Oxygen/Hydrogen Chemistry in the Inner Comae of Active Comets

Abstract

We have constructed a concentric-shell, one-dimensional kinetic model that examines the chemistry of hydrogen and oxygen species in detail. We have studied the effects of the reactions of the reactive OH, O(3 P), and O(1 D) species with themselves and with the abundant stable molecules in the inner coma of moderately and highly active comets. We find that the reactions (1) Oð 1 DÞ þ H 2 O ! 2OH and (2) Oð 3 PÞ þ OH ! O 2 þ H play important roles in the inner comae of active comets. Inclusion of reaction (2) predicts the formation of significant amounts of molecular oxygen. As the densities of O 2 may be as high as 1% those of water in some cases, the possibility of detection exists. We suggest the possibility that the ion O þ 2 may contribute to some previously unassigned features in the optical ion-tail spectra of comets. We also consider the role that reactions of the reactive species might play in the destruction of CO, NH 3 , and organic molecules in the inner coma of the active comet. We find that destruction of formaldehyde, for example, by reaction with OH has a small but essentially negligible effect on the predicted production rate of formaldehyde. Finally, we examine the significance of the reaction of OH with CO in the dense inner coma.

Large Aperture Oi6300 A Observations of Comet Hyakutake: Implications for the Photochemistry of OH and OiProduction in Comet Hale‐Bopp

“…Any of the changes in the OH photochemistry proposed in Table 2 Dalgarno (1984) converted to solar photochemical rates by Huebner et al (1992) describes the measured points well, assuming an average Q(H 2 O) ¼ 3 ; 10 29 s À1 for this FOV. The points are not inconsistent with the dashed line, which is produced using the OH photochemical rates of Glinski et al (2004) and a lower Q(H 2 O) value. The photochemical rates of Morgenthaler et al (2001) result in the dotted line, which matches the outer points well, but begins to fall short of the inner point.…”

“…We use a numeric integrator to convert from particle number density as a function of cometocentric distance to column density as a function of projected cometocentric distance on the sky. This gives us the flexibility to easily include effects, such as quenching, that modify the basic Haser density profile, and experiment with the chemical reactions considered by Glinski et al (2004). We found that over the density ranges represented by our Hyakutake [O i] data, neither quenching nor chemical production of O 2 were significant.…”

“…Like IS, AC did not vary significantly across the broad family of models that fit the radial profiles well. The dashed lines in Figures 3 and 5 were generated using BR3 G and the OH lifetime derived by Glinski et al (2004; see also our Table 2). Particularly over the radial extent covered on March 23 data, the slope of the BR3 G -derived model is clearly flatter than the data.…”

“…The size of the FOVand short exposure times (30 s for March 23 and 10 s for April 9) alleviated the need for active tracking or guiding, even with Hyakutake's fast apparent motion. Huebner et al (1992); M: Morgenthaler et al (2001) values; G: Glinski et al (2004) …”

“…A boost in BR3 to the value BR3 M , ( listed in Table 1) solved the problem, at least for HaleBopp. Subsequent work by Glinski et al (2004) found that the inner coma of Hale-Bopp was dense enough to support an unexpectedly high O 2 production rate by collisions between groundstate oxygen and OH. O 2 photodissociation is very efficient at producing [O i] …”