Pierre Chayer

Overview of the Far Ultraviolet Spectroscopic Explorer Mission

The Far Ultraviolet Spectroscopic Explorer satellite observes light in the far-ultraviolet spectral region, 905-1187 Angstrom, with a high spectral resolution. The instrument consists of four co-aligned prime-focus telescopes and Rowland spectrographs with microchannel plate detectors. Two of the telescope channels use Al :LiF coatings for optimum reflectivity between approximately 1000 and 1187 Angstrom, and the other two channels use SiC coatings for optimized throughput between 905 and 1105 Angstrom. The gratings are holographically ruled to correct largely for astigmatism and to minimize scattered light. The microchannel plate detectors have KBr photocathodes and use photon counting to achieve good quantum efficiency with low background signal. The sensitivity is sufficient to examine reddened lines of sight within the Milky Way and also sufficient to use as active galactic nuclei and QSOs for absorption-line studies of both Milky Way and extragalactic gas clouds. This spectral region contains a number of key scientific diagnostics, including O VI, H I, D I, and the strong electronic transitions of H-2 and HD.

On-Orbit Performance of the Far Ultraviolet Spectroscopic Explorer Satellite

The launch of the Far Ultraviolet Spectroscopic Explorer (FUSE) has been followed by an extensive period of calibration and characterization as part of the preparation for normal satellite operations. Major tasks carried out during this period include the initial coalignment, focusing, and characterization of the four instrument channels and a preliminary measurement of the resolution and throughput performance of the instrument. We describe the results from this test program and present preliminary estimates of the on-orbit performance of the FUSE satellite based on a combination of these data and prelaunch laboratory measurements.

Abundances of Deuterium, Nitrogen, and Oxygen in the Local Interstellar Medium: Overview of First Results from the FUSE Mission

Observations obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) have been used to determine the column densities of D i ,N i, and O i along seven sight lines that probe the local interstellar medium (LISM) at distances from 37 to 179 pc. Five of the sight lines are within the Local Bubble, and two penetrate the surrounding H i wall. Reliable values of N(H i) were determined for five of the sight lines from Hubble Space Telescope (HST) data, International Ultraviolet Explorer (IUE) data, and published Extreme Ultraviolet Explorer (EUVE) measurements. The weighted mean of D i/H i for these five sight lines is ð1:52 � 0:08 Þ� 10 � 5 (1 � uncertainty in the mean). It is likely that the D i/H i ratio in the Local Bubble has a single value. The D i/O i ratio for the five sight lines within the Local Bubble is ð3:76 � 0:20 Þ� 10 � 2 .I t is likely that O i column densities can serve as a proxy for H i in the Local Bubble. The weighted mean for O i/H i for the seven FUSE sight lines is ð3:03 � 0:21 Þ� 10 � 4 , comparable to the weighted mean ð3:43 � 0:15 Þ� 10 � 4 reported for 13 sight lines probing larger distances and higher column densities. The FUSE weighted mean of N i/H i for five sight lines is half that reported by Meyer and colleagues for seven sight lines with larger distances and higher column densities. This result combined with the variability of O i/N i (six sight lines) indicates that at the low column densities found in the LISM, nitrogen ionization balance is important. Thus, unlike O i ,N i cannot be used as a proxy for H i or as a metallicity indicator in the LISM. Subject headings: cosmology: observations — Galaxy: abundances — ISM: abundances — ISM: evolution — ultraviolet: ISM

Far Ultraviolet Spectroscopic Explorer Survey of the Local Interstellar Medium within 200 Parsecs

