Goddard High Resolution Spectrograph Observations Research Articles

THE ABUNDANCE OF BORON IN DIFFUSE INTERSTELLAR CLOUDS

We present a comprehensive survey of boron abundances in diffuse interstellar clouds from observations made with the Space Telescope Imaging Spectrograph (STIS) of the Hubble Space Telescope. Our sample of 56 Galactic sight lines is the result of a complete search of archival STIS data for the B II 1362 resonance line, with each detection confirmed by the presence of absorption from O I 1355, Cu II 1358, and Ga II 1414 (when available) at the same velocity. Five previous measurements of interstellar B II from Goddard High Resolution Spectrograph observations are incorporated in our analysis, yielding a combined sample that more than quadruples the number of sight lines with significant boron detections. Our survey also constitutes the first extensive analysis of interstellar gallium from STIS spectra and expands on previously published results for oxygen and copper. The observations probe both high and low-density diffuse environments, allowing the density-dependent effects of interstellar depletion to be clearly identified in the gas-phase abundance data for each element. In the case of boron, the increase in relative depletion with line-of-sight density amounts to an abundance difference of 0.8 dex between the warm and cold phases of the diffuse interstellar medium. The abundance of boron in warm, low-density gas is found to be B/H = (2.4+/-0.6)*10^-10, which represents a depletion of 60% relative to the meteoritic boron abundance. Beyond the effects of depletion, our survey reveals sight lines with enhanced boron abundances that potentially trace the recent production of 11B, resulting from spallation reactions involving either cosmic rays or neutrinos. Future observations will help to disentangle the relative contributions from the two spallation channels for 11B synthesis.

Variable Na i Absorption toward Leonis: Biased Neutral Formation in the Diffuse Interstellar Medium?

We present multiepoch KPNO coude feed telescope observations of interstellar Na I and Ca II absorption toward the bright star ρ Leo. Comparisons of the Na I profiles observed over a period of 8 yr reveal significant temporal variations in the Na I column in at least one component, implying that there is structure at scales on the order of the proper motion (~12 AU). Archival Hubble Space Telescope (HST) Goddard High Resolution Spectrograph observations of the C I fine-structure excitation in the variable component suggest that the density is 20 cm-3, significantly lower than the densities inferred in past H I 21 cm and Na I studies. We suggest that the bulk of the trace neutral species are in the density peaks within an interstellar cloud. The patchy distribution of these species naturally gives rise to the large fluctuations seen on scales of 10-1000 AU in past temporal and binary studies. This picture predicts that the scales over which fluctuations will be observed vary as a function of the ionization rate of a species.

Is There Evidence for Rotation or Horizontal Motions in Hot DB White Dwarfs?

In a recent analysis of Goddard High Resolution Spectrograph observations of three hot DB white dwarfs, Provencal et al. suggested that the C II λ1335 doublet in two such objects is abnormally broadened. This was interpreted as the signature of rotation, or perhaps of pulsation-related horizontal motions, at a velocity of v ~ 60 km s-1. Our reanalysis of these observations shows that a self-consistent analysis of the observed C II profiles is possible without calling on additional broadening mechanisms. There is thus currently no evidence for significant rotation velocities or horizontal motions in hot DB white dwarfs.

Goddard High Resolution Spectrograph Observations of NGC 4151

Goddard High Resolution Spectrograph observations of the nucleus of the bright, nearby Seyfert galaxy NGC 4151 are presented and briefly described.

STIS and GHRS Observations of Warm and Hot Gas Overlying the Scutum Supershell (GS 018−04+44)

