Computer Science
Scientific paper
Jul 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995hst..prop.5884l&link_type=abstract
HST Proposal ID #5884
Computer Science
Hst Proposal Id #5884 Interstellar Medium
Scientific paper
We propose to obtain GHRS echelle observations of interstellar hydrogen and deuterium toward the hot (Teff ~ 50,000 K) white dwarf HZ 43. EUV observations of HZ 43 (d = 65 pc) show it to be located in an unusually low column density direction (N(HI) ~ 6.5 * 10^17 cm^-2). Previous GHRS observations toward late-type stars have demonstrated the capability to study interstellar deuterium Ly-alpha in directions where the total column density is less than ~ 3 * 10^18 cm^-2. However, the derived D/H abundances toward late-type stars includes a systematic uncertainty due to the complex and poorly understood line profile of the chromospheric Ly-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. A second advantage of HZ 43 over late-type stars as a probe of the D/H abundance is its strong EUV and FUV continuum. In particular, the 504-912 Angstrom EUV continuum provides an independent check on the HI column density derived from Ly-alpha, and we will use the FUV continuum to observe interstellar species of small Doppler width, such as Mg II and Fe II, to resolve the velocity structure along the line of sight. Our simulations show that HZ 43 is sufficiently bright to yield a good S/N echelle spectrum at Ly-alpha, and likely provide the most unbiased estimate of D/H in the local interstellar medium.
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