We present FUSE observations of the interstellar gas toward 30 white dwarf and 1 subdwarf (SdO) stars. These sightlines probe the Local Bubble (LB) and the local interstellar medium (LISM) near the LB. Our survey detected only diffuse H_2 molecular clouds along six sightlines. There is no evidence from this study that H_2 exists well inside the perimeter of the LB. The kinematical temperature for H_2 is less than the usual temperature observed in the local interstellar clouds, implying different gas phases in the LISM. The relative abundance ratios of Si II, P II, and Fe II give insight about the dust content. These ratios vary, but are similar to the depletion patterns observed in warm and halo diffuse clouds in more distant sightlines in the Galaxy. The N I/O I and Ar I/O I ratios are significantly subsolar within the LB. Outside the LB a larger scatter is observed from subsolar to solar. Because Ar and N are only weakly depleted into dust grains if at all, the deficiencies of their neutral forms are likely due to photoionization. The evidence for significant ionization of N (and hence Ar) is strengthened by the detection and measurement of N II, which is a dominant ion for this element toward many sightlines. These observations imply that photoionization is the main ionization mechanism in the LISM and do not support the existence of a highly ionized condition in the past. In view of the variations observed in the different atomic and ionic ratios, the photoionization conditions vary significantly in the LB and the LISM. [Abridged.]We present Far Ultraviolet Spectroscopic Explorer observations of the interstellar gas toward 30 white dwarf stars and one subdwarf (SdO) star. These sight lines probe the Local Bubble (LB) and the local interstellar medium (LISM) near the LB. We systematically measure the column densities for the following species: C II, C II*, C III, N I, N II, O I, Ar I, Si II, P II, Fe II, Fe III, and H2. Our survey detected only diffuse H2 molecular clouds (fH2 10-4) along six sight lines. There is no evidence from this study that H2 exists well inside the perimeter of the LB. The kinematical temperature for H2 is less than the usual temperature observed in the local interstellar clouds, implying different gas phases in the LISM. The relative abundance ratios of Si II, P II, and Fe II give insight about the dust content. These ratios vary but are similar to the depletion patterns observed in warm and halo diffuse clouds in more distant sight lines in the Galaxy. The N I/O I and Ar I/O I ratios are significantly subsolar within the LB. Outside the LB a larger scatter is observed from subsolar to solar. Because Ar and N are only weakly depleted into dust grains if at all, the deficiencies of their neutral forms are likely due to photoionization. The evidence for significant ionization of N (and hence Ar) is strengthened by the detection and measurement of N II, which is a dominant ion for this element toward many sight lines. C III appears to be ubiquitous in the LISM toward our sight lines, but C II remains the dominant ionization stage of C. The limits on Fe III/Fe II imply that Fe II is the dominant ion. These observations imply that photoionization is the main ionization mechanism in the LISM and do not support the existence of a highly ionized condition in the past. In view of the variations observed in the different atomic and ionic ratios, the photoionization conditions vary significantly in the LB and the LISM. The cooling rate in the LISM, lc [in ergs s-1 (H I atom)-1], derived from the emission of the C II 157.7 ?m line has a mean value of log lc = -25.59 ? 0.32 dex, very similar to previous determinations.

Deuterium Abundance toward WD 2211–495: Results from the FUSE Mission*

We present a deuterium abundance analysis of the line of sight toward the white dwarf WD 2211-495 observed with the Far Ultraviolet Spectroscopic Explorer (FUSE). Numerous interstellar lines are detected on the continuum of the stellar spectrum. A thorough analysis was performed through the simultaneous fit of interstellar absorption lines detected in the four FUSE channels of multiple observations with different slits. We excluded all saturated lines in order to reduce possible systematic errors on the column density measurements. We report the determination of the average interstellar D/O and D/N ratios along this line of sight at the 95% confidence level: D/O = (4.0 ? 1.2) ? 10-2 and D/N = (4.4 ? 1.3) ? 10-1. In conjunction with FUSE observations of other nearby sight lines, the results of this study will allow a deeper understanding of the present-day abundance of deuterium in the local interstellar medium and its evolution with time.

