A Coupled Chemistry-Emission Model for Atomic Oxygen Green and Red-Doublet Emissions in the Comet C/1996 B2 Hyakutake

Abstract

The green (5577Å) and red-doublet (6300, 6364Å) lines are prompt emissions of metastable oxygen atoms in the 1 S and 1 D states, respectively, that have been observed in several comets. The value of intensity ratio of green to red-doublet (G/R ratio) of 0.1 has been used as a benchmark to identify the parent molecule of oxygen lines as H 2 O. A coupled chemistry-emission model is developed to study the production and loss mechanisms of O( 1 S) and O( 1 D) atoms and the generation of red and green lines in the coma of C/1996 B2 Hyakutake. The G/R ratio depends not only on photochemistry, but also on the projected area observed for cometary coma, which is a function of the dimension of the slit used and geocentric distance of the comet. Calculations show that the contribution of photodissociation of H 2 O to the green (red) line emission is 30 to 70% (60 to 90%), while CO 2 and CO are the next potential sources contributing 25 to 50% (<5%). The ratio of the photo-production rate of O( 1 S) to O( 1 D) would be around 0.03 (± 0.01) if H 2 O is the main source of oxygen lines, whereas it is ∼0.6 if the parent is CO 2 . Our calculations suggest that the yield of O( 1 S) production in the photodissociation of H 2 O cannot be larger than 1%. The model calculated radial brightness profiles of the red and green lines and G/R ratios are in good agreement with the observations made on comet Hyakutake in March 1996.

THE EVOLVING ACTIVITY OF THE DYNAMICALLY YOUNG COMET C/2009 P1 (Garradd)

“…For Garradd, the only direct measurement of the CO 2 production rate was acquired by the Deep Impact spacecraft at 2 AU post-perihelion (Feaga et al 2014). Line ratios of [O i] may be used to determine the column density ratios of H 2 O, CO 2 , and CO (Delsemme 1980;Festou & Feldman 1981;Bhardwaj & Raghuram 2012). However, the release rates of [O i] in the relevant states as well as physical processes in the coma that may affect these lines are not yet completely understood, resulting in large uncertainties in the derived abundances (McKay et al 2012;DeCock et al 2013 and references therein).…”

“…Assuming theoretical reaction rates by Bhardwaj & Raghuram (2012) we used the Very Large Telescope (VLT)/UVES measurements by DeCock et al (2013) to derive relative coma abundances of CO 2 /H 2 O of 34% at −3.25 AU, 18% at −2.9 AU, 10% at −2.5 AU, and finally, around 5% at −2.08 AU. Based on [O i] observations at Apache Point Observatory, McKay et al (2012) suggest constant CO 2 abundances of order 10%-15% within 2 AU pre-and post-perihelion.…”

A photochemical model of ultraviolet atomic line emissions in the inner coma of comet 67P/Churyumov-Gerasimenko

“…Our model calculations show that photodissociation of O 2 and H 2 O can populate oxygen atoms directly in 0 D state (see Figure 4). Based on our previous O( 1 D) model calculations done for 67P/C-G and several other comets (Raghuram et al, 2016;Bhardwaj and Raghuram, 2012;Raghuram and Bhardwaj, 2013), we argue that the resonance fluorescence is not a significant emission process for O I 1152 Å line in comets. Moreover, the radiative lifetime of O( 1 D) is about 110 s, unlike C( 1 D) state (which is 4077 s, Tozzi et al, 1998), during which it has to fluoresce solar photons at wavelength 1152 Å.…”

“…This yield spectrum can be used to calculate the steady state photoelectron flux. More details of the AYS approach and the method of photoelectron computation are given in several previous papers (Singhal and Haider, 1984;Bhardwaj et al, 1990Bhardwaj et al, , 1996Singhal and Bhardwaj, 1991;Bhardwaj and Michael, 1999;Bhardwaj, 2003Bhardwaj, , 1999Haider and Bhardwaj, 2005;Bhardwaj and Jain, 2009;Bhardwaj and Raghuram, 2012). The photoelectrons are degraded based on the local energy deposition approximation, hence this method of calculation of suprathermal electron flux is valid within the inner coma where the strong collisional coupling between neutrals and electrons exist.…”

Europa’s Optical Aurora

“…There are no experimental cross-sections in the literature for production of O( 1 D) from the electron-impact dissociation of H 2 O. Bhardwaj and Raghuram (2012) use analytical calculations by Jackman et al (1977) to estimate a branching ratio for oxygen produced by this process, and we adopt their approach here to compute the O( 1 D) rates.…”