Abundances of Deuterium, Nitrogen, and Oxygen toward HZ 43A: Results from the FUSE Mission (original) (raw)
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A Comparative Study of the Mass Distribution of Extreme-Ultraviolet-selected White Dwarfs
The Astrophysical Journal, 1999
We present new determinations of effective temperature, surface gravity, and masses for a sample of 46 hot DA white dwarfs selected from the Extreme Ultraviolet Explorer (EUVE) and ROSAT Wide Field Camera bright source lists in the course of a near-infrared survey for low-mass companions. Our analysis, based on hydrogen non-LTE model atmospheres, provides a map of LTE correction vectors, which allow a thorough comparison with previous LTE studies. We find that previous studies underestimate both the systematic errors and the observational scatter in the determination of white dwarf parameters obtained via fits to model atmospheres. The structure of very hot or low-mass white dwarfs depends sensitively on their history. To compute white dwarf masses, we thus use theoretical mass-radius relations that take into account the complete evolution from the main sequence. We find a peak mass of our white dwarf sample of 0.59 Msolar, in agreement with the results of previous analyses. However, we do not confirm a trend of peak mass with temperature reported in two previous analyses. Analogous to other EUV-selected samples, we note a lack of low-mass white dwarfs and a large fraction of massive white dwarfs. Only one white dwarf is likely to have a helium core. While the lack of helium white dwarfs in our sample can be easily understood from their high cooling rate, and therefore low detection probability in our temperature range, this is not enough to explain the large fraction of massive white dwarfs. This feature very likely results from a decreased relative sample volume for low-mass white dwarfs caused by interstellar absorption in EUV-selected samples. Spectral observations reported here were obtained with the Multiple Mirror Telescope, a joint facility of the University of Arizona and the Smithsonian Institution, and with the Bok telescope at the Steward Observatory of the University of Arizona.
The Astrophysical Journal, 1999
We discuss the Hubble Space T elescope Space Telescope Imaging Spectrograph UV echelle spectrum of the hot DA white dwarf REJ 1032]532. The interstellar data from this spectrum are presented by Holberg and coworkers. In this paper we discuss a number of strong photospheric features due to C, N, and Si that are present in the REJ 1032]532 spectrum. While the inferred heavy element content of REJ 1032]532 roughly matches the predictions of radiative levitation for carbon and silicon, the observed nitrogen abundance greatly exceeds predictions by a factor of 50. The observed shapes of the N V lines provide the Ðrst evidence, at UV wavelengths, of heavy element stratiÐcation in a hot DA white dwarf. Homogeneous models are unable to reproduce the shape of the REJ 1032]532 N V lines, nor can they account for the relatively low degree of EUV opacity in the star. We present a simple stratiÐed nitrogen model that resolves these problems. The high degree of stratiÐcation in REJ 1032]532 is the signature of ongoing mass loss in this star. The radial velocity of REJ 1032]532 obtained with Space Telescope Imaging Spectrograph di †ers by 44 km s~1 from that obtained from the Balmer H I lines with the Multiple Mirror Telescope. This suggests that REJ 1032]532 is likely a member of a binary system containing either a late M star or another white dwarf.
The Astrophysical Journal, 2000
We present an analysis of the Far Ultraviolet Spectroscopic Explorer (FUSE ) spectra of the little-known southern nova-like cataclysmic variable, BB Doradus. The spectrum was obtained as part of our Cycle 8 FUSE survey of highdeclination nova-like stars. The FUSE spectrum of BB Dor, observed in a high state, is modeled with an accretion disk with a very low inclination (possibly lower than 10 ). Assuming an average white dwarf (WD) mass of 0.8 M leads to a mass accretion rate of 10 À9 M yr À1 and a distance on the order of $650 pc, consistent with the extremely low Galactic reddening in the direction of BB Dor. The spectrum presents some broad and deep silicon and sulfur absorption lines, indicating that these elements are overabundant by 3 and 20 times solar, respectively.
The D/H Ratio in the Interstellar Medium toward the White Dwarf PG0038+199
We determine the D/H ratio in the interstellar medium toward the DO white dwarf PG 0038+199 using spectra from the Far Ultraviolet Spectroscopic Explorer (FUSE), with ground-based support from Keck HIRES. We employ curve of growth, apparent optical depth and profile fitting techniques to measure column densities and limits of many other species (H 2 , Na I, C I, C II, C III, N I, N II, O I, Si II, P II, S III, Ar I and Fe II) which allow us to determine related ratios such as D/O, D/N and the H 2 fraction. Our efforts are concentrated on measuring gas-phase D/H, which is key to understanding Galactic chemical evolution and 1 This paper is dedicated in memory of Ervin J. Williger, father of the first author, who passed away on 2003 September 13. His enthusiastic support and encouragement were essential to its successful completion.
