Model for Cameron-band emission in comets: a case for the EPOXI mission target comet 103P/Hartley 2

Abstract

The CO₂ production rate has been derived in comets using Cameron-band (a³Π→X¹Σ) emission of CO molecules, assuming that photodissociative excitation of CO₂ is the main production mechanism of CO in the a³Π metastable state. We have developed a model for the production and loss of CO(a³Π), which has been applied to comet 103P/Hartley 2: the target of the EPOXI mission. Our model calculations show that photoelectron impact excitation of CO and dissociative excitation of CO₂ can together contribute about 60-90 per cent to Cameron-band emission. The modelled brightness of (0-0) Cameron-band emission on comet Hartley 2 is consistent with Hubble Space Telescope observations for 3-5 per cent CO₂ (depending on the model input solar flux) and 0.5 per cent CO relative to water, where the photoelectron impact contribution is about 50-75 per cent. We suggest that estimation of CO₂ abundances on comets using Cameron-band emission may be reconsidered. We predict a height-integrated column brightness of the Cameron band of ∼1300 Rayleigh during the EPOXI mission encounter period.