5. Concluding remarks
McCullough (1992) finds all Copernicus and IUE ISM deuterium abundance measurements to be consistent with . This value was confirmed for the local interstellar cloud by GHRS observations towards Aur (Linsky et al. 1995) and HR 1099 (Piskunov et al. 1997) yielding and respectively. For a summary of GHRS measurements see Linsky (1998). McCullough's list of included data contains 6 measurements towards nearby stars ( pc) observed only in Ly and 8 Copernicus observations of each 2 to 5 Lyman lines towards more distant stars ( pc). The mean of the results of the latter observations is . Our result based on 5 deuterium lines lies below this value though it is consistent within its uncertainty.
Recent determinations of for example by Vidal-Madjar et al. (1998) towards G191-B2B using the GHRS (), Gölz et al. (1998) in a preliminary analysis of ORFEUS II observations of BD +28 4211 () or Jenkins et al. (1999) using IMAPS spectra of Ori indicate smaller values towards early type stars with pc. As Vidal-Madjar et al. and Jenkins et al. state, a possible explanation may be blending of the Local Interstellar Cloud (LIC) contribution with that of more distant deuterium-poor clouds. Small scale spatial inhomogeneities of could be the result of the production of significant amounts of deuterium in stellar flares, as Mullan & Linsky (1999) suggest. This discussion remains open since Sahu et al. (1999) found towards G191-B2B from a STIS spectrum, a value consistent with a homogeneous in the LISM.
However this may be, the various determinations of all are consistent with . The uncertainty in each individual value is influenced by the spectral resolution of the instrument, by the number of D lines used, and by the actual D column density. Our result from the new instrument ORFEUS using 5 lines at 30 km s-1 resolution adds to these determinations.
© European Southern Observatory (ESO) 1999
Online publication: November 23, 1999