A Time-Dependent Model of Cometary Molecular Sulphur Fluorescence Spectra

The quantity and quality of high-resolution spectra of comets Hyakutake and Hale-Bopp provide a unique opportunity to advance our knowledge of parent species in comets and thereby gain clues to understanding the origins of the solar system and of life. To interpret the extensive multispectral observations that show spatial variations in the coma, a detailed model that combines time-dependent fluorescence calculations with a global coma model based on gas dynamics and chemistry has been developed. Using this versatile model, a systematic study of the vibrational-rotational spectrum of S_2 is undertaken, including relevant effects, such as, metastable states, optical depth, collisions, non-equilibrium state superposition, the Swings and Greenstein effects, and the number density profile within the coma due to dynamics and chemistry. The molecule S_2, having a short lifetime, is concentrated in the inner-most region of the coma where dynamics and chemistry are most important. S_2 was first discovered serendipitously in comet IRAS-Araki-Alcock at its closest approach to Earth (Delta = 0.032 AU) with an abundance of about 0.1% relative to water. With its close approach to the Earth (Delta = 0.1 AU), the detection of S_2 in comet Hyakutake was made at a level of about 0.01% relative to water. Synthetic fluorescence spectra of S_2 are calculated as a function of time after release from the nucleus to compare with observations and to investigate the importance of the above effects on the scale length and abundance of S_2 in comets Hyakutake and IRAS-Araki-Alcock.