We present Space Telescope Imaging Spectrograph (STIS) and Goddard High Resolution Spectrograph (GHRS) observations of interstellar UV absorption toward HD 177989, a B0 III star at 4.9 kpc in the direction l = 178, b = -119. The line of sight passes through the high-latitude ejecta of the Scutum supershell (GS 018-06+44), which is ~5° in diameter extending ~7° below the Galactic plane at a kinematic distance of ~3.5 kpc in the Scutum spiral arm. The observations with the STIS E140H and GHRS echelle B gratings provide far- and middle-UV spectra at resolutions (FWHM) of ~3 km s-1 and a signal-to-noise ratio (S/N) of ~20:1 to 50:1. The observations reveal strong and broad absorption in the lines of Si IV and C IV centered on LSR velocities of +18 and +42 km s-1 and weaker absorption from these ions near -50 and -13 km s-1. Weak absorption by N V extends over the full velocity range traced by Si IV and C IV. The +18 km s-1 high-ionization absorption likely occurs in gas ~400 pc below the Sagittarius spiral arm, while the extremely strong +42 km s-1 absorption occurs in highly ionized gas in the Scutum supershell at a distance of ~700 pc below the Galactic plane. The properties of the highly ionized gas associated with the Scutum supershell are similar to the gas found in radio loops I and IV; in both cases there is a strong enhancement in the column density of C IV without a corresponding increase in the column density of N V, which causes N(C IV)/N(N V) to be among the largest measured in the interstellar medium. The low-ionization absorption lines of N I, S II, Si II, and Fe II produce narrow absorption features at +37, +40 km s-1 and +55, +60 km s-1. The strength and kinematic properties of these absorption features bear no resemblance to those expected for the high-latitude neutral cloud seen in the H I 21 cm line. This may be due to the relatively low angular resolution (FWHM ~21') of the 21 cm observations. The kinematic relationships among the high-ionization and low-ionization absorption lines observed in the UV suggest a related origin in a hot-warm gas interface region. We are possibly seeing the warm gas in the swept-up shell surrounding a region where hot gas is being vented into the halo. In the warm gas, N(N I)/N(S II) ~0.01 solar, which implies a similar value for N(H0)/[N(H0) + N(H+)]. The warm neutral and ionized gas in the matter overlying the Scutum supershell has values of Si /S and Fe /S roughly similar to those found in the warm neutral medium of the Galactic disk in the vicinity of the Sun. While there has been grain processing in the ejecta of the Scutum supershell, the processing has not been complete. Based on observations of interstellar C IV and Si IV at high S/N and high resolution toward four very distant stars, we determine that highly ionized gas absorption components occur at a frequency of ~1 component kpc-1. The strongest components are associated with lines of sight that pass over or under spiral arms or that pass though Galactic supershells.

Space Telescope Imaging Spectrograph Echelle Observations of the Seyfert Galaxy NGC 4151: Physical Conditions in the Ultraviolet Absorbers

We have examined the physical conditions in intrinsic UV-absorbing gas in the Seyfert galaxy NGC 4151, using echelle spectra obtained with the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope on 1999 July 19. We confirm the presence of the kinematic components detected in earlier Goddard High Resolution Spectrograph (GHRS) observations, all of which appear to be outflowing from the nucleus, as well as a new broad absorption feature at a radial velocity of -1680 km s-1. The UV continuum of NGC 4151 was a factor of about 4 lower than in observations taken over the previous 2 yr, and we argue that the changes in the column density of the low-ionization absorption lines associated with the broad component at -490 km s-1 reflect the decrease in the ionizing flux. Most of the strong absorption lines (e.g., N V, C IV, Si IV, etc.) from this component are saturated but show substantial residual flux in their cores, indicating that the absorber does not fully cover the source of emission. Our interpretation is that the unocculted light is due to scattering by free electrons from an extended region, which reflects continuum, emission lines, and absorption lines. For the first time in such a study, we have been able to constrain the densities for this kinematic component and several others based on the strength of absorption lines from metastable states of C III and Fe II and/or the ratios of ground and fine structure lines of O I, C II, and Si II. We have generated a set of photoionization models that successfully match not only the ionic column densities for each component during the present low-flux state but also those seen in previous high-flux states with the GHRS and STIS, confirming that the absorbers are photoionized and respond to the changes in the continuum flux. Based on the model parameters (ionization parameter and density), we have been able to map the relative radial positions of the absorbers. We find that the absorbing gas decreases in density with distance. Finally, none of the UV absorbers is of sufficiently large column density or high enough ionization state to account for the observed X-ray absorption, while the scatterer is too highly ionized. Hence, the X-ray absorption must arise in a separate component of circumnuclear gas.