Deuterium Abundance toward G191‐B2B: Results from the FUSE Mission

High-resolution spectra of the hot white dwarf G191-B2B, covering the wavelength region 905-1187 A ˚ , were obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE). These data were used in conjunction with existing high-resolution Hubble Space Telescope Space Telescope Imaging Spectrograph (STIS) obser- vations to evaluate the total H i ,D i ,O i, and N i column densities along the line of sight. Previous determina- tions of N(D i) based upon GHRS and STIS observations were controversial as a result of the saturated strength of the D i Lyline. In the present analysis the column density of D i has been measured using only the unsaturated Lyand Lylines observed by FUSE. A careful inspection of possible systematic uncertainties tied to the modeling of the stellar continuum or to the uncertainties in the FUSE instrumental characteristics has been performed. The column densities derived are log NðD i Þ¼ 13:40 � 0:07, log NðO i Þ¼ 14:86 � 0:07, and log NðN i Þ¼ 13:87 � 0:07, quoted with 2 � uncertainties. The measurement of the H i column density by profile fitting of the Lyline has been found to be uncertain. If additional weak, hot interstellar components are added to the three detected clouds along the line of sight, the H i column den- sity can be reduced quite significantly, even though the signal-to-noise ratio and spectral resolution at Ly� are excellent. The new estimate of N(H i) toward G191-B2B reads log NðH i Þ¼ 18:18 � 0:18 (2 � ), so that the average D/H ratio on the line of sight is D=H ¼ 1:66 þ0:9 � 0:6 � 10 � 5 (2 � ). Subject headings: ISM: abundances — ISM: clouds — stars: individual (G191-B2B) — ultraviolet: ISM

Structural parameters of the hot pulsating B subdwarf PG 1219+534 from asteroseismology

Over the last several years, we have embarked on a long term effort to exploit the strong potential that hot B subdwarf (sdB) pulsators have to offer in terms of asteroseismology. This effort is multifaceted as it involves, on the observational front, the acquisition of high sensitivity photometric data supplemented by accurate spectroscopic measurements, and, on the theoret- ical and modeling fronts, the development of appropriate numerical tools dedicated to the asteroseismological interpretation of the seismic observations. In this paper, we report on the observations and thorough analysis of the rapidly pulsating sdB star (or EC 14026 star) PG 1219+534. Our model atmosphere analysis of the time averaged optical spectrum of PG 1219+534 obtained at the new Multiple Mirror Telescope (MMT) leads to estimates of Teff = 33 600 ± 370 K and log g = 5.810 ± 0.046 (with log N(He)/N(H) = −1.49 ± 0.08), in good agreement with previous spectroscopic measurements of its atmospheric parameters. This places PG 1219+534 right in the middle of the EC 14026 instability region in the log g − Teff plane. A standard Fourier analysis of our high signal-to-noise ratio Canada-France-Hawaii Telescope (CFHT) light curves reveals the presence of nine distinct harmonic oscillations with periods in the range 122−172 s, a significant improvement over the original detection of only four periods by Koen et al. (1999, MNRAS, 305, 28). On this basis, we have carried out a detailed asteroseismic analysis of PG 1219+534 using the well-known forward method and assuming that the observed modes have � ≤ 3. Our analysis leads ob- jectively to the identification of the (k, � ) indices of the nine periods observed in the star PG 1219+534, and to the determination of its structural parameters. The periods all correspond to low-order acoustic modes with adjacent values of k and with � = 0, 1, 2, and 3. They define a band of unstable modes, in close agreement with nonadiabatic pulsation theory. Furthermore, the average dispersion between the nine observed periods and the periods of the corresponding nine theoretical modes of the optimal model is only ∼0.6%, comparable to the results of a similar analysis carried out by Brassard et al. (2001) on the rapid sdB pulsator PG 0014+067. On the basis of our combined spectroscopic and asteroseismic analysis, the inferred global structural parameters of PG 1219+534 are Teff = 33 600 ± 370 K, log g = 5.8071 ± 0.0057, log Menv/M∗ = −4.254 ± 0.147, M∗ = 0.457 ± 0.012 M� , R/R� = 0.1397 ± 0.0028, and L/L� = 22.01 ± 1.85. Combined with detailed model atmosphere calculations, we estimate, in addition, that this star has an absolute visual magnitude MV = 4.62 ± 0.06 and is located at a distance d = 531 ± 23 pc (using V = 13.24 ± 0.20). Finally, if we interpret the absence of fine structure (frequency multiplets) as indicative of a slow rotation rate of that star, we further find that PG 1219+534 rotates with a period longer than 3.4 days, and has a maximum rotational broadening velocity of V sin i < 2. 1k m s −1 .

