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Astron. Astrophys. 338, 694-712 (1998)

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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

1 Institut d'Astrophysique de Paris, CNRS, 98 bis boulevard Arago, F-75014 Paris, France
2 DARC, UPR-176 CNRS, Observatoire de Paris-Meudon, F-92195 Meudon Cédex, France
3 Universität Kiel, Germany
4 CEA/DSM/DAPNIA, Service d'Astrophysique, Saclay, F-91191 Gif-sur-Yvette, France
5 School of Physics, University of New South Wales, Sydney, NSW 2052, Australia

Received 27 April 1998 / Accepted 2 July 1998

Abstract

We present high resolution ([FORMULA][FORMULA]) 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 )=2.4[FORMULA]cm-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 [FORMULA]=1.12[FORMULA], 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 [FORMULA] in the Local Interstellar Cloud in which the solar system is embedded. This shows that the D/H ratio varies by at least [FORMULA] 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 [FORMULA], and in such a way that the D/H ratio as averaged over all three components remains at the above value, i.e. [FORMULA].

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

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© European Southern Observatory (ESO) 1998

Online publication: September 14, 1998
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