Hubble Space Telescope Goddard High Research Articles

FUSEOBSERVATIONS OF A FULL ORBIT OF SCORPIUS X-1

We obtained UV spectra of X-ray binary Scorpius X-1 in the 900-1200 A range with the Far Ultraviolet Spectroscopic Explorer over the full 0.79 day binary orbit. The strongest emission lines are the doublet of O VI at 1032,1038 A and the C III complex at 1175 A. The spectrum is affected by a multitude of narrow interstellar absorption lines, both atomic and molecular. Examination of line variability and Doppler tomograms suggests emission from both the neighborhood of the donor star and the accretion disk. Models of turbulence and Doppler broadened Keplerian disk lines Doppler shifted with the orbit of the neutron star added to narrow Gaussian emission lines with undetermined Doppler shift fit the data with consistent values of disk radius, inclination, and radial line brightness profile. The Doppler shift of the narrow component with the orbit suggests an association with the donor star. We test our line models with previously analyzed near UV spectra obtained with the Hubble Space Telescope Goddard High Resolution Spectrograph and archival spectra obtained with the HST Cosmic Origins Spectrograph.

The Nature and Origin of the Nonvoid Lyα Cloud Population

I continue my study of the low-redshift Lyα cloud population. Previous work showed how galaxy catalogs could be used to attribute relative degrees of isolation to low-redshift Lyα clouds found in Hubble Space Telescope-Goddard High Resolution Spectrograph spectra. This enabled the separation of clouds into two distinct populations corresponding to two distinct environments, variously characterized as void/unshocked and nonvoid/shocked. Void clouds have a steep equivalent width distribution (i.e., many smaller absorbers), while nonvoid clouds have a flat distribution. I show that N-body/hydro simulations of Lyα clouds are inconsistent with observations of the clouds as a function of their environments. Simulations fail to predict the existence of significant numbers of detectable void clouds and incorrectly predict the characteristics of nonvoid clouds. Implicated in this failure is the so-called fluctuating Gunn-Peterson approximation (FGPA), which envisions that Lyα absorbers are formed in the large-scale structures of coalescing matter. A recent paper (of mine) has modeled the void cloud population as subgalactic perturbations that have expanded in response to reionization. It is notable that success in this modeling was contingent on using the more massive isothermal halo in place of the standard Navarro, Frenk, & White halo, for it was found that gravitational restraint on evaporation of baryons is key to producing detectable void absorbers. In this paper I extend my modeling of Lyα clouds to nonvoid clouds, using the same basic cloud model. In the case of voids, clouds are in a quiescent environment, while nonvoid clouds are thought of as void clouds that have accreted to the denser, turbulent intergalactic medium surrounding galaxies and are thus subjected to bow shock stripping. Model void clouds are analytically shock-stripped, and a column density spectrum (CDS) is derived, based on the same halo velocity distribution function as that used to explain the void CDS. The nonvoid CDS produced by shocked subgalactic clouds are found to be capable of producing an excellent fit to the observed nonvoid CDS without recourse to the FGPA mechanism.

Hubble Space TelescopeSpectroscopy of the Dwarf Nova RX Andromedae. I. The Underlying White Dwarf

We obtained Hubble Space Telescope Goddard High Resolution Spectrograph phase-resolved spectroscopic observations of the dwarf nova RX Andromedae at three times in its outburst cycle: (1) near the end of an extraordinarily deep and long dwarf nova quiescence, 3 months after the last outburst; (2) during the rise to outburst; and (3) near the end of a decline from outburst. The spectral wavelength range covered was 1149-1435 A. All of the spectra are dominated by absorption lines with weak to moderately strong emission wings due to the continued presence of disk material. Uncertainties in line velocities preclude a K1 determination or mass information. Our best-fitting model yielded Twd/1000 = 34.0 ± 0.1 K, log g = 8.0 ± 0.1, and Vrot= 600 km s-1. The Teff value is very similar to the Teff of the white dwarf in U Geminorum, but the rotational velocity appears to be higher than U Gem's value. We report approximate subsolar chemical abundances of carbon and silicon for RX And with C being 0.05 times solar and Si being 0.1 times solar while other elements are at essentially their solar values. However, accurate abundances are complicated by line emission, and we cannot exclude the possibility that the abundances of all species are essentially at the solar values. We see no evidence of thermonuclear-processed abundance ratios. If the white dwarf mass is 0.8 M☉, then the corresponding white dwarf cooling age, 4 × 106 years, is a lower limit to the age of this cataclysmic variable (CV). If the peculiar line features seen in the spectrum on the late decline from outburst are inverse P Cygni in nature, then infall velocities of ~ 2000 km s-1 are indicated during the decline from outburst. We compare the surface properties of the RX And white dwarf with the properties of other CV degenerates studied to date with the Hubble Space Telescope, the Hopkins Ultraviolet Telescope, and the International Ultraviolet Explorer.

