2D photochemical model for forbidden oxygen line emission for comet 1P/Halley

MNRAS 462, S116–S123 (2016) doi:10.1093/mnras/stw2150 Advance Access publication 2016 August 30 2D photochemical model for forbidden oxygen line emission for comet 1P/Halley G. Cessateur,1‹ J. De Keyser,1 R. Maggiolo,1 M. Rubin,2 G. Gronoff,3,4 A. Gibbons,1,5 E. Jehin,6 F. Dhooghe,1 H. Gunell,1 N. Vaeck5 and J. Loreau5 1 Space Physics Division, Royal Belgian Institute for Space Aeronomy, Ringlaan 3, B-1180 Brussels, Belgium 2 Physikalisches Institut, University of Bern, Sidlerstr. 5, CH-3012 Bern, Switzerland 3 Science Directorate, Chemistry and Dynamics Branch, NASA Langley Research Center, Hampton, VA 23681-2199, USA 4 SSAI, Hampton, VA 23681-2199, USA 5 Service de Chimie Quantique et Photophysique, Université Libre de Bruxelles, Av. F. D. Roosevelt 50, B-1050 Brussels, Belgium 6 Institut d’Astrophysique, de Géophysique et Océanographie, Université de Liège, Allée du 6 août 17, B-4000 Liège, Belgium Accepted 2016 August 29. Received 2016 August 22; in original form 2016 June 15 We present here a 2D model of photochemistry for computing the production and loss mechanisms of the O(1 S) and O(1 D) states, which are responsible for the emission lines at 577.7, 630, and 636.4 nm, in case of the comet 1P/Halley. The presence of O2 within cometary atmospheres, measured by the in situ Rosetta and Giotto missions, necessitates a revision of the usual photochemical models. Indeed, the photodissociation of molecular oxygen also leads to a significant production of oxygen in excited electronic states. In order to correctly model the solar ultraviolet (UV) flux absorption, we consider here a 2D configuration. While the green to red-doublet ratio is not affected by the solar UV flux absorption, estimates of the red-doublet and green lines emissions are, however, overestimated by a factor of 2 in the 1D model compared to the 2D model. Considering a spherical symmetry, emission maps can be deduced from the 2D model in order to be directly compared to ground and/or in situ observations. Key words: molecular processes – methods: numerical – comets: general. 1 I N T RO D U C T I O N Comets are usually considered as the best preserved objects in the Solar system since its formation 4.6 billion years ago. Their study could bring us valuable information regarding the composition of the primitive solar nebula. The recent discoveries of the Rosetta mission, currently orbiting around the comet 67P/Churyumov– Gerasimenko (hereafter 67P; Glassmeier et al. 2007), have shed a new light on our current knowledge regarding cometary composition. Specifically, the presence of molecular oxygen in the inner coma, in significant abundances relative to water (3.80 ± 0.85 per cent for 67P) was reported by Bieler et al. (2015) measured by the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA; Balsiger et al. 2007). The presence of O2 has also been confirmed with a reinterpretation of the Giotto data obtained during the flyby of the comet 1P/Halley (Rubin et al. 2015), with a 3.7 ± 1.7 per cent abundance relative to water. These results strongly suggest that molecular oxygen is in fact a common species in cometary atmospheres. Current modelling of the oxygen line emissions therefore has to be revised, in order to take the presence of molecular  E-mail: oxygen into account. Cessateur et al. (2016) explored the impact of the presence of molecular oxygen on the red-doublet (at 630 and 636.4 nm) and green (at 577.5 nm) line emissions for 67P. In this paper, we perform a similar study for the comet 1P/Halley and we extend the model by considering a 2D approach. The excited oxygen states come mainly from the photodissociation of H2 O, CO2 , O2 , and CO as suggested both by remote observations of atomic oxygen lines (see e.g. Decock et al. 2015; McKay et al. 2015), and by modelling (see e.g. Bhardwaj & Raghuram 2012, and references therein). The oxygen states of interest are O(1 D) (leading to emissions at 630 and 636.4 nm), and O(1 S) with a deactivation towards the oxygen state O(1 D) through radiative emission at 557.7 nm. We will focus on the impact of the presence of O2 on the green to red-doublet emission intensity ratio (G/R) as a function of the cometocentric distance, traditionally used to determine the abundances of the major oxygen-bearing volatile components in cometary atmospheres (Decock et al. 2015), in the case of 1P/Halley. After briefly introducing the 1D photochemical model used for 67P to assess the red-doublet and green line emissions, we will focus on a 2D approach in order to better take the solar ultraviolet (UV) flux absorption into account. Using a spherical symmetry, emission maps according to different observation angles can be deduced from the 2D model. We furthermore  C 2016 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society ABSTRACT G/R ratio for 1P/Halley S117 discuss the impact of the water production rate on the 1D and 2D approaches. We will discuss the outcomes of the 2D model while using two different outgassing speed profiles. Finally, we briefly discuss the cross-section uncertainties relative to CO2 on the G/R ratio. 2 P H OT O C H E M I S T RY– E M I S S I O N C O U P L E D MODEL 1 d 2 (r Ni v(r)) = Pi − Li , r 2 dr (1) where Pi is the production term, Li the loss term, and v(r) the velocity of the excited oxygen atom. The dominant source of O(1 D) and O(1 S) states is the photodissociation by the solar UV flux of the oxygen-bearing volatile components, as discussed by Decock et al. (2015). We consider here the usual species such as H2 O, CO2 , and CO. In the case of 67P (see Cessateur et al. 2016), this list of species had to be completed with O2 , which has been detected in significant abundance (3.80 ± 0.85 per cent relative to water) within the cometary atmosphere of 67P (Bieler et al. 2015). A new interpretation of the Giotto data has been performed to investigate the presence of O2 during the 1P/Halley flyby in 1986. Rubin et al. (2015) demonstrate the presence of molecular oxygen with a significant abundance of about 3.7 ± 1.7 per cent relative to water using the data from the Neutral Mass Spectrometer (Krankowsky et al. 1986), on board the Giotto spacecraft (Reinhard 1986). This makes molecular oxygen the third most abundant species behind water and CO (13.1 per cent relative to water), and before CO2 (2.5 per cent relative to water) for 1P/Halley. However, the DSMC model from Rubin et al. (2011) does not provide the velocity for O2 , but it does so for methanol (CH3 OH) that has a similar molar mass as O2 . The radial profiles for these four species are displayed in Fig. 1. The reaction rates for the photodissociation due to the solar UV flux are computed for each altitude within the cometary atmosphere. Because of the solar UV flux absorption, reaction rates are indeed not constant. Regarding the loss reactions, we consider collisions and radiative decay, which produce the gree (...truncated)