The Fourth Positive System of Carbon Monoxide in the Hubble Space Telescope Spectra of Comets

TL;DR

This paper models the Fourth Positive System of CO in HST spectra of comets to determine CO abundances, revealing a wide range of CO/H2O ratios and identifying a hydrogen line pumped by solar radiation.

Contribution

It develops a detailed fluorescence model for CO in comet spectra, incorporating optical depth effects and providing the first identification of a solar-pumped hydrogen line in comet spectra.

Findings

CO abundances relative to water range from 0.44% to 20.9%.

The model confirms a predominantly native source of CO in observed comets.

First identification of a solar HI Lyman-beta pumped hydrogen line in a comet spectrum.

Abstract

The rich structure of the Fourth Positive System (A-X) of carbon monoxide accounts for many of the spectral features seen in long slit HST-STIS observations of comets 153P/Ikeya-Zhang, C/2001 Q4 (NEAT), and C/2000 WM1 (LINEAR), as well as in the HST-GHRS spectrum of comet C/1996 B2 Hyakutake. A detailed CO fluorescence model is developed to derive the CO abundances in these comets by simultaneously fitting all of the observed A-X bands. The model includes the latest values for the oscillator strengths and state parameters, and accounts for optical depth effects due to line overlap and self-absorption. The model fits yield radial profiles of CO column density that are consistent with a predominantly native source for all the comets observed by STIS. The derived CO abundances relative to water in these comets span a wide range, from 0.44% for C/2000 WM1 (LINEAR), 7.2% for 153P/Ikeya-Zhang, 8.8% for C/2001 Q4 (NEAT) to 20.9% for C/1996 B2 (Hyakutake). The subtraction of the CO spectral features using this model leads to the first identification of a molecular hydrogen line pumped by solar HI Lyman-beta longward of 1200A in the spectrum of comet 153P/Ikeya-Zhang. (Abridged)