Reanalysis of the LLS in QSO HS 1700+6416

Abstract

We take advantage of the unique information provided by the recent HST observations of three Lyman limit systems toward QSO HS 1700+6416 by Reimers et al. (1992) to constrain photoionization models of the absorbing clouds. Constant density models are ruled out. However, the observed column densities (and in particular the large N(HeI)/N(HI) column density ratio) can be reproduced with a model in which the medium is considered inhomogeneous with two phases of different densities. The oxygen and nitrogen abundances are found to be about, 0.1 solar, the carbon abundance may be slightly smaller. The presence of a break of a factor ten in the ionizing spectrum at the HeII edge is not ruled out by these observations. It is clear that the high ionization phase could be thermally ionized.