The Abundance of Interstellar Boron

We use new Space Telescope Imaging Spectrograph (STIS) and archival Goddard High Resolution Spectrograph (GHRS) observations to study interstellar B II λ1362 and O I λ1355 absorption along seven sight lines. Our new column density measurements, combined with measurements of four sight lines from the literature, allow us to study the relative B/O abundances over a wide range of interstellar environments. We measure sight-line-integrated relative gas-phase abundances in the range [B/O] = -1.00 to -0.17, and our data show that the B/O abundances are anticorrelated with average sight line densities over the range lognH ≈ -1.3 to +0.7. Detailed comparisons of the B II and O I line shapes show that the B/O ratio is significantly higher in warm interstellar clouds than in cool clouds. These results are consistent with the incorporation of boron into dust grains in the diffuse ISM. Since boron is likely incorporated into grains, we derive a lower limit to the present-day total (gas + dust) interstellar boron abundance of B/H (2.5 ± 0.9) × 10-10. The effects of dust depletion and ionization differences from element to element will make it very difficult to reliably determine 11B/10B along most interstellar sight lines.

The Large Magellanic Cloud Eclipsing Binary HV 2274: Fundamental Properties and Comparison with Evolutionary Models

We are carrying out an international, multiwavelength program to determine the fundamental properties and independent distance estimates of selected 14th to 15th magnitude eclipsing binaries in the Large and Small Magellanic Clouds (LMC and SMC). Eclipsing binaries with well-defined double-line radial velocity curves and light curves provide valuable information on orbital and physical properties of their component stars. These properties include, among other characteristics, stellar mass and radius. These can be measured with an accuracy and directness unachievable by any other means. The study of stars in the LMC and SMC, where the metal abundances are significantly lower than solar (by one-third to one-tenth) provides an important opportunity to test opacities and stellar atmosphere, interior, and evolution models. For the first time, we can also measure direct mass-luminosity relations for stars outside our Galaxy. In a previous paper we demonstrated how a precise distance to the LMC—corresponding to (V0 - Mv)LMC = 18.30 ± 0.07 mag—could be determined using the 14th magnitude LMC eclipsing binary HV 2274. In this paper we concentrate on the determination of the orbital and physical properties of HV 2274 and its component stars from analyses of light curves and new radial velocity curves formed from Hubble Space Telescope (HST)/Goddard High-Resolution Spectrograph observations. HV 2274 (B1-2 IV-III + B1-2 IV-III; Vmax ≈ +14.2; P = 5.73 days) is a particularly appealing star because it is a detached binary that has an eccentric orbit (e = 0.136) and shows rapid apsidal motion. The results of these analyses yield reliable masses and absolute radii, as well as other physical and orbital properties of the stars and the system. From UV/optical spectrophotometry (1150-4820 A) of HV 2274 obtained with the HST Faint Object Spectrograph, the temperatures and the metallicity ([Fe/H] = -0.45 ± 0.06) of the stars were found, as well as the interstellar extinction of the system. The values of the mass, absolute radius, and effective temperature for the primary and secondary stars are 12.2 ± 0.7 M☉, 9.9 ± 0.2 R☉, 23000 ± 180 K and 11.4 ± 0.7 M☉, 9.0 ± 0.2 R☉, 23110 ± 180 K, respectively. The age of the system (τ = 17 ± 2 Myr), helium abundance (Y = 0.26 ± 0.03), and a lower limit of the convective core overshooting parameter of αov ≈ 0.2 were obtained by fitting the stellar data with evolution models of Claret & Gimenez. HV 2274 has a relatively well determined (and fast) apsidal motion period of U = 123 ± 3 yr. From an analysis of apsidal motion, additional information and constraints on the structure of the stars can be obtained. The apsidal motion analysis corroborates that some amount of convective core overshooting (αov between 0.2 and 0.5) is needed.

