Atomic Sulfur in Cometary Comae Based on UV Spectra of the S I Triplet near 1814 A

Details Atomic Sulfur in Cometary Comae Based on UV Spectra of the S I Triplet near 1814 A Atomic Sulfur in Cometary Comae Based on UV Spectra of the S I Triplet near 1814 A

Jan 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997icar..125..164m&link_type=abstract

Icarus, Volume 125, Issue 1, pp. 164-194.

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Scientific paper

We present a detailed study of the S I triplet emissions near 1814 A in a variety of comets. Even in a moderately dense cometary coma the ground-state component (1807 A) of the triplet turns out to be more or less optically deep. Therefore, any straightforward interpretation of the measured S I flux in terms of a g factor and a column density is incorrect. We combine a Monte Carlo radiative transfer model with a detailed, nonequilibrium fluorescence model of sulfur. Radiative processes and neutral-neutral collisions, as well as initial dissociative excitation, are investigated. Our model considers multiple scattering of photons in a radially expanding coma. The phase space consists of a radial bulk velocity superimposed by an isotropic, three-dimensional, and Maxwellian-distributed random velocity. Lacking highly resolved, reliable spatial maps of sulfur we concentrate on spherically symmetric and hemispherical comae We analyze the S I triplet of 19 different comets which have been observed with the International Ultraviolet Explorer (IUE) and the Faint Object Spectrograph (FOS) aboard the Hubble Space Telescope. Radical CS, which is also detectable in the UV, we attribute to its likely parent molecule CS_2. Assuming sulfur observed in excess to sulfur from CS_2 originates from either H_2S or nuclear S, we obtain total sulfur abundances relative to water between ~0.001 and ~0.01. Cometary sulfur stored in volatile species appears to be depleted compared with solar abundances. Spectra of Comet 1P/1982 U1 (Halley) reveal that Q(CS_2)/Q(H_2O) increases from 0.0005 at R ~ 1.9 AU to 0.0030 at R ~ 0.8 AU. The ratio Q(H_2S)/Q(H_2O) strongly varies with time and ranges from ~0.001 to ~0.01, where the (apparent) H_2S production rate, Q(H_2S), also includes sulfur parents other than H_2S. We find that much of the apparent variations in Q(H_2S)/Q(H_2O) might be explained by the different lifetimes of the species coupled with the variation of outgassing due to nuclear rotation. For Comets 109P/1992 S2 (Swift-Tuttle), C/1990 K1 (Levy), C/1989 X1 (Austin), and 1P/1982 U1 (Halley), a direct comparison of our apparent H_2S abundances with microwave and in situ observations is possible. This comparison suggests yet another or more sulfur parents with a relatively short lifetime on the order of 10^4 sec or less. For two given apertures (IUE and FOS) look-up tables are provided that enable an approximate determination of sulfur production rates as a function of the observed flux, the parent molecule, and the Sun-comet-Earth geometry.

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