Model for atomic oxygen visible line emissions in Comet C/1995 O1 Hale-Bopp

Abstract

We have recently developed a coupled chemistry-emission model for the green (5577 Å) and red-doublet (6300, 6364 Å) emissions of atomic oxygen on Comet C/1996 B2 Hyakutake. In the present work we applied our model to Comet C/1995 O1 Hale-Bopp, which had an order of magnitude higher H₂O production rate than Comet Hyakutake, to evaluate the photochemistry associated with the production and loss of O(¹S) and O(¹D) atoms and emission processes of green and red-doublet lines. We present the wavelength-dependent photo-attenuation rates for different photodissociation processes forming O(¹S) and O(¹D). The calculated radiative efficiency profiles of O(¹S) and O(¹D) atoms show that in Comet Hale-Bopp the green and red-doublet emissions are emitted mostly above radial distances of 103 and 104 km, respectively. The model calculated [OI] 6300 Å emission surface brightness and average intensity over the Fabry-Pérot spectrometer field of view are consistent with the observation of Morgenthaler et al. (Morgenthaler, J.P. et al. [2001]. Astrophys. J. 563, 451–461), while the intensity ratio of green to red-doublet emission is in agreement with the observation of Zhang et al. (Zhang, H.W., Zhao, G., Hu, J.Y. [2001]. Astron. Astophys. 367 (3), 1049–1055). In Comet Hale-Bopp, for cometocentric distances less than 105 km, the intensity of [OI] 6300 Å line is mainly governed by photodissociation of H₂O. Beyond 105 km, O(¹D) production is dominated by photodissociation of the water photochemical daughter product OH. Whereas the [OI] 5577 Å emission line is controlled by photodissociation of both H₂O and CO₂. The calculated mean excess energy in various photodissociation processes show that the photodissociation of CO₂ can produce O(¹S) atoms with higher excess velocity compared to the photodissociation of H₂O. Thus, our model calculations suggest that involvement of multiple sources in the formation of O(¹S) could be a reason for the larger width of green line than that of red-doublet emission lines observed in several comets.