J. L. Linsky

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.

New mass-loss measurements from astrospheric Lyα absorption

Measurements of stellar mass-loss rates are used to assess how wind strength varies with coronal activity and age for solar-like stars. Mass loss generally increases with activity, but we find evidence that winds suddenly weaken at a certain activity threshold. Very active stars are often observed to have polar starspots, and we speculate that the magnetic field geometry associated with these spots may be inhibiting the winds. Our inferred mass-loss/age relation represents an empirical estimate of the history of the solar wind. This result is important for planetary studies as well as solar/stellar astronomy, since solar wind erosion may have played an important role in the evolution of planetary atmospheres.

The On-Orbit Performance of the Space Telescope Imaging Spectrograph

The Space Telescope Imaging Spectrograph (STIS) was successfully installed into the Hubble Space Telescope (HST) in 1997 February, during the second HST servicing mission, STS-82. STIS is a versatile spectrograph, covering the 115-1000 nm wavelength range in a variety of spectroscopic and imaging modes that take advantage of the angular resolution, unobstructed wavelength coverage, and dark sky offered by the HST. In the months since launch, a number of performance tests and calibrations have been carried out and are continuing. These tests demonstrate that the instrument is performing very well. We present here a synopsis of the results to date.

An IUE Atlas of Pre-Main-Sequence Stars. II. Far-Ultraviolet Accretion Diagnostics in T Tauri Stars

We use our ultraviolet (UV) atlas of premain-sequence stars constructed from all useful, short- wavelength, low-resolution spectra in the International Ultraviolet Explorer (IUE) satellite Final Archive to analyze the short-wavelength UV properties of 49 T Tauri stars (TTSs). We compare the line and continuum —uxes in these TTSs with each other and with previously published parameters of these systems, including rotation rate, infrared excess, and mass accretion rate. The short-wavelength contin- uum in the classical TTSs (CTTSs) appears to originate in a D10,000 K optically thick plasma, while in the naked TTSs (NTTSsstars without dusty disks) the continuum appears to originate in the stellar atmosphere. We show that all of the TTSs in our sample lie in the regime of ii saturated ˇˇ magnetic activity due to their small Rossby numbers. However, while some of the TTSs show emission line surface —uxes consistent with this saturation level, many CTTSs show signi—cantly stronger emission than pre- dicted by saturation. In these stars, the emission line luminosity in the high ionization lines present in the spectrum between 1200 and 2000 correlates well with the mass accretion rate. Therefore, we con- Ae clude that the bulk of the short-wavelength emission seen in CTTSs results from accretion related pro- cesses and not from dynamo-driven magnetic activity. Using CTTSs with known mass accretion rates, we calibrate the relationship between and to derive the mass accretion rate for some CTTSs M0 L CIV which for various reasons have never had their mass accretion rates measured. Finally, several of the CTTSs show strong emission from molecular hydrogen. While emission from cannot form in gas at a H 2 temperature of D105 K, the strength of the molecular hydrogen emission is nevertheless well correlated with all the other emissions displayed in the IUE short-wavelength bandpass. This suggests that the H 2 emission is in fact —uorescent emission pumped by the emission (likely Lya) from hotter gas. Subject headings: accretion, accretion diskscircumstellar matterstars: activity ¨ stars: premain-sequence

