Revised chemical models of ζ Oph based on HST data

Abstract

Hubble Space Telescope observations by Savage, Cardelli and Sofia (1992) and Cardelliet al. (1993) have led to improved gas phase abundances for many elements in the diffuse cloud towards the star ζ Oph. Most remarkably, it was found that oxygen is much more strongly depleted than previousCopernicus observations indicated. As a consequence, chemical models of the ζ Oph cloud are severely affected by the drop in the observed oxygen abundance by almost a factor of two; some previous model calculations for the ζ Oph cloud failed to reproduce-even approximately-the observed high CO column density. The model calculations for the ζ Oph cloud developed by Wagenblast (1992) in which the abundance of all observed neutral molecules could be reproduced have been revised and it is found that oxygen and nitrogen hydrides are required to be formed efficiently on the surface of grains; further, there are indications for a high cosmic ray ionization rate of the order 10^‑16 s^‑1.