GHRS Observations of Cool, Low‐Gravity Stars. V. The Outer Atmosphere and Wind of the Nearby K Supergiant λ Velorum

UV spectra of λ Velorum taken with the Goddard High Resolution Spectrograph (GHRS) on the Hubble Space Telescope are used to probe the structure of the outer atmospheric layers and wind and to estimate the mass-loss rate from this K5 Ib-II supergiant. VLA radio observations at λ = 3.6 cm are used to obtain an independent check on the wind velocity and mass-loss rate inferred from the UV observations. Parameters of the chromospheric structure are estimated from measurements of UV line widths, positions, and fluxes and from the UV continuum flux distribution. The ratios of optically thin C II] emission lines indicate a mean chromospheric electron density of log Ne ≈ 8.9 ± 0.2 cm-3. The profiles of these lines indicate a chromospheric turbulence (v0 ≈ 25-36 km s-1), which greatly exceeds that seen in either the photosphere or wind. The centroids of optically thin emission lines of Fe II and of the emission wings of self-reversed Fe II lines indicate that they are formed in plasma approximately at rest with respect to the photosphere of the star. This suggests that the acceleration of the wind occurs above the chromospheric regions in which these emission line photons are created. The UV continuum detected by the GHRS clearly traces the mean flux-formation temperature as it increases with height in the chromosphere from a well-defined temperature minimum of 3200 K up to about 4600 K. Emission seen in lines of C III] and Si III] provides evidence of material at higher than chromospheric temperatures in the outer atmosphere of this noncoronal star. The photon-scattering wind produces self-reversals in the strong chromospheric emission lines, which allow us to probe the velocity field of the wind. The velocities to which these self-absorptions extend increase with intrinsic line strength, and thus height in the wind, and therefore directly map the wind acceleration. The width and shape of these self-absorptions reflect a wind turbulence of ≈9-21 km s-1. We further characterize the wind by comparing the observations with synthetic profiles generated with the Lamers et al. Sobolev with Exact Integration (SEI) radiative transfer code, assuming simple models of the outer atmospheric structure. These comparisons indicate that the wind in 1994 can be described by a model with a wind acceleration parameter β ~ 0.9, a terminal velocity of 29-33 km s-1, and a mass-loss rate ~ 3 × 10-9 M☉ yr-1. Modeling of the 3.6 cm radio flux observed in 1997 suggests a more slowly accelerating wind (higher β) and/or a higher mass-loss rate than inferred from the UV line profiles. These differences may be due to temporal variations in the wind or from limitations in one or both of the models. The discrepancy is currently under investigation.

Models of X‐Ray Photoionization in LMC X‐4: Slices of a Stellar Wind

We show that the orbital variation in the UV P Cygni lines of the X-ray binary LMC X-4 results when X-rays photoionize nearly the entire region outside of the X-ray shadow of the normal star. We fit models to Goddard High Resolution Spectrograph (GHRS) observations of N V and C IV P Cygni line profiles. Analytic methods assuming a spherically symmetric wind show that the wind velocity law is well fitted by v (1 - 1/r)?, where ? is likely ?1.4-1.6 and definitely less than 2.5. Escape probability models can fit the observed P Cygni profiles and provide measurements of the stellar wind parameters. The fits determine LX/ = 2.6 ? 0.1 ? 1043 ergs s-1 M -1? yr, where LX is the X-ray luminosity and is the mass-loss rate of the star. Allowing an inhomogeneous wind improves the fits. IUE spectra show greater P Cygni absorption during the second half of the orbit than during the first. We discuss possible causes of this effect.

Goddard High‐Resolution Spectrograph Observations of the BiiiResonance Doublet in Early B Stars: Abundances and Isotope Ratios

