Jean Dupuis

Hot White Dwarfs in the Extreme Ultraviolet Explorer Survey. II. Mass Distribution, Space Density, and Population Age

We present new effective temperature and surface gravity determinations for a sample of 90 hot white dwarfs detected in the Extreme Ultraviolet Explorer (EUVE) all-sky survey. The measurements, based on spectroscopy of the Balmer line series obtained at the Michigan-Dartmouth-MIT Observatory, Mount Stromlo Observatory, Lick Observatory, and Cerro Tololo Inter-American Observatory, are used to constrain the space density as well as the population age and mass distribution of a sample of 110 EUV-selected DA white dwarfs in the solar neighborhood. We find a mass spectrum narrowly peaked over 0.56 M☉, indicative of a C-O core with a thin hydrogen layer, and a significant population of 10 ultramassive (M ≥ 1.1 M☉) white dwarfs; we also find that all objects fall between effective temperatures of ≈ 25,000 and ≈ 75,000 K and that most are younger than 30 Myr. Using Woods evolutionary models we determine a DA white dwarf birthrate in the solar neighborhood of (0.7-1.0) × 10-12 pc-3 yr-1. Although most objects are on normal C-O cooling tracks, we suggest that a few low-mass white dwarfs and the population of ultramassive white dwarfs may follow different paths with, respectively, He, and, possibly, O-Ne-Mg cores.

CallFUSE Version 3: A Data Reduction Pipeline for the Far Ultraviolet Spectroscopic Explorer

Since its launch in 1999, the Far Ultraviolet Spectroscopic Explorer (FUSE) has made over 4900 observations of some 2500 individual targets. The data are reduced by the principal investigator team at the Johns Hopkins University and archived at the Multimission Archive at STScI (MAST). The data reduction software package, called CalFUSE, has evolved considerably over the lifetime of the mission. The entire FUSE data set has recently been reprocessed with CalFUSE version 3.2, the latest version of this software. This paper describes CalFUSE version 3.2, the instrument calibrations on which it is based, and the format of the resulting calibrated data files.

Far-Ultraviolet Performance of the Berkeley Spectrograph duringthe ORFEUS-SPAS II Mission*

The Berkeley spectrograph aboard the ORFEUS telescope made its second flight on the 14 day ORFEUS-SPAS II mission of the Space Shuttle Columbia in 1996 November/December. Approximately half of the available observing time was dedicated to the Berkeley spectrograph, which was used by both principal and guest investigators. The spectrographs full bandpass is 390-1218 A; here we discuss its in-flight performance at far-ultraviolet wavelengths, at which most of the observations were performed. The instruments effective area peaks at 8.9±0.5 cm2 near 1020 A, and the mean spectral resolution is 95 km s-1 FWHM for point sources. Over most of the spectral range, the typical nighttime background event rate in each spectral resolution element was ~0.003 s-1. Simultaneous background observations of an adjacent blank field were provided through a secondary, off-axis aperture. The Berkeley spectrographs unique combination of sensitivity and resolution provided valuable observations of approximately 105 distinct astronomical targets, which ranged in distance from the Earths own Moon to some of the brightest active galactic nuclei.

FUSE determination of a low deuterium abundance along an extended sight line in the Galactic disk

We present a study of the deuterium abundance along the extended sight line toward HD 90087 with the Far Ultraviolet Spectroscopic Explorer (FUSE). HD 90087 is a O9.5 III star located in the Galactic disk at a distance of ~2.7 kpc away from the Sun. Both in terms of distance and column densities, HD 90087 has the longest and densest sight line observed in the Galactic disk for which a deuterium abundance has been measured from ultraviolet absorption lines so far. Because many interstellar clouds are probed along this sight line, possible variations in the properties of individual clouds should be averaged out. This would yield a deuterium abundance that is characteristic of the interstellar medium on scales larger than the Local Bubble. The FUSE spectra of HD 90087 show numerous blended interstellar and stellar features. We have measured interstellar column densities of neutral atoms, ions, and molecules by simultaneously fitting the interstellar absorption lines detected in the different FUSE channels. As far as possible, saturated lines were excluded from the fits in order to minimize possible systematic errors. IUE (International Ultraviolet Explorer) archival data are also used to measure neutral hydrogen. We report D/O = (1.7 ± 0.7) × 10-2 and D/H = (9.8 ± 3.8) × 10-6 (2 σ). Our new results confirm that the gas-phase deuterium abundance in the distant interstellar medium is significantly lower than the one measured within the Local Bubble. We supplement our study with a revision of the oxygen abundance toward Feige 110, a moderately distant (~200 pc) sdOB star, located ~150 pc below the Galactic plane. Excluding saturated lines from the fits of the FUSE spectra is critical; this led us to derive an O I column density about 2 times larger than the one previously reported for Feige 110. The corresponding updated D/O ratio on this sight line is D/O = (2.6 ± 1.0) × 10-2 (2 σ), which is lower than the one measured within the Local Bubble. The data set available now outside the Local Bubble, which is based primarily on FUSE measurements, shows a contrast between the constancy of D/O and the variability of D/H. As oxygen is considered to be a good proxy for hydrogen within the interstellar medium, this discrepancy is puzzling.