An Optical Region Elemental Abundance Analysis of the HgMn Type Star HR 7775

Optical region spectra of the chemically peculiar HgMn type star HR 7775 have been analyzed for the determination of elemental abundances and spectral line identification as a complementary study to the analysis of Hubble Space Telescope Goddard High Resolution Spectrograph spectra. The spectra were obtained with the SOFIN echelle spectrograph on the Nordic Optical Telescope at nominal resolving powers of R = 25,000, 80,000, and 170,000 and signal-to-noise ratios typically in excess of 100 : 1. The overall nature of the elemental abundance distribution is similar to other cool, HgMn stars; somewhat enhanced abundances for the iron-group elements, with progressively stronger enhancements for heavier elements. Of particular note are the presence of lines from the rare-earth spectra Nd III and Pr III, and the heavy elements Pt, Au, Hg, Tl, and Bi. The isotopic mixture for several lines of mercury and platinum has been investigated. For mercury a possible difference is found between the mixtures as derived from lines of Hg I and Hg II. Three lines of Pt II display the same mixture, which is different from that found from lower excitation lines at ultraviolet wavelengths. We present new laboratory Fourier Transform spectrometer measurements for isotopic and hyperfine structure components of Hg I λ4046 and 4358 and Hg II λ3984 A.

A Uniform Analysis of Boron in F and G Disk Dwarfs fromHubble Space TelescopeArchival Spectra

The boron abundance is derived for 14 near-solar metallicity F and G dwarfs from the archived data set of spectra obtained with the Hubble Space Telescope Goddard High Resolution Spectrograph in the region of the B I resonance line near 2497 A. These abundances are derived in a uniform way with respect to recent work on solar boron. The synthetic spectra of the B I region were calculated by means of an improved linelist to fit the spectra of the stars spanning a range in Teff between 5650 and 6700 K and metallicity between [Fe/H] = -0.75 and +0.15. The synthetic spectra were computed considering all major sources of metal opacities and, in particular, an updated lower value of Mg I b-f opacity at 2500 A was used in the calculations. Adopting Li and Be abundances from the literature, significant B depletions (~0.3-0.5 dex) are found only for the two stars with the most extreme Li depletions (≥2.5 dex) and Be depletions (≥1.5 dex), although the trend of B versus Be indicates that mild B depletions may exist in many of the stars. These results are in concordance with the Boesgaard et al. conclusion that a slow mixing in the outer envelope is responsible for depletions of Li, Be, and B. Stars with less severe Li and Be depletions exhibit roughly solar [B/Fe] ratios. A comparison of the boron abundances from the disk stars (with undepleted boron) with metal-poor halo dwarfs reveals that B is linearly related to Fe over the entire range of metallicities from [Fe/H] ~ -2.5 to 0.1. The B/Be ratio found for stars with near-solar metallicities and no Li depletion is B/Be = 27 ± 5, equal to the solar ratio (B/Be = 23).

Digging Deeper in the Coronal Graveyard: Postscript

An X-ray upper limit—from the ROSAT High Resolution Imager—is presented for Corvi (HD 105707), a K2 giant that lies in the noncoronal zone of the Hertzsprung-Russell diagram, but which previously was detected by the Hubble Space Telescope Goddard High Resolution Spectrograph in far-ultraviolet coronal proxies. The new measurement strengthens the anticorrelation between coronal X-rays and λ1380 carbon monoxide emission among the red giants.

