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Astron. Astrophys. 352, 287-296 (1999)

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5. Concluding remarks

McCullough (1992) finds all Copernicus and IUE ISM deuterium abundance measurements to be consistent with [FORMULA]. This value was confirmed for the local interstellar cloud by GHRS observations towards [FORMULA] Aur (Linsky et al. 1995) and HR 1099 (Piskunov et al. 1997) yielding [FORMULA] and [FORMULA] respectively. For a summary of GHRS measurements see Linsky (1998). McCullough's list of included data contains 6 measurements towards nearby stars ([FORMULA] pc) observed only in Ly [FORMULA] and 8 Copernicus observations of each 2 to 5 Lyman lines towards more distant stars ([FORMULA] pc). The mean of the results of the latter observations is [FORMULA]. Our result based on 5 deuterium lines lies below this value though it is consistent within its uncertainty.

Recent determinations of [FORMULA] for example by Vidal-Madjar et al. (1998) towards G191-B2B using the GHRS ([FORMULA]), Gölz et al. (1998) in a preliminary analysis of ORFEUS II observations of BD +28 4211 ([FORMULA]) or Jenkins et al. (1999) using IMAPS spectra of [FORMULA] Ori [FORMULA] indicate smaller values towards early type stars with [FORMULA] 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 [FORMULA] 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 [FORMULA] towards G191-B2B from a STIS spectrum, a value consistent with a homogeneous [FORMULA] in the LISM.

However this may be, the various determinations of [FORMULA] all are consistent with [FORMULA]. 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.

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© European Southern Observatory (ESO) 1999

Online publication: November 23, 1999