The first detection of ionized helium in the local ISM - EUVE and IUE spectroscopy of the hot DA white dwarf GD 246

We report observations of the extreme ultraviolet spectrum of the hot degenerate star GD 246 obtained with the EUVE. Our initial attempt at modeling the photospheric emission from the white dwarf reveals a relatively uncontaminated pure H spectrum in the range above 200 A, allowing a study of interstellar continuum absorption features in the line of sight of GD 246. Modeling of the He I autoionization transition discussed by Rumph et al. (1993), and the EUV continuum using the white dwarf as a source of background radiation provides measurements of both neutral and, for the first time, singly ionized He column densities in the local ISM (LISM). We estimate the He ionization fraction He II/(He I + He II) at roughly 25 percent with a total He column of 1.40-1.65 x 10 exp 18/sq cm. We have measured and compared H I column densities from the saturated Ly-alpha ISM absorption in IUE high-dispersion spectroscopy and from EUV continuum absorption: the two measurements are in good agreement with a total H column of 1.2-1.6 x 10 exp 19/sq cm. We discuss some implications for the nature of the LISM, particularly in the context of current models of the EUV radiation field.

O/H in the local bubble

We present new measurements of the oxygen gas-phase abundance along four sight lines probing gas inside the Local Bubble, using data obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE )a nd theHubble Space Telescope(HST).CombiningthesewithsevenpreviouslypublishedN(Oi)andN(Hi)measurements,wedetermine a mean O/H ratio for the Local Bubble, (O/H)LB ¼ (3:45 � 0:19);10 � 4 (1 � in the mean). Our result is virtually identical to O/H ¼ (3:43 � 0:15);10 � 4 derived from data for sight lines probing gas well outside the Local Bubble. In contrast to the D/O and D/H ratios, which seem to have different values beyond the Local Bubble, our results showthattheO/HratioisconstantandsinglevaluedbothinsideandoutsidetheLocalBubbleforlow-densitysight lines with column densities up to log N(H) ’ 21. In addition, the similarity of the two values above suggests that the net effect of the uncertain O i f-values in the FUSE bandpass is not significant. Taking into account the latest determinations of the solar abundance of oxygen, our result implies that � 25% of the oxygen in the Local Bubble is depleted onto dust grains. The similarity of the value for O/H along low average density sight lines in the Local Bubble with that of denser sight lines beyond may permit a limit on the diluting effects of infalling low-metallicity gas. Subject headingg ISM: abundances — ISM: evolution — stars: individual (WD 0050� 332, WD 0232+305, WD 1254+223, WD 2152� 548) — ultraviolet: ISM

Abundance of Elements beyond the Iron Group in Cool DO White Dwarfs

We report the presence of elements beyond the iron group in the atmospheres of the cool DO white dwarfs HD 149499 B and HZ 21. Photospheric lines of germanium (Z = 32), arsenic (33), selenium (34), tin (50), tellurium (52), iodine (53), and perhaps bromine (35) are observed in ultraviolet spectra of HD 149499 B obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE), the Goddard High Resolution Spectrograph (GHRS), and the International Ultraviolet Explorer. Germanium, arsenic, and tellurium are also observed in FUSE and GHRS spectra of HZ 21. Light elements such as carbon, silicon, phosphorus, and sulfur are present in the atmospheres of both stars. Nitrogen is detected in HZ 21 but not in HD 149499 B. We calculated a grid of synthetic spectra covering the range of effective temperatures, surface gravities, and element abundances appropriate for a detailed line profile analysis. The measured mass fractions of elements heavier than iron reveal overabundances ranging from about a factor of 3 to a factor of 1000 relative to the Sun. The heavy-element enrichment in both stars could be the result of slow neutron capture nucleosynthesis, which would have occurred during the asymptotic giant branch phase of their evolution.

Adding More Mysteries to the DA White Dwarf GD 394

We present spectroscopic and timing analyses of the hot DA white dwarf GD 394 showing abundance inhomogeneities across its surface. Lick Hamilton echelle, International Ultraviolet Explorer (IUE), HST GHRS, and Hopkins Ultraviolet Telescope (HUT) spectra show dominant Si III and Si IV features, while Extreme Ultraviolet Explorer (EUVE) spectra show evidence of a host of trace element opacities. We report the detection of Si III λ4552 triplet with a measured radial velocity of 27 km s-1 in agreement with far ultraviolet (FUV) line velocities, but indicating a silicon abundance (Si/H = 2 × 10-5) a factor of 10 larger than measured in the FUV range [Si/H = (2-7) × 10-6]. Effective temperature measurements based on Lyman continuum (EUVE) and Lyman line series (HUT) are systematically cooler (ΔT ~ -4000 K) than measurements based on medium-dispersion Balmer line spectroscopy, an effect attributed to yet unidentified opacities. A timing analysis of EUVE deep-survey and scanner time series, as well as spectrometer data, shows GD 394 to be variable in the extreme ultraviolet (EUV) with an amplitude of 25% and a period of 1.150 ± 0.003 days. The EUV variability suggests abundance inhomogeneities in the atmosphere, and we explore different models to explain its origin.