The Diffuse Interstellar Clouds toward 23 Orionis

Spectra obtained with the Hubble Space Telescope Goddard High Resolution Spectrograph are combined with high-resolution optical spectra and UV spectra from Copernicus to study the abundances and physical conditions in the diffuse interstellar clouds seen along the line of sight to the star 23 Ori. Multiple absorption components are present for each of several distinct types of gas, which are characterized by different relative abundance and depletion patterns and physical conditions.?????Strong low-velocity (SLV) absorption, due to cool, moderately dense neutral gas and representing about 92% of the total N(H I), is seen for various neutral and singly ionized species at +20 km s-1 v? +26 km s-1. Most typically severely depleted species are less depleted by factors of 2-4, compared to the cold, dense cloud pattern found, for example, in the main components toward ? Oph.For the two strongest SLV components, T ~ 100 K and the thermal pressure log (nHT) ~ 3.1 cm-3 K; we thus have nH ~ 10-15 cm-3 and a total thickness of 12-16 pc. The adopted average SLV electron density, ne = 0.15 ? 0.05 cm-3, implies a relatively large ne/nH ~ 0.01 and thus some ionization of hydrogen in these predominantly neutral components.?????Weaker low-velocity (WLV) absorption, probably largely due to warmer neutral gas, is seen primarily for various singly ionized species at 0 km s-1 v? +30 km s-1. The depletions in the WLV gas are typically less severe by a factor of 2-3 than in the SLV gas and are somewhat similar to the warm cloud pattern seen in lines of sight with low reddening, low mean density, and/or low molecular fraction. If T ~ 3000 K for the WLV components, then we have log(nHT) ~ 4.7-4.8 cm-3 K, nH ~ 15-20 cm-3, ne ~ 0.2 cm-3, ne/nH ~ 0.01, and a total thickness of 0.7-0.9 pc.?????Absorption from a number of singly and doubly ionized species, perhaps due to a radiative shock, is seen at -108 km s-1 v? -83 km s-1. While the depletions in these ionized components are uncertain owing to unobserved ionization stages, aluminum (typically severely depleted) is probably depleted there by only a factor ~3, even at cloud velocities in excess of 100 km s-1. The individual high-velocity components typically have T ~ 8000 ? 2000 K, ne = nH ~ 0.4-0.5 cm-3, thermal pressure log(2neT) ~ 3.7-4.0 cm-3 K, and thicknesses of order 0.1 pc.?????Weak absorption components from ionized (H II) gas are seen in C II, Mg II, and Si III at intermediate velocities (-43 km s-1 v? -4 km s-1). Broad, weak absorption from the higher ions S III, C IV, Si IV, and N V is centered at -5 km s-1 v? +6 km s-1. No obvious absorption is discerned from a circumstellar H II region around 23 Ori itself.?????The large range in ne (from 0.04 to 0.95 cm-3) derived independently from nine pairs of neutral and singly ionized species in the SLV gas suggests that additional processes besides simple photoionization and radiative recombination affect the ionization balance. Charge exchange with protons may reduce the abundances of S I, Mn I, and Fe I; dissociative recombination of CH+ may help to enhance C I. The large apparent fractional ionization in the SLV and WLV gas may be due to an enhanced flux of X-rays in the Orion region, to mixing of neutral and ionized gas at the boundary of the Orion-Eridanus bubble, or perhaps (in part) to charge exchange between singly ionized atomic species and large molecules (in which case the true ne would be somewhat smaller). Comparisons of the SLV depletions and nH with those found for the strong component B (v? ~ -14 km s-1) blend toward ? Oph hint at a possible relationship between depletion and local density for relatively cold interstellar clouds. Calcium appears to be more severely depleted in warm, low density gas than has generally been assumed.?????An appendix summarizes the most reliable oscillator strengths currently available for a number of the interstellar absorption lines analyzed in this work.

