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

Abstract

We present high resolution (Delta lambda = 3.7 km.s^{-1}) 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, D i, N i, O i, Si ii and Si iii. We detect three absorbing clouds, and derive a total Hi\ column density N(Hi)=2.4m0.1 \times10¹⁸cm^{-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(D i)_total/N(Hi)_total=1.12m 0.08 \times 10^{-5}, in disagreement with the previously reported value of the local D/H as reported by Linsky et al. (1995) toward Capella. We re-analyze the GHRS data of the Capella line of sight, and confirm their estimate, as we find (D/H)_Capella=1.56m 0.1 \times 10^{-5} in the Local Interstellar Cloud in which the solar system is embedded. This shows that the D/H ratio varies by at least im30% 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 0.9\times10^{-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=1.12\times10^{-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. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Hubble Space Telescope Science Institute which is operated by the Association of Universities for Research in Astronomy Inc., under NASA contract NAS5-26555.