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

The Goddard High Resolution Spectrograph: In-orbit performance

The in-orbit performance of the Goddard High Resolution Spectrograph onboard the Hubble Space Telescope (HST) is presented. This report covers the pre-COSTAR period, when instrument performance was limited by the effects of spherical aberration of the telescopes primary mirror. The digicon detectors provide a linear response to count rates spanning over six orders of magnitude, ranging from the normal background flux of 0.01 counts diode -1 s-1 to values larger than 104 counts diode-1 s-1. Scattered light from the first-order gratings is small and can be removed by standard background subtraction techniques. Scattered light in the echelle mode is more complex in origin, but it also can be accurately removed. Data have been obtained over a wavelength range from below 1100 A to 3300 A, at spectral resolutions as high as R = lambda/delta-lambda = 90,000. The wavelength scale is influenced by spectrograph temperature, outgassing of the optical bench, and interaction of the magnetic field within the detector with the earths magnetic field. Models of these effects lead to a default wavelength scale with an accuracy better than 1 diode, corresponding to 3 km s-1 in the echelle mode. With care, the wavelength scale can be determined to an accuracy of 0.2 diodes. Calibration of the instrument sensitivity functions is tied into the HST flux calibration through observations of spectrophotometric standard stars. The measurements of vignetting and the echelle blaze function provide relative photometric precision to about 5% or better. The effects of fixed-pattern noise have been investigated, and techniques have been devised for recognizing and removing it from the data. The ultimate signal-to-noise ratio achievable with the spectrograph is essentially limited only by counting statistics, and values approaching 1000:1 have been obtained.

The Goddard High Resolution Spectrograph: Instrument, goals, and science results

The Goddard High Resolution Spectrograph (GHRS), currently in Earth orbit on the Hubble Space Telescope (HST), operates in the wavelength range of 1150-3200A with spectral resolutions (lambda/delta-lambda) of approximately 2 X 103, 2 X 104, and 1 X 105. This paper describes the instrument and its development from inception, its current status, the approach to operations, representative results in the major areas of the scientific goals, and prospects for the future.

Abundances of Deuterium, Nitrogen, and Oxygen toward HZ 43A: Results from the FUSE Mission

We present an analysis of interstellar absorption along the line of sight to the nearby white dwarf star HZ43A. The distance to this star is 68±13 pc, and the line of sight extends toward the north Galactic pole. Column densities

Deuterium Abundance toward WD 1634–573: Results from the FUSE Mission*

We use Far Ultraviolet Spectrocopic Explorer (FUSE) observations to study interstellar absorption along the line of sight to the white dwarf WD1634-573 (d=37.1+/-2.6 pc). Combining our measurement of D I with a measurement of H I from Extreme Ultraviolet Explorer data, we find a D/H ratio toward WD1634-573 of D/H=(1.6+/-0.5)e-5. In contrast, multiplying our measurements of D I/O I=0.035+/-0.006 and D I/N I=0.27+/-0.05 with published mean Galactic ISM gas phase O/H and N/H ratios yields D/H(O)=(1.2+/-0.2)e-5 and D/H(N)=(2.0+/-0.4)e-5, respectively. Note that all uncertainties quoted above are 2 sigma. The inconsistency between D/H(O) and D/H(N) suggests that either the O I/H I and/or the N I/H I ratio toward WD1634-573 must be different from the previously measured average ISM O/H and N/H values. The computation of D/H(N) from D I/N I is more suspect, since the relative N and H ionization states could conceivably vary within the LISM, while the O and H ionization states will be more tightly coupled by charge exchange.We use Far Ultraviolet Spectroscopic Explorer (FUSE) observations to study interstellar absorption along the line of sight to the white dwarf WD 1634-573 (d = 37.1 ± 2.6 pc). Combining our measurement of D I with a measurement of H I from Extreme Ultraviolet Explorer data, we find a D/H ratio toward WD 1634-573 of D/H = (1.6 ± 0.5) × 10-5. In contrast, multiplying our measurements of D /O = 0.035 ± 0.006 and D /N = 0.27 ± 0.05 with published mean Galactic interstellar medium (ISM) gas-phase O/H and N/H ratios yields D/HO = (1.2 ± 0.2) × 10-5 and D/HN = (2.0 ± 0.4) × 10-5, respectively. Note that all uncertainties quoted above are 2 σ. The inconsistency between D/HO and D/HN suggests that either the O I/H I or the N I/H I ratio toward WD 1634-573 must be different from the previously measured average ISM O/H and N/H values. The computation of D/HN from D I/N I is more suspect, since the relative N and H ionization states could conceivably vary within the local ISM, while the O and H ionization states will be more tightly coupled by charge exchange.