1998A&A...338..694V

We present high resolution (Δλ≃3.7km/s) HST-GHRS observations of the DA white dwarf G191-B2B, and derive the interstellar D/H ratio on the line of sight. We have observed and analysed simultaneously the interstellar lines of HI, DI, NI, OI, SiII and SiIII. We detect three absorbing clouds, and derive a total HI column density N(HI)=2.4±0.1x10¹⁸cm^–2, confirming our Cycle 1 estimate, but in disagreement with other previous measurements. We derive an average D/H ratio over the three absorbing clouds N(Di)_total/N(HI)_total=1.12±0.08x10^–5, in disagreement with the previously reported value of the local D/H as reported by Linsky et al. (1995ApJ...451..335L) toward Capella. We re-analyze the GHRS data of the Capella line of sight, and confirm their estimate, as we find (D/H)_Capella=1.56±0.1x10^–5 in the Local Interstellar Cloud in which the solar system is embedded. This shows that the D/H ratio varies by at least ∼30% within the local interstellar medium. Furthermore, the Local Interstellar Cloud is also detected toward G191-B2B, and we show that the D/H ratio in this component, toward G191-B2B, can be made compatible with that derived toward Capella. However, this comes at the expense of a much smaller value for the D/H ratio as averaged over the other two components, of order 0.9x10^–5, and in such a way that the D/H ratio as averaged over all three components remains at the above value, i.e. (D/H)_Total=1.12x10^–5. We thus conclude that, either the D/H ratio varies from cloud to cloud, and/or the D/H ratio varies within the Local Interstellar Cloud, in which the Sun is embedded, although our observations neither prove nor disprove this latter possibility.