Science with the Hubble Space Telescope -- II
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GHRS Observations of Interstellar Deuterium toward HZ 43

Wayne Landsman
Hughes STX, NASA/GSFC, Code 681, Greenbelt, MD 20771 USA

Ulysses J. Sofia
National Research Council, NASA/GSFC, Code 681, Greenbelt, MD 20771 USA

P. Bergeron
Département de Physique, Université de Montréal, C.P. 6128, Succ. Centre-Ville, Montréal, Québec, Canada, H3C 3J7

Abstract:

We present GHRS echelle observations of interstellar hydrogen and deuterium toward the hot white dwarf HZ 43. Previous observations of Capella and other late-type stars have shown the utility of GHRS for the study of interstellar deuterium in directions where the total column density is less than cm. However, the derived D/H abundances toward late-type stars include a systematic uncertainty due to the complex and poorly known line profile of the chromospheric Lyman alpha emission. In contrast, the spectrum of HZ 43 has been successfully fit with a pure hydrogen model atmosphere at all wavelengths at which it has been observed (Napiwotzki et al. 1993). In addition, the strong EUV continuum of HZ 43 allows an independent estimate of the total H I column density (Dupuis et al. 1995).

Our GHRS observations were completed on 1 Aug 1995, and the Ly spectrum shows a well defined interstellar deuterium feature. The spectrum can be fit using a model with (1) a stellar Ly profile computed from a pure hydrogen NLTE atmosphere, (2) a deuterium abundance and thermal broadening parameter consistent with those derived toward Capella, and (3) a hydrogen column density consistent with that derived from EUVE observations of HZ 43.

Introduction

An exciting early result from HST was the determination of the deuterium abundance in the local interstellar medium from a high S/N GHRS spectrum of the Ly emission of Capella (Linsky et al. 1993). The derived value of D/H = (Linsky et al. 1995) provides an important anchor point for studies of cosmology and galactic chemical evolution. Capella is by far the brightest Ly emission source in the sky (Landsman & Simon 1993), and GHRS observations of additional lines of sight are unlikely to improve on the precision of the D/H measurement toward Capella. However, there are at least two reasons for studying deuterium in other lines of sight. First, there may be some evidence for variations of D/H in the local ISM (Ferlet et al. 1996) which, if confirmed, would complicate the use of deuterium as a cosmological probe. Second, the derived hydrogen column density toward Capella may be subject to systematic errors because of the difficulty of modeling the heavily saturated interstellar H I absorption against an unknown intrinsic chromospheric emission profile. The probable detection of a ``hydrogen wall'' in the heliosphere from Ly observations of Cen (Linsky & Wood 1996), provides an additional complication in the interpretation of the interstellar H I profile toward nearby stars.

HZ 43 is a bright, hot (T K) white dwarf located at a distance of 65 pc in an exceptionally low column density direction toward the North Galactic Pole. GHRS observations of HZ 43 may provide the most unbiased probe of D/H in the local interstellar medium for following reasons:

• The interstellar H I column density toward HZ 43 can be well determined from its strong EUV continuum (Dupuis et al. 1995). As noted above, it is the determination of the hydrogen rather than the deuterium column density that is usually the major uncertainty in the determination of D/H in the local ISM.

• The H I column density toward HZ 43 is known to be unusually low. From their EUVE observations, Dupuis et al. (1995) derive N(H I ) cm. The low column density decreases the probability of multiple interstellar components existing along the line of sight, and places the deuterium feature closer to the linear part of the curve of growth.

• The spectrum of HZ 43 can be accurately modeled. Observations in the optical, ultraviolet, EUV, and X-ray have all been successfully fit with a pure hydrogen atmosphere (Napiwotzki et al. 1993, Barstow et al. 1995). In contrast, white dwarfs with an appreciable metal opacity, such as G191-B2B, will have a larger uncertainty both in the interstellar hydrogen column derived from their EUV spectra (Dupuis et al. 1995), and in the accuracy of the computed stellar profile in the NLTE core of Ly (Lanz & Hubeny 1995).

Observations

Our GHRS observations of HZ 43 were completed on 1 Aug 1995. Along with the large-aperture, Echelle-A spectra of Ly (6 orbits), we also obtained spectra of Fe II 2382 (1 orbit) and Mg II 2800 (1 orbit) with the Echelle-B grating, and a small-aperture spectrum of Ly and N I 1200 (1 orbit) with the G160M grating. The spectra of the narrow metal lines show only a single interstellar component toward HZ 43.

The echelle Ly spectrum shows a well-defined interstellar deuterium feature (Figure 1). An excellent model fit to the Ly data is possible using almost no free parameters. The fit shown in Figure 1 was obtained using (1) a stellar profile computed using a pure hydrogen model atmosphere with T = 49,000 K, and = 7.7, as derived by Napiwotzki et al. (1993), (2) a hydrogen column density (N(H I ) = cm) consistent with the EUVE observations of Dupuis et al. (1995), and (3) a deuterium abundance (D/H = ) and thermal broadening parameter (T = 7000 K) consistent with the values determined toward Capella by Linsky et al. (1995). Figure 1 shows that a fit of the red wing of the interstellar H I profile is possible only if the core of the stellar Ly profile is computed in NLTE. (The NLTE model for HZ 43 was kindly computed for us by T. Lanz.) The stellar radial velocity ( km s) required to fit the NLTE core is in reasonable agreement with a recent radial velocity measurement of HZ 43 by Reid (1996) using Balmer line profiles.

Conclusions

Our analysis of the GHRS observations of HZ 43 has thus far yielded the following preliminary conclusions:

• The high opacity in the central Å of a white dwarf Ly \ profile requires the use of a NLTE model atmosphere. This is true even in the case of HZ 43, where observations at all other wavelengths have been successfully fit with a pure hydrogen LTE model atmosphere.

• The GHRS Ly profile of HZ 43 can be fit by a model with D/H = (consistent with the value derived toward Capella), and N(H I ) = cm (consistent with the value derived from EUVE observations of HZ 43).

• Unlike the GHRS observations of late-type stars, the accuracy of the deuterium abundance toward HZ 43 is likely limited by the modest S/N of the current GHRS data, rather than by possible systematic errors. Additional echelle observations with STIS should improve the S/N while providing simultaneous coverage of the N I 1200 triplet, which is a valuable tracer of component structure of the neutral ISM (e.g., Ferlet et al. 1996).

 
Figure: At left is the Echelle-A spectrum of HZ 43, showing the interstellar deuterium feature near 1215.32Å, and strong geocoronal emission through the large aperture. A model fit using an NLTE stellar profile (dotted line) provides a better fit to the red wing of the interstellar H I profile than does a LTE model (dashed line). The interstellar parameters for both fits are identical, and described in the text. At right, the GHRS spectrum is multiplied by for an assumed H I column density of cm, in order to reconstruct the intrinsic stellar profile. The NLTE model (dotted line) provides a much better fit than the LTE model (dashed line) in the region between 1215.8Å, and 1216.0Å.

References:

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W. Landsman, U. J. Sofia, P. BergeronLandsman, Sofia, and BergeronGHRS Observations of Interstellar Deuterium toward HZ 43