The Boron Abundance of the Orion G‐Dwarf Member BD −05o1317

The boron abundance is derived for the young Orion solar-type member BD -05°1317 from Hubble Space Telescope Goddard High Resolution Spectrograph spectra of the B I resonance lines near 2497 A. The abundance of log (B) = 2.1 ± 0.2 is ~0.6-0.7 dex below solar. As this young pre-main-sequence star has an undepleted lithium abundance, such a low boron abundance in a near-solar metallicity star is unexpected. No simple explanation apparently accounts for the low boron abundance observed for this star. A comparison of the boron and oxygen abundances in BD -05°1317 with those of other near-solar metallicity disk stars suggests that there might be a large scatter (~0.8 dex) in the boron abundance at a given oxygen abundance. Such a scatter also has been reported in the literature for beryllium at a given metallicity.

Mercury in the HgMn Stars χ Lupi and HR 7775

Observations of mercury lines in the HgMn stars chi Lupi and HR 7775 made with the Hubble Space Telescope Goddard High Resolution Spectrograph are presented and analyzed. In chi Lupi we find that all observed lines are consistent with the same isotopic mixture (essentially pure (204)Hg). Strong ionization anomalies are present, with UV Hg I lines being too weak and Hg III lines too strong for the abundance derived from lines of the majority ionization state, Hg rr. Observations of mercury in I;IR 7775 show less extreme isotope and ionization anomalies. We find that the ionization anomaly in the Hg I resonance lines can be plausibly explained as a non-LTE effect, but the same non-LTE calculations show that the Hg III ionization anomaly in chi Lupi cannot be explained in this way. Radiative force calculations show that the observed mercury abundance cannot be supported in the atmosphere by the radiative forces alone. We suggest that weak mixing brings mercury into the line-forming region from below the photosphere, while a wind of order 10(-14) M. yr(-1) supports a cloud of Hg III at very small optical depths. (Less)

The Physical Characteristics of the Small-Scale Interstellar Structure toward μ Crucis

We present Hubble Space Telescope Goddard High Resolution Spectrograph echelle observations of multiple interstellar lines of C I, Mg I, Cr II, and Zn II toward both stars in the μ Crucis binary system. Despite large differences in the profiles of the neutral species, no significant variations between the stars are seen in the Cr II and Zn II line profiles. In particular, the Zn II absorption observed at -8.6 km s-1 toward μ Cru is constant despite greatly enhanced columns of the neutral species at this velocity toward μ1 Cru. An analysis of the fine-structure excitation of C I in this cloud implies that the density is n<250 cm−3. From the lack of variation in the (optical) Na I D2 line profiles toward μ1 and μ2 Cru in spectra taken 21 months apart, we can place a lower limit to the size of the structures of ~10 AU. These results are discussed in the context of recent radio and optical studies of apparently pervasive high-density small-scale interstellar structure.

The Far‐Ultraviolet Oxygen Airglow of Europa and Ganymede

Far-UV spectra of Europa and Ganymede, acquired by the Hubble Space Telescope Goddard High Resolution Spectrograph, indicate that, in addition to faintly reflected sunlight, both satellites emit O I 1304 A and O I 1356 A airglow radiation. The observed brightnesses of the reflected solar C II 1335 A feature indicate that the disk-averaged albedos of Europa and Ganymede are about 1.5% and 2.6%, respectively. Airglow emissions from both satellites are characterized by the flux ratio F(1356 A)/F(1304 A) of roughly 1-2, diagnostic of dissociative electron impact excitation of O2. Inferred O2 vertical column densities are in the range (2.4-14) × 1014 cm-2 for Europa and (1-10) × 1014 cm-2 for Ganymede. The observed double-peaked profile of Ganymede's O I 1356 A feature indicates a nonuniform spatial emission distribution that suggests two distinct and spatially-confined emission regions, consistent with the satellite's north and south poles.