Boron abundances and isotope ratios in two early B stars have been measured by using the Hubble Space Telescope's Goddard High-Resolution Spectrograph (GHRS) in order to observe the B III resonance doublet near 2066 A. We also report new experimental and theoretical results for the wavelengths, isotope shift, and hyperfine structure of these boron lines. In HD 886 (γ Pegasi; B2 IV), and HD 35299 (B1 V) the abundance of boron is about a factor of 2 lower than that inferred from solar system meteorites. The 11B/10B isotope ratio in HD 886 is determined to be 4.7, and for HD 35299 a value of 3.7 is found. These values are consistent with the solar system isotope ratio. In a third star, HD 3360 (ζ Cassiopeiae; B2 IV), our GHRS observations show that the boron abundance is a factor of 40 below the solar system abundance. Our results for HD 886 and HD 35299 appear to be consistent with little dispersion in the initial boron-to-oxygen ratio among Population I stars and a universal 11B/10B isotope ratio. However, the abundance measured for HD 3360 shows that either some stars suffer significant boron depletion while still in the core-hydrogen burning phase of their evolution, or there is indeed a very wide variation in initial Population I boron abundances. A much-improved understanding of boron astration in early B stars will be required before this question can be resolved.

The Gaseous Extent of Galaxies and the Origin of Lyα Absorption Systems. IV. Lyα Absorbers Arising in a Galaxy Group

We present new Goddard High Resolution Spectrograph observations of Lyα absorption lines associated with a group of galaxies toward the QSO 1545+2101. We have identified eight distinct Lyα absorption features in the spectrum of QSO 1545+2101 at a mean redshift of ⟨z⟩ = 0.2648 with a velocity dispersion of 163 km s-1. A group of galaxies is detected in the vicinity of this QSO at a mean redshift of ⟨z⟩ = 0.2645 and velocity dispersion of 239 km s-1. The identification of discrete absorption systems indicates that they arise in clouds of neutral hydrogen rather than in a diffuse intragroup medium. Our analysis suggests that the Lyα absorption lines are associated with individual galaxies in the group, although a one-to-one relationship between absorbers and galaxies is difficult to establish in such a dense environment.

Very High Resolution Ultraviolet Spectroscopy of a Chemically Peculiar Star: Results of the χ Lupi Pathfinder Project

We summarize here the results of a major eight-year investigation of the extraordinarily detailed UV spectrum of the sharp-lined, nonmagnetic, main-sequence, chemically peculiar star χ Lupi (B9.5p HgMn + A2 Vm). The UV observations are composed of 345 Å of the spectrum acquired with the Goddard High Resolution Spectrograph (GHRS) on board the Hubble Space Telescope at an average resolution of 0.023 Å. The complete set of echelle spectrograms is presented as an atlas in a companion paper. These data were supplemented by optical-wavelength spectra obtained at the Anglo-Australian Telescope. Quantitatively accurate analysis and theoretical interpretation of these data required major improvements in the accuracy and completeness of available atomic data—wavelengths, transition probabilities, hyperfine structure, and isotope shifts—for the lowest ionization states of many elements. A large, international group of theoretical and experimental atomic physicists has collaborated in this investigation, and their results are summarized or referenced in this paper. In turn, the GHRS observations of χ Lupi have become a useful source of data for atomic spectroscopy, displaying many transitions that are difficult to observe in a laboratory setting. Measured abundances or upper limits are presented for 72 ions of 51 chemical elements, spanning the periodic table. We have confirmed and refined previously identified isotopic abundance anomalies in mercury and platinum and have discovered similar isotopic anomalies in thallium and, tentatively, in lead. Large discrepancies among the LTE abundances derived, using a chemically homogeneous model atmosphere, from two or three ionization states of the same element are found to be common. In some cases these are due to departures from LTE in the ionization equilibria, but the largest such discrepancies probably result from chemical stratification within the photosphere. We find qualitative trends in the abundances of the elements that clearly signify radiatively driven diffusion and gravitational settling as the primary mechanism producing abundance anomalies. However, detailed non-LTE diffusion calculations for mercury and thallium show that there is insufficient unsaturated radiative force within the chemically enriched atmosphere to sustain the observed huge overabundances of these elements in equilibrium with gravity. Either other hydrodynamic processes, such as slow mass motions or unexpectedly strong stellar winds must assist radiation pressure in supporting the enriched material, or the observed abundance patterns simply provide a snapshot in time of a nonequilibrium, time-variable phenomenon.

