Detection of spatial variations in the (D/H) ratio in the local interstellar medium

Abstract

We present high spectral resolution HST-GHRS observations of the 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 we measure an average (D/H) ratio: N(DI)_Total / N(HI)_Total = 1.12 +/- 0.08 x 10**-5, in disagreement with the value of the D/H ratio as measured toward Capella by Linsky et al. (1993, 1995). We re-analyze the GHRS data of the Capella line of sight, and confirm their estimate, as we find: (D/H)_Capella = 1.56 +/- 0.1 x 10**-5, in the Local Interstellar Cloud in which the Sun is embedded. This shows that the D/H ratio varies by at least 30% within the local interstellar medium (not to be confused with the Local Interstellar Cloud). Furthermore, the Local Interstellar Cloud is also detected toward our target 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 0.9 x 10**-5, and in such a way that the D/H ratio, as averaged over all three components, remains at the above value, i.e. 1.12 x 10**-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 we note that our observations neither prove nor disprove this latter possibility.