwotzki 1997). This is because in such hot objects, the populations of the ions are mainly determined by the intense radiation field despite the high gravities.

The FUSE spectrum shows features of hydrogen, helium, and metals (e.g., C, O, Fe, and S), with O vi λλ1032, 1038 being especially prominent. Test models indicated that the solar abundance ratio for H/He (as found by Mèndez et al. 1985) was adequate. We thus constructed a grid of solar abundance models varying T_eff and log g, treating hydrogen and helium in NLTE to calculate the structure of the atmosphere. Once T_eff and log g were determined adequately, they were held fixed, and the CNO elements were varied individually in NLTE to constrain their abundances and ensure that neglect of their NLTE treatment did not alter the derived T_eff and log g.

The atomic data used come from TOPBASE, the database of the Opacity Project (Cunto et al. 1993). TLUSTY makes use of the concept of "superlevels," where levels of similar energy are grouped together and treated as a single level in the rate equations (after Anderson 1989). The number of levels+superlevels used for the NLTE model ions were H i (8+1), He i (24+0), He ii (20+0), C iii (34+12), C iv (35+2), N iv (15+8), N v (21+4), O iv (39+31), O v (34+6), and O vi (15+5). Ne, Na, Mg, Si, S, Ar, Ca, and Fe were allowed to contribute to the total number of particles and charge, but their opacity contribution was neglected in the model atmosphere calculation. We have adopted SYNSPEC's values for solar abundances, which are taken from Grevesse & Sauval (1998).

As previously mentioned, a solar hydrogen‐to‐helium ratio appeared adequate to fit the He ii and H Lyman spectrum. The gravity was constrained by fitting mainly the wings of these features, as demonstrated in Figure 3. To determine the effective temperature, the FUV–UV continuum shape, as well as FUV spectral features of hydrogen, helium and metals (mainly oxygen and carbon), were used as diagnostics. The parameters of our best‐fit model for the CSPN of Lo 1 are T_eff = 120 ± 10 kK, log g = 6.7^+0.3_-0.7 cm s⁻². Solar values for the metal abundances were found to be adequate, except for oxygen, for which the solar value underproduced the strong O vi λλ1032, 1038 feature (shown in Fig. 4). We found that an oxygen abundance enriched 5 times with respect to the solar value (X_O = 5 X_⊙ by mass) produced a good fit; however, some of the other oxygen features then appeared a bit strong. For temperatures T_eff≃100 - 120 kK, the O vi feature is at its strongest, thus higher or lower temperatures require an even greater oxygen enrichment. Unless otherwise stated, the model spectrum shown in the figures and what we refer to as "our model" has the parameters T_eff = 120 kK, log g = 6.7 cm s⁻², X_O = 5 X_⊙, with the abundances of all other elements set to their solar values.

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3.2. Modeling H₂ and H i Absorption Toward Lo 1

Our derived model parameters for the central star and sight‐line hydrogen are presented in Table 3. Scaling our model flux to the observed flux yields R_*/D, the ratio of the stellar radius to the distance. This value, using a distance of D = 800 pc (Ishida & Weinberger 1987), yields a radius of R_*≃0.037 R_⊙ and a corresponding luminosity of L≃250 L_⊙. The model flux then yields a corresponding visual magnitude of V = 15.5 mag, in good agreement with the measured value of V = 15.4 mag (Kaler & Lutz 1985). Because we can only constrain the gravity rather loosely, we cannot derive a meaningful value for the mass of the central star without appealing to stellar evolution tracks. As discussed in § 3.1, the O vi doublet may indicate an oxygen enriched atmosphere. Usually, in CSPN, oxygen enrichment is associated with helium‐rich objects (i.e., helium burners) and is often accompanied by carbon enrichment. However, the abundances of the other elements in Lo 1 do not appear to be much different than the solar values, which is characteristic of many H‐burning CSPN. Therefore, we compared our derived effective temperature and luminosity with both the hydrogen‐ and helium‐burning (solar abundance) tracks of Vassiliadis & Wood (1994). The H‐burning tracks indicate a current core mass of M_c = 0.633 M_⊙, and an initial mass of 2.0 M_⊙, with the uncertainties of our parameters encompassing the M_init = 1.5 M_⊙, M_c = 0.597 M_⊙ and M_init = 2.5 M_⊙, M_c = 0.677 M_⊙ tracks as well. So, from the H‐burning evolutionary models, we derive M_init = 2.0 ± 0.5 M_⊙, M_c = 0.63 ± 0.04 M_⊙, and a post‐AGB age τ_evol∼60 kyr. In a similar fashion, we derive M_init = 1.5 ± 0.5 M_⊙, M_c = 0.60 ± 0.04 M_⊙, τ_evol∼70 kyr from comparison with the He‐burning tracks.

The derived stellar parameters of Lo 1 are similar to those of the hotter, higher gravity O(H) stars in the sample of central stars for old PN classified by Napiwotzki (1999). Thus, we confirm the Mèndez et al. (1985) classification of Lo 1 as a hgO(H) star.

Mèndez et al. (1985) performed a non‐LTE analysis of its optical spectrum and obtained T_eff = 65 ± 10 kK, log g = 5.7 ± 0.3 cm s⁻², and Y = 0.10 ± 0.03 for the central star. We have calculated a TLUSTY model with these parameters and find it fails to match the FUV data for multiple reasons. When this model is scaled to match the UV continuum level, it significantly underproduces the FUV continuum level. It also fails to duplicate many of the FUV diagnostics, most notably the strong O vi λλ1032, 1038 feature. Similarly, Hoare et al. (1996), from an analysis of the optical and extreme ultraviolet spectra of the CSPN NGC 1360, found a temperature significantly higher than the results of Mèndez et al. (1985), which were based on optical data only. Our significantly higher derived temperature and gravity illustrate the importance of considering the FUV wavelength regime when modeling such hot CSPN, where they emit the majority of their observable flux (i.e., longward of the Lyman limit) as well as display their strongest stellar features.

We have analyzed FUV and UV spectra of Lo 1, a hot CSPN notable for its relatively high galactic latitude and thus having a minimal reddening. Its FUSE spectrum, aside from showing hydrogen and helium lines, shows strong O vi λλ1032, 1038 signatures, perhaps indicating an oxygen‐enriched object.

We have modeled the FUSE and IUE spectrum of this object to determine parameters T_eff = 120 kK, log g = 6.7 cm s⁻², R_* = 0.04 R_⊙, L = 250 L_⊙, and M≃0.6 M_⊙. The temperature is much higher than that derived by Mèndez et al. (1985) from an optical‐line analysis (T_eff = 65 ± 10 kK), and illustrates the importance of the FUV–UV range in the analysis of hot CSPN. These parameters confirm the Mèndez et al. (1985) classification of Lo 1 as a high‐gravity O(H) star. Comparison of our parameters to evolutionary tracks indicate a post‐AGB age of ∼65 kyr. We also measure v_rad≃100 km s⁻¹ for the Lo 1 PN system.