Hubble Space TelescopeGoddard High Resolution Spectrograph Observations of the Be + sdO BinaryφPersei

Mass transfer during the evolution of intermediate-mass stars in a close binary system can result in a rejuvenated and spun-up secondary star (which may appear as a rapidly rotating Be star) orbiting an unseen, stripped-down, remnant companion. One of the best candidates for such a system is the long- period (127 days) binary / Per. Here we present new Hubble Space Telescope Goddard High Resolution Spectrograph spectra of / Per in several UV regions that show clearly for the -rst time the spectral signature of the faint remnant companion. We derive a double-lined solution for the radial velocity curve that yields masses of 9.3 ^ 0.3 and 1.14 ^ 0.04 for the Be star and companion, respectively. A M _ M _ Doppler tomographic reconstruction of the secondary spectrum shows a rich spectrum dominated by sharp Fe IV and Fe V lines, similar to those observed in hot sdO stars. Non-LTE spectrum synthesis indicates that the subdwarf has temperature kK and gravity log g 4.2 ^ 0.1 and that the T eff 53 ^ 3 subdwarfEtoEBe star Nux ratio is 0.165 ^ 0.006 and 0.154 ^ 0.009 for the 1374 and 1647 regions, Ae respectively. The spectrum of the Be primary appears normal for a very rapidly rotating early B-type star, but we argue that the star is overluminous for its mass (perhaps owing to accretion-induced mixing). Additional sharp lines of Fe IV appear when the companion is in the foreground, and we show that these form in a heated region of the Be starIs disk that faces the hot subdwarf. Subject headings: stars: binaries: spectroscopic E stars: emission-line, Be E stars: evolution E stars: individual (/ Persei, HD 10516) E stars: subdwarfs

Rotationally Broadened Synthetic Spectra for HST and IUE Observations of Exposed White Dwarfs in Cataclysmic Variables11Based on observations with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5‐26555.

ABSTRACTIn many dwarf novae during quiescence, the underlying white dwarf dominates the flux in the far ultraviolet. By observing these exposed degenerate stars with the Hubble Space Telescope Goddard High Resolution Spectrograph and Faint Object Spectrograph (HST GHRS and FOS), the white‐dwarf rotation rates can be determined or constrained by fitting synthetic spectra broadened by rotation. Using TLUSTY and SYNSPEC, we present a grid of rotationally broadened white‐dwarf synthetic spectra, convolved with the resolution of the HST GHRS (G140L, G160M), FOS (G130H), and IUE low‐resolution short‐wavelength prime (SWP) spectra, computed for log g = 8, solar composition, Teff = 15,000, 25,000, and 50,000 K, and rotational velocities of 0–5000 km s−1. We examine the predicted appearance of accreted solar‐composition white‐dwarf atmospheres in the context of determining or constraining the white‐dwarf rotational velocities. We present model grids that demonstrate this capability. We present the first application of these grids to the SWP low‐resolution spectra of the white dwarf in the nova‐like variable TT Ari, exposed during the very low brightness state of that system. We also rule out the possibility that the absence of Lyα absorption in many cataclysmic variables in quiescence (specifically those containing white dwarfs with Teff = 15,000–20,000 K) could be due to rotational velocities of the white dwarf near breakup.

Atomic Data for the ReiiUV 1 Multiplet and the Rhenium Abundance in the HgMn‐Type Star χ Lupi

We report new laboratory spectroscopic analyses that have been conducted to acquire wavelength and oscillator strength data for lines of the Re II UV 1 multiplet. Wavelengths were determined to a sub-milliangstrom accuracy for the hyperfine and isotopic structure components from laboratory spectra obtained with the Lund VUV Fourier Transform Spectrometer. An absolute atomic transition probability is reported for Re II 2275.253 A, obtained from its upper level lifetime and an experimentally determined branching fraction. The radiative lifetime of the z 7Po2 level was found to be 4.47 ± 0.22 ns by the method of laser-induced fluorescence. With the new atomic data, synthetic spectra were generated to fit a Hubble Space Telescope Goddard High Resolution Spectrograph echelle mode spectrum of the chemically peculiar HgMn-type star χ Lupi. The weakness of the λ2275.253 A feature only allowed for the determination of an upper limit to the rhenium abundance, which was found to be consistent with the meteoritic value. Thus, in χ Lupi, rhenium appears to be several orders of magnitude less abundant than the apparent surficial abundances of the slightly heavier elements Pt, Au, Hg, and Tl.