Astronomy & Astrophysics, 2004
One of the strongest absorption lines observed in far-ultraviolet FUSE spectra of many PG 1159 stars remained unidentified up to now. We show that this line, located at 973.3 Å, stems from Ne . We also present new optical highresolution spectra of PG 1159 stars, obtained with the ESO VLT, which cover the Ne 3644 Å line and a newly identified Ne multiplet in the 3850-3910 Å region. We compare the observed neon lines with NLTE models and conclude a substantial neon overabundance in a number of objects. Although a detailed analysis is still to be performed in order to compare quantitatively the abundances with evolutionary theory predictions, this corroborates the idea that the PG 1159 stars and their immediate progenitors, the [WC]-type nuclei of planetary nebulae, display intershell matter of their precursor AGB stars. Possibly as the consequence of a late He-shell flash, H-deficient and (s-processed) Fe-depleted matter, that is strongly enriched by 3α-processed elements (C, O, Ne), is dredged up to the surface. Hence, a detailed study of the element abundance patterns in these peculiar stars gives the unique possibility to probe mixing and nucleosynthesis processes in the precursor AGB stars.
The Astrophysical Journal, 2003
We report on new F USE far-UV spectroscopy of the most metal-poor blue compact dwarf galaxy I Zw 18. The new data represent an improvement over previous F USE spectra by a factor of 1.7 in the signal-to-noise. Together with a larger spectral coverage (λλ = 917−1188Å), this allows us to characterize absorption lines in the interstellar medium with unprecedented accuracy. The kinematics averaged over the large sampled region shows no clear evidence of gas inflows or outflows. The H i absorption is interstellar with a column density of 2.2 + 0.6 − 0.5 × 10 21 cm −2 . A conservative 3σ upper limit of 5.25 × 10 14 cm −2 is derived for the column density of diffuse H 2 . From a simultaneous fitting of metal absorption lines in the interstellar medium, we infer the following abundances: [Fe/H] = − 1.76 ± 0.12, [O/H] = − 2.06 ± 0.28, [Si/H] = − 2.09 ± 0.12, [Ar/H] = − 2.27 ± 0.13, and [N/H] = − 2.88 ± 0.
High-resolution FUSE and HST ultraviolet spectroscopy of the white dwarf central star of Sh 2-216
Astronomy and Astrophysics, 2007
Context. We perform a comprehensive spectral analysis of LS V +46 • 21 in order to compare its photospheric properties to theoretical predictions from stellar evolution theory as well as from diffusion calculations. Aims. LS V +46 • 21 is the DAO-type central star of the planetary nebula Sh 2−216. High-resolution, high-S/N ultraviolet observations obtained with FUSE and STIS aboard the HST as well as the optical spectrum have been analyzed in order to determine the photospheric parameters and the spectroscopic distance. Methods. We performed a detailed spectral analysis of the ultraviolet and optical spectrum by means of state-of-the-art NLTE modelatmosphere techniques. Results. From the N IV -N V, O IV -O VI, Si IV -Si V, and Fe V -Fe VII ionization equilibria, we determined an effective temperature of (95 ± 2) kK with high precision. The surface gravity is log g = 6.9 ± 0.2. An unexplained discrepancy appears between the spectroscopic distance d = 224 +46 −58 pc and the parallax distance d = 129 +6 −5 pc of LS V +46 • 21. For the first time, we have identified Mg IV and Ar VI absorption lines in the spectrum of a hydrogen-rich central star and determined the Mg and Ar abundances as well as the individual abundances of iron-group elements (Cr, Mn, Fe, Co, and Ni). With the realistic treatment of metal opacities up to the iron group in the model-atmosphere calculations, the so-called Balmer-line problem (found in models that neglect metal-line blanketing) vanishes. Conclusions. Spectral analysis by means of NLTE model atmospheres has presently arrived at a high level of sophistication, which is now hampered largely by the lack of reliable atomic data and accurate line-broadening tables. Strong efforts should be made to improve upon this situation.
