The Lyman and Werner band systems of deuterated molecular hydrogen (HD) occur in the far UV range below 120 nm. This spectral window is now open at moderate resolution and high sensitivity with the FUSE satellite. FUSE spectra of hot stars with high extinction through translucent clouds will give access to the deuterium abundance inside molecular clouds where D is essentially in the form of HD. Measurement of HD/H2 ratio becomes thus a new powerful method to evaluate the D/H ratio in the interstellar medium.

An example is given with the FUSE spectrum of the high extinction O9III star HD 73882 (EB–V = 0.7). Very preliminary analysis and an estimate of the HD/H2 ratio are presented.

Introduction

It has long been recognized that the primordial abundance of deuterium represents the most sensitive probe of the baryonic density Ωb of the Universe (see, e.g., Schramm & Turner 1998; Olive et al. 1999). On the other hand, abundance of deuterium at any epoch is a lower limit to its primordial abundance, since deuterium is destroyed, not created, in stars of any mass. For this reason, deuterium abundance is also an efficient tracer of the universal star formation rate. Unfortunately, the evolution of deuterium abundance from the primordial to the solar metallicity is still unclear.

Measurements of the atomic D/H ratio have been performed in different astrophysical sites, namely in moderate to high redshift quasar absorbers, in the presolar nebula and in the local interstellar medium (for reviews see, e.g., Ferlet & Lemoine 1996; Linsky 1998; Vidal-Madjar et al. 1998a; Lemoine et al. 1999).