Deuterated molecular hydrogen in the Galactic ISM

New observations along seven translucent sightlines

¹ Institut d'Astrophysique de Paris, CNRS/UPMC, 98bis boulevard Arago, 75014 Paris, France e-mail: sylvestre.lacour@obspm.fr
² Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA
³ Observatoire de Paris-Meudon, LESIA, 5 place Jules Janssen, 92195 Meudon, France
⁴ Onsala Space Observatory, 43992 Onsala, Sweden
⁵ Observatoire de Paris-Meudon, LUTH, 5 place Jules Janssen, 92195 Meudon, France
⁶ Department of Astronomy and Astrophysics, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637, USA

Received: 2 July 2004
Accepted: 4 October 2004

Abstract

We present column density measurements of the HD molecule in the interstellar gas toward 17 Galactic stars. The values for the seven most heavily reddened sightlines, with , are derived from observations with the Far Ultraviolet Spectroscopic Explorer (FUSE). The other ten values are from a reanalysis of spectra obtained with Copernicus. In all cases, high-resolution ground-based observations of  and/or the CH molecule were used to constrain the gas velocity structure and to correct for saturation effects. Comparisons of the column densities HD, CH, CN, and  in these 17 sightlines indicate that HD is most tightly correlated with CH. Stringent lower limits to the interstellar D/H ratio, derived from the HD/2H₂ ratio, range from  to . Our results also suggest that the HD/H₂ ratio increases with the molecular fraction f(H₂) and that the interstellar D/H ratio might be obtained from HD by probing clouds with f(H. Finally, we note an apparent relationship between the molecular fractions of hydrogen and deuterium.