arXiv subjects

J. B. Holberg

Publications and source records attributed to J. B. Holberg.

42 records · Page 3Linked to original sources

arXiv2000

STIS observations of five hot white dwarfs

We present some early results from a study of five hot DA white dwarf stars, based on spectra obtained using STIS. All show multiple components in one or more of the strong resonance absorption lines typically associated with the stellar photosphere (e.g. C IV, Si IV, N V and O V). Possible relationships between the non-photospheric velocity components and the interstellar medium or local stellar environment, are investigated, including contributions from gravitational redshifting.

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arXiv2000

The discovery of photospheric nickel in the hot DO white dwarf REJ0503-289

We present the first evidence for the direct detection of nickel in the photosphere of the hot DO white dwarf REJ0503$-$289. While this element has been seen previously in the atmospheres of hot H-rich white dwarfs, this is one of the first similar discoveries in a He-rich object. Intriguingly, iron, which is observed to be more abundant than Ni in the hot DA stars, is not detected, the upper limit to its abundance (Fe/He$=10^{-6}$) implying a Fe/Ni ratio a factor 10 lower than seen in the H-rich objects (Ni/He$=10^{-5}$ for REJ0503$-$289). The abundance of nickel and various other elements heavier than He were determined from GHRS spectra. We used two completely independent sets of NLTE model atmospheres which both provide the same results. This not only reduces the possibility of systematic errors in our analysis but is also an important consistency check for both model atmosphere codes. We have also developed a more objective method of determining $T_{\rm eff}$ and log g, from the He lines in the optical spectrum, in the form of a formal fitting of the line profiles to a grid of model spectra, an analogue of the standard procedure utilising the Balmer lines in DA white dwarfs. This gives the assigned uncertainties in $T_{\rm eff}$ and log g a firm statistical basis and allows us to demonstrate that inclusion of elements heavier than H, He and C in the spectral calculations, exclusively considered in most published optical analyses, yields a systematic downward shift in the measured value of $T_{\rm eff}$.

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arXiv1999

The D/H Ratio in Interstellar Gas Towards G191-B2B

We reinvestigate the question of spatial variation of the local D/H abundance, using both archival GHRS spectra, and new echelle spectra of G191-B2B obtained with the Space Telescope Imaging Spectrograph (STIS) aboard HST. Our analysis uses stratified line-blanketed non-LTE model atmosphere calculations to determine the shape of the intrinsic WD Lyman-alpha profile and estimate the WD photospheric contamination of the interstellar lines. Although three velocity components were reported previously towards G191-B2B, we detect only two velocity components. The first component is at V(hel) ~ 8.6 km/s and the second at V(hel) ~ 19.3 km/s, which we identify with the Local Interstellar Cloud (LIC). From the STIS data we derive D/H = 1.60(+0.39,-0.27)X10^-5 for the LIC component, and D/H > 1.26X10^-5 for the 8.6 km/s component (uncertainties denote 2-sigma or 95% confidence limits). The STIS data provide no evidence for local or component-to-component variation in the D/H ratio. Despite using two velocity components for the profile fitting and using a more physically realistic WD Lyman-alpha profile for G191-B2B, our re-analysis of the GHRS data indicates a component-to-component variation as well as a variation of the D/H ratio in the LISM, neither of which are supported by the newer STIS data. We believe the most probable cause for this difference is the characterization of the background due to scattered light in the GHRS and STIS spectrographs. The two-dimensional MAMA detectors of STIS measure both the spatial and wavelength dependences of scattered light, allowing more accurate scattered light corrections than was possible with GHRS.

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arXiv1999

Evidence for the stratification of Fe in the photosphere of G191-B2B

The presence of heavy elements in the atmospheres of the hottest H-rich DA white dwarfs has been the subject of considerable interest. While theoretical calculations can demonstrate that radiative forces, counteracting the effects of gravitational settling, can explain the detections of individual species, the predicted abundances do not accord well with observation. However, accurate abundance measurements can only be based on a thorough understanding of the physical structure of the white dwarf photospheres, which has proved elusive. Recently, the availability of new non-LTE model atmospheres with improved atomic data has allowed self-consistent analysis of the EUV, far UV and optical spectra of the prototypical object G191$-$B2B. Even so, the predicted and observed stellar fluxes remain in serious disagreement at the shortest wavelengths (below $\approx 190$Å), while the inferred abundances remain largely unaltered. We show here that the complete spectrum of G191$-$B2B can be explained by a model atmosphere where Fe is stratified, with increasing abundance at greater depth. This abundance profile may explain the difficulties in matching observed photospheric abundances, usually obtained by analyses using homogeneous model atmospheres, to the detailed radiative levitation predictions. Particularly as the latter are only strictly valid for regions deeper than where the EUV/far UV lines and continua are formed. Furthermore, the relative depletion of Fe in the outer layers of the atmosphere may be evidence for radiatively driven mass loss in G191$-$B2B.

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arXiv1998

A search for hidden white dwarfs in the ROSAT EUV survey II: Discovery of a distant DA+F6/7V binary system in a direction of low density neutral hydrogen

We report the results of our final search for hot white dwarfs in unresolved, Sirius-type, binary systems with IUE. One new system, RE J0500-364 (DA+F6/7V), has been identified. This star appears to lie at a distance of between 500-1000pc, making it one of the most distant white dwarfs, if not the most distant, to be detected in the EUV surveys. The very low line-of-sight neutral hydrogen volume density to this object could place a lower limit on the length of the Beta CMa interstellar tunnel of diffuse gas, which stretches away from the Local Bubble in a similar direction to RE J0500-364.

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arXiv1996

The White Dwarf Distance to the Globular Cluster NGC 6752 (and its Age) with the Hubble Space Telescope (Based on observations made with the NASA/ESA Hubble Space Telescope (HST))

Deep Hubble Space Telescope (HST) observations with WFPC2 of the nearby globular cluster NGC 6752 have allowed us to obtain accurate photometry for the cluster white dwarfs (WD). A sample of local WDs of known trigonometric parallax and mass close to that of the cluster WDs have also been observed with WFPC2. Matching the cluster and the local WD sequences provides a direct measure of the distance to the cluster: $(m-M)_\circ=13.05$, with an uncertainty less than $\pm0.1$ mag which allows a substantial reduction in the uncertainty in the age of the cluster. Indeed, coupling this value of the cluster distance to the cluster metallicity, helium abundance and $α$-element enhancement [$α$/Fe]=0.5 yields an age of 15.5 Gyr and 14.5 Gyr using evolutionary models that do not include or do include helium diffusion, respectively. The uncertainty affecting these age determinations is $\sim 10\%$. The majority of the cluster WDs appear to be of the DA variety, while the color-magnitude location of two WDs is consistent with the DB type. This suggests a cluster DB/DA ratio similar to that of WDs in the solar neighborhood.

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