Interstellar Deuterium, Nitrogen, and Oxygen Abundances toward GD 246, WD 2331–475, HZ 21, and Lanning 23: Results from the FUSE Mission*

The interstellar abundances of D i ,N i, and O i in the local ISM are studied using high-resolution spectra of four hot white dwarfs. The spectra of GD 246, WD 2331� 475, HZ 21, and Lan 23 were obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) in the wavelength range 905–1187 A ˚ . The line of sight to GD 246 probes the Local Interstellar Cloud and at least one other H i cloud inside the Local Bubble, which contains most of the gas seen along this line of sight. The column densities of Hi ,C ii*, Sii, and Siii are measured using archival Hubble Space Telescope STIS echelle-mode observations. The H i column density is deter... . .. . .. . .. . .. ... ...

Testing the forward modeling approach in asteroseismology II. Structure and internal dynamics of the hot B subdwarf component in the close eclipsing binary system PG 1336-018

Aims. We present a stringent test on the forward modeling technique in asteroseismology by confronting the predictions of a detailed seismic analysis of the pulsating subdwarf component in the unique close eclipsing binary system PG 1336−018 with those derived independently from modeling the binary light curve of the system. We also take advantage of the observed rotationally-split rich period spectrum to investigate the internal dynamics of the pulsating component in this system expected to be tidally locked. Methods. We carry out numerical exercises based on the double optimization technique that we developed within the framework of the forward modeling approach in asteroseismology. We use a recently updated version that now incorporates the effects of stellar rotation on the pulsation properties. We thus search in parameter space for the optimal model that objectively leads to the best simultaneous match of the 25 periods (including rotationally-split components) observed in PG 1336−018. For the first time, we also attempt to precisely reconstruct the internal rotation profile of the pulsator from its oscillations. Results. Our principal result is that our seismic model, which closely reproduces the observed periods, is remarkably consistent with one of the best-fitting possible solutions uncovered independently from the binary light curve analysis, in effect pointing to the correct

FUSE Determination of Abundances in Long-Period Pulsating V1093 Her (PG 1716+426) Stars*

We present an analysis of the Far Ultraviolet Spectroscopic Explorer (FUSE) spectra of five V1093 Her (PG 1716+426) stars, the subgroup of hydrogen-rich subdwarf B stars that exhibit very low amplitude, long-period luminosity variations. Our primary aim is to investigate whether these stars display abundances which differ from those observed in the nonvariable sdB stars and also in the shorter period V361 Hya variables. For the light elements and for those beyond the iron peak, our abundances are consistent with the trends observed in earlier studies. For the important iron peak elements Ti, V, Cr, Mn, Fe, Co, and Ni, which are thought to be directly linked to the driving mechanism in both long-period and short-period variables, the abundances determined in the V1093 Her stars appear very homogeneous and exhibit mild enrichments (by factors of 2-5 for Cr, Mn, Fe, and Co) over the solar value. However, these abundances do not differ appreciably from those measured in a sample consisting of constant stars and one short-period pulsator. The implications of these findings for current models which involve both diffusion processes and stellar winds to account for the driving of nonradial pulsations in sdB stars are discussed.