The D/H Ratio toward PG 0038+199* **

We determine the D/H ratio in the interstellar medium toward the DO white dwarf PG0038+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 (H2, NaI, CI, CII, CIII, NI, NII, OI, SiII, PII, SIII, ArI and FeII) which allow us to determine related ratios such as D/O, D/N and the H2 fraction. Our efforts are concentrated on measuring gas-phase D/H, which is key to understanding Galactic chemical evolution and comparing it to predictions from Big Bang nucleosynthesis. We find column densities log N(HI) = 20.41+-0.08, log N(DI)=15.75+-0.08 and log N(H2) = 19.33+-0.04, yielding a molecular hydrogen fraction of 0.14+-0.02 (2 sigma errors), with an excitation temperature of 143+-5K. The high HI column density implies that PG0038+199 lies outside of the Local Bubble; we estimate its distance to be 297 (+164,-104)pc (1 sigma). D/[HI+2H2] toward PG0038+199 is 1.91(+0.52,-0.42) e-5 (2 sigma). There is no evidence of component structure on the scale of Delta v>8 km/s based on NaI, but there is marginal evidence for structure on smaller scales. The D/H value is high compared to the majority of recent D/H measurements, but consistent with the values for two other measurements at similar distances. D/O is in agreement with other distant measurements. The scatter in D/H values beyond ~100pc remains a challenge for Galactic chemical evolution.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 the column densities and limits of many other species (H2, 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 allows us to determine related ratios such as D/O, D/N, and the H2 fraction. Our efforts are concentrated on measuring gas-phase D/H, which is key to understanding Galactic chemical evolution, and comparing it to predictions from big bang nucleosynthesis. We find column densities log N(H ) = 20.41 ± 0.08, log N(D ) = 15.75 ± 0.08, and log N(H2) = 19.33 ± 0.04, yielding a molecular hydrogen fraction of 0.14 ± 0.02 (2 σ errors), with an excitation temperature of 143 ± 5 K. The high H I column density implies that PG 0038+199 lies outside of the Local Bubble; we estimate its distance to be 297 pc (1 σ). [D + HD]/[H + 2H2] toward PG 0038+199 is 1.91 × 10-5 (2 σ). There is no evidence of component structure on the scale of Δv > 8 km s-1, based on Na I, but there is marginal evidence for structure on smaller scales. The D/H value is high compared to the majority of recent D/H measurements but consistent with the values for two other measurements at similar distances. D/O is in agreement with other distant measurements. The scatter in D/H values beyond ~100 pc remains a challenge for Galactic chemical evolution.

Far Ultraviolet Spectroscopic Explorer Observation of the Ultramassive White Dwarf PG 1658+441

We present an analysis of the Far Ultraviolet Spectroscopic Explorer (FUSE) spectrum of the ultramassive (M = 1.31 M☉), magnetic (Bs = 2.3 MG) white dwarf PG 1658+441. The far-ultraviolet (FUV) spectrum exhibits very broad Lyman lines and quasi-molecular Lyβ satellites, but weak Lyγ satellites may also be present. PG 1658+441 is the hottest white dwarf known to show these satellite features. We fit the Lyman lines with stellar models and obtain atmospheric parameters consistent with a published analysis of the Balmer lines. By averaging results obtained for the different FUSE segments, we determine Teff = 29,620 ± 500 K and log g = 9.31 ± 0.07. The models match the data over large portions of the spectrum, but discrepancies remain near the satellite features. Finally, no trace elements have been identified in the FUV spectrum, and we provide abundance upper limits for C, N, Si, and P.We present an analysis of the Far Ultraviolet Spectroscopic Explorer (FUSE) spectrum of the ultramassive (M = 1.31 M⊙), magnetic (Bs = 2.3 MG) white dwarf PG 1658+441. The far ultraviolet (FUV) spectrum exhibits very broad Lyman lines and quasi-molecular Lyman β satellites, but weak Lyman γ satellites may also be present. PG 1658+441 is the hottest white dwarf known to show these satellite features. We fit the Lyman lines with stellar models and obtain atmospheric parameters consistent with a published analysis of the Balmer lines. By averaging results obtained for the different FUSE segments, we determine Teff = 29, 620 ± 500 K and log g = 9.31 ± 0.07. The models match the data over large portions of the spectrum but discrepancies remain near the satellite features. Finally, no trace elements have been identified in the FUV spectrum, and we provide abundance upper limits for C, N, Si, and P. also with the Department of Physics and Astronomy, University of Victoria, PO Box 3055, Station Csc, Victoria, BC V8W 3P6, Canada. also with Observatoire de Paris-Meudon, LERMA, F-92195 Meudon Principal Cedex, France.