ITAL]Hubble[/ITAL] [ITAL]Space[/ITAL] [ITAL]T[/ITAL][ITAL]elescope[/ITAL] Wide Field Planetary Camera 2 Observations of η Carinae

Hubble Space Telescope (HST) continuum and emission-line images of η Carinae obtained with the Planetary Camera (PC) of the Wide Field Planetary Camera 2 show a startling wealth of detailed structure in the expanding Homunculus Nebula and outer debris field. Using a combination of dithering and image deconvolutions, we have achieved a resolution of about 50 mas over the ~30'' field of view covered by the PC. The complex network of bright cells and reddish, dusty lanes that lace the surfaces of the bipolar lobes of the Homunculus resemble similar structures observed in recent HST images of planetary nebulae. The contrast between these dust lanes and the adjacent bright cells, together with the dimensions of these features, is used to estimate the opacity and density of the obscuring material. Though radiation pressure is expected to accelerate the dust lanes by less than 10–20 km s-1 with respect to the 650 km s-1 expansion of the Homunculus, it may be the source of the instability responsible for their formation, and we discuss the formation of the bright cells and dusty filaments in this context. We present new data on the debris field beyond the Homunculus. This region contains a set of high-velocity (or streamers) that show bipolar kinematics that associate them with the radially expanding lobes of the Homunculus and extend away from the central star like shrapnel from an explosion with trailing strings. The whiskers are several arcseconds long but often less than 01 wide, with some having length-to-width ratios exceeding 100. They are also remarkably uniform in brightness. We observe excess UV light several arcseconds northwest of the central star in the vicinity of a recent burst of radio emission. This blue appears to emanate from the equatorial region between the bipolar lobes. Though we cannot confirm temporal variability of this excess light, it may be connected with a burst of ultraviolet radiation that escaped the central region and ionized gas in this direction, perhaps generating radio-wavelength hydrogen recombination line emission. The near-ultraviolet glow in our images may be produced by fluorescent Fe II emission lines in the bandpass of the F336W filter, as seen in our recent HST Goddard High Resolution Spectrograph observations of this region.

Search for Interstellar Water in the Translucent Molecular Cloud toward HD 154368

We report an upper limit of 9 x 10(exp 12)/sq cm on the column density of water in the translucent cloud along the line of sight toward HD 154368. This result is based on a search for the C-X band of water near 1240 A carried out using the Goddard High-Resolution Spectrograph (GHRS) of the Hubble Space Telescope. Our observational limit on the water abundance, together with detailed chemical models of translucent clouds and previous measurements of OH along the line of sight, constrains the branching ratio in the dissociative recombination of H3O(+) to form water. We find at the 3 sigma level that no more than 30% of dissociative recombinations of H3O(+) can lead to H2O. The observed spectrum also yielded high-resolution observations of the Mg II doublet at 1239.9 A and 1240.4 A, allowing the velocity structure of the dominant ionization state of magnesium to be studied along the line of sight. The Mg II spectrum is consistent with GHRS observations at lower spectral resolution that were obtained previously but allow an additional velocity component to be identified.

HubbleSpaceTelescopeUltraviolet Imaging and High‐Resolution Spectroscopy of Water Photodissociation Products in Comet Hyakutake (C/1996 B2)