The Masses and Radii of the Eclipsing Binary F Aurigae

We present a full determination of the fundamental stellar and orbital parameters of the eclipsing binary ζ Aurigae (K4 Ib + B5 V) using recent observations with the Hubble Space Telescope Goddard High Resolution Spectrograph (GHRS) and the Mark III long-baseline optical interferometer. The information obtained from spectroscopic and interferometric measurements is complementary, and the combination permits a complete determination of the stellar masses, the absolute semimajor axis of the orbit, and the distance. A complete solution requires that both components be visible spectroscopically, and this has always been difficult for the ζ Aur systems. The ζ Aur K star primary presents no difficulty, and accurate radial velocities are readily obtainable in the optical. However, the B star secondary is more problematic. Ground-based radial velocity measurements are hampered by the difficulty of working with the composite spectrum in the blue-violet region, the small number of suitable lines in the generally featureless optical spectrum of the B star, and the great width of the few available lines (the Balmer lines of hydrogen and a few weak He I lines) due to rapid rotation. We avoid the worst of these problems by using GHRS observations in the ultraviolet, where the K star Flux is negligible and the intrinsic B star spectrum is more distinctive, and obtain the most accurate determination of the B star radial velocity amplitude to date. We also analyze published photometry of previous eclipses and near-eclipse phases of ζ Aur in order to obtain eclipse durations, which fix the length of the eclipse chord and therefore determine the orbit inclination. The long-baseline interferometry (LBI) yields, in conjunction with the spectroscopic solution, the distance to the system and thus the absolute stellar radius of the resolved K supergiant primary star, ζ Aur A. The secondary is not resolved by LBI, but its angular (and absolute) radius is found by Fitting the model stellar flux plus an interstellar extinction model to the flux-calibrated GHRS data. We Find MK = 5.8 ± 0.2 M☉, MB = 4.8 ± 0.2 M☉, RK = 148 ± 3 R☉ and RB = 4.5 ± 0.3 RB for the masses and radii of the ζ Aur stars. We determine the distance to ζ Aur to be 261 ± 3 pc. Additionally, we refine the stellar parameters of the B star secondary presented in the 1995 spectroscopic study of Bennett, Brown, & Linsky. We also determine the effective temperature of the K star primary using values of the bolometric flux, angular diameter, and interstellar extinction derived in this study. The positions of the ζ Aur stars on the theoretical H-R diagram are compared to current evolutionary model tracks, and the resulting good agreement provides a strong check of the internal self-consistency of this analysis and the accuracy of the theoretical models. The ζ Aurigae stars are confirmed to be coeval with an age of 80 ± 15 Myr.

Abundance and Isotopic Anomalies of Thallium in the Atmosphere of the HgMn Star chi LUPI

We announce the detection of two fully resolved hyperfine components of the intercombination line of Tl II (Z = 81) at 1908.6 Angstrom in the ultraviolet spectrum of the sharp-lined, nonmagnetic, chemically peculiar star chi Lupi (BB.5pHgMn + A2 Vm). The wavelengths and profiles of these absorption lines strongly suggest that they arise from the heaviest isotope, Tl-205. The echelle observations were obtained with the Hubble Space Telescope Goddard High Resolution Spectrograph (HST/GHRS). Spectrum synthesis fits to the line profiles yield a logarithmic overabundance of thallium, relative to the solar system abundance, of +3.8 dex. We believe this to be the first detection of thallium in a star other than the Sun. It is also the first time hyperfine structure has been resolved in the satellite ultraviolet spectrum of an astrophysical source. The large overabundance and the presence of only the heaviest isotope of the element continue the pattern observed previously in chi Lup for platinum (Z = 78), gold (Z = 79), and mercury (Z = 80). (Less)

