Detection of spatial variations in the (D/H) ratio in the local interstellar medium *
Alfred Vidal-Madjar 1,
Martin Lemoine 2,
Roger Ferlet 1,
Guillaume Hébrard 1,
Detlev Koester 3,
Jean Audouze 1,
Michel Cassé 4, 1,
Elisabeth Vangioni-Flam 1 and
John Webb 5
Received 27 April 1998 / Accepted 2 July 1998
We present high resolution () 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 H I , D I , N I , O I , Si II and Si III . We detect three absorbing clouds, and derive a total H I column density N(H I )=2.4cm-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 =1.12, 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 in the Local Interstellar Cloud in which the solar system is embedded. This shows that the D/H ratio varies by at least 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 , and in such a way that the D/H ratio as averaged over all three components remains at the above value, i.e. .
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.
Key words: stars: individual: G191-B2B ISM: abundances ISM: atoms and ions cosmology: observations ultraviolet: ISM
* 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.
Send offprint requests to: A. Vidal-Madjar
© European Southern Observatory (ESO) 1998
Online publication: September 14, 1998