Comet Hyakutake (C/1996 B2) provided a target of opportunity for performing a systematic study of water photodissociation products in which we obtained data from three instruments on the Hubble Space Telescope (HST). The HST Goddard High Resolution Spectrograph (GHRS) was used to measure the line profile of hydrogen Lyα (H Lyα) at six locations around the coma of the comet, ranging from the nucleus to a displacement of 100,000 km, and covering different directions compared with the comet-sun line. GHRS yielded line profiles with a spectral resolution (FWHM ~4 km s-1) that was a factor of 2-3 better than any previous H Lyα or Hα ground-based measurements. The Wide Field Planetary Camera 2 (WFPC2) and the Woods filter were used to obtain H Lyα images of the inner coma. The faint object spectrograph (FOS) was used to determine the OH production rate and monitor its variation throughout the HST observing sequence. The GHRS H Lyα line profiles show the behavior of a line profile that is optically thick in the core for positions near the nucleus (<5000 km) and gradually becoming more optically thin at larger displacements and lower column abundances. A composite H Lyα image constructed from four separate WFPC2 exposures is consistent with the relative fluxes seen in GHRS observations and clearly shows the dayside enhancement of a solar illuminated optically thick coma. These data were analyzed self-consistently to test our understanding of the detailed physics and chemistry of the expanding coma and our ability to obtain accurate water production rates from remote observations of gaseous hydrogen (H) and hydroxyl (OH), the major water dissociation products. Our hybrid kinetic/hydrodynamic model of the coma combined with a spherical radiative transfer calculation is able to account for (1) the velocity distribution of H atoms, (2) the spatial distribution of the H Lyα emission in the inner coma, and (3) the absolute intensities of H and OH emissions, giving a water production rate of (2.6 ± 0.4) × 1029 s-1 on 1996 April 4.

The Definitive Abundance of Interstellar Oxygen

Using the Goddard High Resolution Spectrograph (GHRS) onboard HST, we have obtained high S/N echelle observations of the weak interstellar O I 1356 A absorption toward the stars Gamma Cas, Epsilon Per, Delta Ori, Epsilon Ori, 15 Mon, Tau CMa, and Gamma Ara. In combination with previous GHRS measurements in six other sightlines (Zeta Per, Xi Per, Lambda Ori, Iota Ori, Kappa Ori, and Zeta Oph), these new observations yield a mean interstellar gas-phase oxygen abundance (per 10$^6$ H atoms) of 10$^6$ O/H = 319 +/- 14. The largest deviation from the mean is less than 18%, and there are no statistically significant variations in the measured O abundances from sightline to sightline and no evidence of density-dependent oxygen depletion from the gas phase. Assuming various mixtures of silicates and oxides, the abundance of interstellar oxygen tied up in dust grains is unlikely to surpass 10$^6$ O/H $\approx$ 180. Consequently, the GHRS observations imply that the total abundance of interstellar oxygen (gas plus grains) is homogeneous in the vicinity of the Sun and about 2/3 of the solar value of 10$^6$ O/H = 741 +/- 130. This oxygen deficit is consistent with that observed in nearby B stars and similar to that recently found for interstellar krypton with GHRS. Possible explanations for this deficit include: (1) early solar system enrichment by a local supernova, (2) a recent infall of metal-poor gas in the local Milky Way, or (3) an outward diffusion of the Sun from a smaller galactocentric distance.

The Local ISM and Its Interaction with the Winds of Nearby Late‐Type Stars

We present new Goddard High-Resolution Spectrograph (GHRS) observations of the Ly-alpha and Mg II absorption lines seen toward the nearby stars 61 Cyg A and 40 Eri A. We use these data to measure interstellar properties along these lines of sight and to search for evidence of circumstellar hydrogen walls, which are produced by collisions between the stellar winds and the Local InterStellar Medium (LISM). We were able to model the Ly-alpha lines of both stars without hydrogen-wall absorption components, but for 61 Cyg A the fit required a stellar Ly-alpha, line profile with an improbably deep self-reversal, and for 40 Eri A the fit required a very low deuterium-to-hydrogen ratio that is inconsistent with previous GHRS measurements. Since these problems could be rectified simply by including stellar hydrogen-wall components with reasonable attributes, our preferred fits to the data include these components. We have explored several ways in which the hydrogen-wall properties measured here and in previous work can be used to study stellar winds and the LISM. We argue that the existence of a hydrogen wall around 40 Eri A and a low H I column density along that line of sight imply that either the interstellar density must decrease toward 40 Eri A or the hydrogen ionization fraction (chi) must increase. We find that hydrogen-wall temperatures are larger for stars with faster velocities through the LISM. The observed temperature-velocity relation is consistent with the predictions of hydromagnetic shock jump conditions. More precise comparison of the data and the jump conditions suggests crude upper limits for both chi and the ratio of magnetic to thermal pressure in the LISM (alpha): chi less than 0.6 and alpha less than 2. The latter upper limit corresponds to a limit on the LISM magnetic field of B less than 5 micro G. These results imply that the plasma Mach number of the interstellar wind flowing into the heliosphere is M(sub A) greater than 1.3, which indicates that the collision is supersonic and that there should therefore be a bow shock outside the heliopause in the upwind direction. Finally, we estimate stellar wind pressures (P sub wind) from the measured hydrogen-wall column densities. These estimates represent the first empirical measurements of wind properties for late-type main-sequence stars. The wind pressures appear to be correlated with stellar X-ray surface fluxes, F(x), in a manner consistent with the relation P(wind) varies as F(x)(exp -1/2), a relation that is also consistent with the variations of P(sub wind) and F(sub x) observed during the solar activity cycle. If this relation can in fact be generalized to solar-like stars, as is suggested by our data, then it is possible to estimate stellar wind properties simply by measuring stellar X-rays. One implication of this is that stellar wind pressures and mass-loss rates are then predicted to increase with time, since F(sub x) is known to decrease with stellar age.