An HST study of galactic inerstellar zinc and chromium

We present a survey of interstellar Zn II and Cr II absorption extracted from the Hubble Space Telescope Goddard High Resolution Spectrograph (HST GHRS) data archive. We find clear evidence for an enhanced depletion of Zn from the gas phase with increasing fractional abundance of molecular hydrogen f(H2). Our lower limit to the Galactic interstellar metallicity is approximately 65% of the solar value as determined by the measured Zn abundances in the lowest f(H2) sightlines, (N(Zn)/N(H(sup 0)(sub tot)) = -0.19 +/- 0.04. The correspondingly high depletion of Cr with respect to solar (N(Cr/N(H(sup 0)(sub tot)) = -1.44 +/- 0.26 indicates that there are significant amounts of dust present in these lines of sight. The Galactic abundances of Zn and Cr in the ISM provide a fundamental reference point which is used to understand the metal enrichment and dust formation history of damped Lyman alpha QSO absorption-line systems, normally believed to arise from intervening precursors to modern disk galaxies. Although the spread in Zn abundances is large for both the local ISM and in damped Lyman alpha systems, we still find a substantial difference (factor of 4-10) in metallicity between the two sets. This survey and future observations of more distant objects which probe the full extent of the Milky Way halo provide a more complete picture of the enrichment and depletion characteristics of present-day galaxies.

Hubble Space Telescope high resolution spectroscoy of the exposed white dwarf in the dwarf nova VW Hydri in quiescence: A rapidly rotating white dwarf

We obtained a far-ultraviolet spectrum of the dwarf nova VW Hyi in quiescence, with the Hubble Space Telescope Goddard High Resolution Spectrograph covering the region of the Si iv lambda(lambda)1393, 1402 resonance doublet. The broad, shallow Si iv doublet feature is fully resolved, has a total equivalent width of 2.8 A, and is the first metal absorption feature to be clearly detected in the exposed white dwarf. Our synthetic spectral analysis, using a model grid constructed with the code TLUSTY, resulted in a reasonable fit to a white dwarf photosphere with T(sub eff) = 22,000 +/- 2000 K, log g = 8.0 +/- 0.3, an approximately solar Si/H abundance, and a rotational velocity, v sin i approximately equal to 600 km/s. This rotation rate, while not definitive because it is based upon just one line transition, is 20% of the Keplerian (breakup) velocity of the white dwarf and hence does not account for the unexpectedly low boundary-layer luminosity inferred from the soft-X-ray/extreme ultra-violet bands where most of the boundary-layer luminosity should be radiated. The predicted boundary-layer luminosity for a 0.6 solar mass white dwarf accreting at the rate 10(exp -10) solar mass/yr and rotating at 600 km/s, corresponding to VW Hyi in quiescence, is 2 x 10(exp 32) ergs/s when proper account is taken of the rotational kinetic energy going into spinning up the white dwarf. If the boundary-layer area is equal to that of the white dwarf, then T(sub bl) = 24,000 K. This is essentially identical to the photspheric luminosity and temperature determined in far-ultraviolet photospheric analyses. If the boundary-layer area is 10(exp -3) of the white dwarf surface area, then T(sub bl) = 136,000 K.

Photometric Characteristics of Data from the Hubble Space Telescope Goddard-High Resolution Spectrograph

The G-HRS is one of four axial scientific instruments which will fly aboard the Hubble Space Telescope (ref 1,2). It will produce spectroscopic observations in the 1050 A ≤ λ ≤ 3300 A region with greater spectral, spatial and temporal resolution than has been possible with previous space-based instruments. Five first order diffraction gratings and one Echelle provide three modes of spectroscopic operation with resolving powers of R = λ/ΔΔ = 2000, 20000 and 90000. Two magnetically focused, pulse-counting digicon detectors, which differ only in the nature of their photocathodes, produce data whose photometric quality is usually determined by statistical noise in the signal (ref 3). Under ideal circumstances the signal to noise ratio increases as the square root of the exposure time. For some observations detector dark count, instrumental scattered light or granularity in the pixel to pixel sensitivity will cause additional noise. The signal to noise ratio of the net spectrum will then depend on several parameters, and will increase more slowly with exposure time. We have analyzed data from the ground based calibration programs, and have developed a theoretical model of the HRS performance (ref 4). Our results allow observing and data reduction strategies to be optimized when factors other than photon statistics influence the photometric quality of the data.