Ultraviolet absorption studies of H 2O and other species in comet comae with satellite telescope-spectrometers

Abstract

Ultraviolet absorption by H 2O and other species in the comae of comets could be detected by studying, with satellite telescope-spectrometers, the occultation of hot stars by comets. Such observations could produce the first direct detection of H 2O, the fundamental parent molecule in comet comae, and give measures of molecular level populations. The first instrument suitable for such observations will be the High Resolution Spectrograph on Space Telescope and, therefore, we consider its capabilities. We have used a Haser model to estimate the molecular column densities and to predict equivalent widths for lines of H 2O, OH, CO, and O as functions of time and angular distance from a comet with a high H 2O production rate. We have determined the minimum detectable equivalent widths, and therefore, the maximum angular separation from such a comet at which H 2O, OH, and CO could be studied. A conservative, statistical estimate shows that comets with high water production rates should pass near enough to about 10 to 100 stars suitable for absorption studies of the C X band of H 2O (1240 Å). Estimated equivalent widths for CO, OH, and the resonance lines of C and O indicate that these species may also be detected.