The Abundance of Interstellar Nitrogen

Using the HST Goddard High Resolution Spectrograph (GHRS), we have obtained high S/N echelle observations of the weak interstellar N I 1160, 1161 A absorption doublet toward the stars Gamma Cas, Lambda Ori, Iota Ori, Kappa Ori, Delta Sco, and Kappa Sco. In combination with a previous GHRS measurement of N I toward Zeta Oph, these new observations yield a mean interstellar gas phase nitrogen abundance (per 10$^6$ H atoms) of 10$^6$ N/H = 75 +/- 4. There are no statistically significant variations in the measured N abundances from sightline to sightline and no evidence of density-dependent depletion from the gas-phase. Since N is not expected to be depleted much into dust grains in these diffuse sightlines, its gas-phase abundance should reflect the total interstellar abundance. Consequently, the GHRS observations imply that the abundance of interstellar nitrogen (gas plus grains) in the local Milky Way is about 80% of the solar system value of 10$^6$ N/H = 93 +/- 16. Although this interstellar abundance deficit is somewhat less than that recently found for oxygen and krypton with GHRS, the solar N abundance and the N I oscillator strengths are too uncertain to definitively rule out either a solar ISM N abundance or a 2/3 solar ISM N abundance similar to that of O and Kr.

Temperatures and Altitudes of Jupiter's Ultraviolet Aurora Inferred from GHRS Observations with theHubble Space Telescope

We observed the jovian UV auroral regions with the Goddard high resolution spectrograph (GHRS) on board the Hubble Space Telescope (HST) on Apr. 29, May 2, and June 10, 1995. Observations of target areas were made in pairs in the two wavelength ranges 1257–1293 Å and 1587–1621 Å. Spectra in the long wavelength range are dominated by emissions of the H2Lyman band system and show well separated rotational features, which we have used to determine the temperatures of the auroral emission regions. Spectra in the short wavelength range are mostly due to emission in the H2Lyman and Werner band systems, but their intensities are reduced by hydrocarbon absorption. The brightest spectral pair was observed toward an area with longitude 155° and jovicentric latitude 58° when the central meridian longitudes (CMLs) were 191° and 203°. This area was found to be bright in our previous HST observations in 1993 and in HST faint object camera images. Assuming that electron impact excitation is the major source of the jovian aurora, we estimate total emission rates in the Lyman band system of about 270 and 46 kR for the long and short wavelength spectra of the pair, respectively. The attenuation of emission rate in the short wavelength spectrum implies a methane column density of about 3 × 1016cm−2, and a temperature of about 450 K is inferred from the long wavelength spectrum of the brightest pair. For all six pairs of observed spectra, we estimate methane column densities in the range (1–7) × 1016cm−2, which, when compared to a standard mid-latitude model, corresponds to a pressure range from a few μbar to a few tens of μbar. The temperatures derived are in the range 400–850 K with a possible tendency toward lower temperatures for higher methane column densities. This tendency and the uncertainty in the temperatures derived may indicate that the temperatures increases rapidly with altitude around the methane homopause in the auroral regions.