The Astrophysical Journal, 2003
We have carried out an investigation of the abundance of deuterium along two extended sight lines through the interstellar medium (ISM) of the Galactic disk. The data include Far Ultraviolet Spectroscopic Explorer (FUSE) observations of HD 195965 (B1Ib) and HD 191877 (B0V), as well as Space Telescope Imaging Spectrograph (STIS) observations of HD 195965. The distances to HD 195965 and HD 191877, derived from spectroscopic parallax, are 794 ± 200 pc and 2200 ± 550 pc, respectively, making these the longest Galactic disk sight lines in which deuterium has been investigated with FUSE. The FUSE spectra contain all of the H I Lyman series transitions (and the corresponding D transitions) except Lyα. The higher Lyman lines clearly show the presence of deuterium. We use a combination of curve of growth analyses and line profile fitting to determine the D I abundance toward each object. We also present column densities for O I and N I toward both stars, and H I measured from Lyα absorption in the STIS spectrum of HD 195965. Toward HD 195965 we find D/H=(0.85± 0.34 0.24) × 10 −5 (2σ), O/H=(6.61± 1.03 1.11) × 10 −4 , and N/H=(7.94± 1.69 1.34) × 10 −5. Toward HD 191877 we find D/H=(0.78± 0.52 0.25) × 10 −5 (2σ) and N/H=(6.76± 2.22 1.97) × 10 −5. The O I column density toward HD 191877 is very uncertain. Our preferred value gives O/H=(3.09± 1.98 0.98) × 10 −4 , but we cannot rule out O/H values as low as O/H=1.86×10 −4 , so the O/H value for this sight line should be taken with caution. The D/H ratios along these sight lines are lower than the average value of (1.52 ± 0.15) × 10 −5 (2σ in the mean) found with FUSE for the local interstellar medium (∼ 37 to 179 pc from the Sun). These observations lend support to earlier detections of variation in D/H over distances greater than a few hundred pc. The O/H ratio toward HD 195965 is supersolar. This star is part of an OB association, so there may be local enrichment by nearby massive stars. The D/H and O/H values measured along these sight lines support the expectation that the ISM is not well mixed on distances of ∼ 1000 pc. These observations demonstrate that although D/H studies through Lyman absorption may become impractical at d>2500 pc and log N (H I)>21, D/H studies in the distance range from 500 to 2500 pc may be very useful for investigating mixing and chemical evolution in the ISM.
Astronomical Journal, 1997
We present optical identifications of 30 previously unidentified extreme ultraviolet (EUV) and soft x-ray sources coinciding with the position of faint sources detected during the all-sky surveys of the Extreme Ultraviolet Explorer (EUVE) in the 58-174 Â (0.071-0.214 keV) band and of the ROSAT Position Sensitive Proportional Counter (PSPC) in the 5-120 Â (0.1-2.4 keV x-ray) band. We present low-resolution spectra of the possible optical counterparts of the x-ray and EUVE sources obtained with 1.5 m telescope at CTIO in 1995 May and December and 1996 February. The optical counterparts for 23 sources are identified as late-type (dKe and dMe) stars, and 4 are identified as new DA white dwarfs. All spectra of sources identified as late-type stars show the Balmer series and Ca H and K fines in emission, indicating the signature of chromospheric activity. For the white dwarfs, the joint detection by the ROSAT PSPC and the EUVE Lexan band indicates that they have a low-abundance of metals and that their EUV emission is mostly attenuated by the local interstellar medium (ISM).
We present a detailed analysis of a large spectroscopic and photometric sample of DZ white dwarfs based on our latest model atmosphere calculations. We revise the atmospheric parameters of the trigonometric parallax sample of Bergeron, Leggett, & Ruiz (12 stars) and analyze 147 new DZ white dwarfs discovered in the Sloan Digital Sky Survey. The inclusion of metals and hydrogen in our model atmosphere calculations leads to different atmospheric parameters than those derived from pure helium models. Calcium abundances are found in the range from log (Ca/He) = −12 to −8. We also find that fits of the coolest objects show peculiarities, suggesting that our physical models may not correctly describe the conditions of high atmospheric pressure encountered in the coolest DZ stars. We find that the mean mass of the 11 DZ stars with trigonometric parallaxes, M = 0.63 M ⊙ , is significantly lower than that obtained from pure helium models, M = 0.78 M ⊙ , and in much better agreement with the mean mass of other types of white dwarfs. We determine hydrogen abundances for 27% of the DZ stars in our sample, while only upper limits are obtained for objects with low signal-to-noise ratio spectroscopic data. We confirm with a high level of