IUE observations of H Lyman-α in comet P/Giacobini-Zinner

Abstract

Comet P/Giacobini-Zinner (1985 XIII) was observed during the flyby of the International Cometary Explorer (ICE) satellite and during the 3 preceding months by the International Ultraviolet Explorer (IUE) satellite. Models for the hydrogen abundance and spatial distribution, which were previously used to reproduce the observed spatial distribution of the wide-field Lyman-α comae of Comets Kohoutek and Halley measured by rocket and spacecraft borne instruments, have now been successfully applied to these IUE observations. Vectorial models, which implicitly assume production of cometary OH by dissociation of water molecules, have been routinely used to infer global water production rates from various nucleus-centered IUE observations of comets. However, because of modeling complexities, a large base of Lyman-α observations has not generally been analyzed. One of these modeling complexities is that often the H atom column densities are high enough so that the coma is optically thick to solar Lyman-α radiation which cannot be treated by the standard optically thin models (Haser, vectorial, or Monte Carlo). A spherical radiative transfer model, adapted for application to the H coma, has been used to analyze the IUE observations. This analysis yields water production rates in very good agreement with those calculated from vectorial model analysis of OH observations. The H model makes essentially the same physical assumptions as the vectorial model for OH except for optical density. A further interesting aspect of this dataset is that various offset observations of the Lyman-α brightness were made on August 12, and September 10 and 11, 1985, at sampling distances of 7,500 to 41,000 km. This is much higher spatial resolution that is typically available from the usual rocketborne cameras which have made most of the wide-field measurements of the H distribution in comets. The same H model, in combination with the radiative transfer calculation, also reproduces the observed spatial distribution of H in the inner coma.