The interstellar D/H ratio toward G191-B2B from GHRS observations.

Abstract

We have observed the DA white dwarf G191-B2B using HST-GHRS to measure the interstellar D/H ratio. In order to derive an accurate structure of the line of sight, we have observed and analysed using a profile fitting procedure the interstellar lines of MgII (2800. and FeII (2344. high resolution using the Echelle B grating, and SiII (1190... (1206. (1334. (1200. (1302 . HI Lyman α (1216. medium resolution using the G160M grating. After an agreement is reached on the structure of the line of sight among the different lines analyzed, we finally detect 3 absorbing clouds whose physical parameters are tightly constrained; we derive a total HI column density N(HI)=2.3+/-0.1x10^18^cm^-2^, in disagreement with those measured previously toward G191-B2B. We derive an average D/H ratio over the three absorbing clouds N(DI)_total_/N(HI)_total_=1.3x10^-5^ as well as individual D/H ratios, D/H_A_=1.^+0.4^_-0.1_x10^-5^, D/H_B_=1.4^+0.1^_-0.3_ x10^-5^, D/H_C_=1.5^+1.0^_-1.0_x10^-5^. We find that our cloud B may be exactly identified with the cloud detected by Linsky et al. (1993) in their analysis of the D/H ratio toward α Aur. When the two evaluations of the D/H ratio in this cloud are matched at D/H=1.4x10^-5^, a value compatible with the quoted error bars, the D/H ratio in our cloud A can hardly exceed 1.x10^-5^, thus suggesting possible variations of the D/H ratio in the local interstellar medium.