On the Effect of Electron Collisions in the Excitation of Cometary HCN

Abstract

The electron-HCN collision rate for the excitation of rotational transitions of the HCN molecule is evaluated in comets C/1995 O1 (Hale-Bopp) and C/1996 B2 (Hyakutake). Based on theoretical models of the cometary atmosphere, we show that collisions with electrons can provide a significant excitation mechanism for rotational transitions in the HCN molecule. Computed values of the cross section e-HCN can be as high as 1:3 ; 10 À12 cm 2 , more than 2 orders of magnitude greater than the commonly assumed HCN-H 2 O cross section. For the ground rotational transitions of HCN, the electron-HCN collision rate is found to exceed the HCN-H 2 O collision rate at distances greater than 3000 km from the cometary nucleus of Hale-Bopp and 1000 km from that of Hyakutake. Collisional excitation processes dominate over radiative excitation processes up to a distance of 160,000 km from the cometary nucleus of Hale-Bopp and 50,000 km from that of Hyakutake. Excitation models that neglect electron collisions can underestimate the HCN gas production rates by as much as a factor of 2.

The Distribution, Excitation, and Formation of Cometary Molecules: Methanol, Methyl Cyanide, and Ethylene Glycol

“…Our observations encompass a region with a H 2 O density of $10 (5.7-7.5) cm À3 (Lovell et al 2004). Therefore, there is sufficient collisional excitation to populate the CH 3 OH levels, and our observations sample molecular abundances and not excitation effects.…”

“…This difference implies either (1) an enhancement of the CH 3 CN column density at smaller coma radii or (2) that the excitation of the transitions we observed of CH 3 CN takes place at a slightly smaller radius, where the density is higher, and the measurements taken with the 12 m telescope are suffering from some beam dilution. Assuming the H 2 O density profile of Lovell et al (2004), the maximum coma radius that lies below the CH 3 CN critical density range of (1:2 5:3) ; 10 6 cm À3 is $10,000 km. This explains why the 30 m CH 3 CN line strengths are higher than the 12 m. Finally, from the nondetection of (CH 2 OH ) 2 in the 12 m observations, it is also apparent that the distribution of (CH 2 OH ) 2 must also be compact in the cometary coma.…”

BIMA Array Detections of HCN in Comets LINEAR (C/2002 T7) and NEAT (C/2001 Q4)

“…Fluorescence effects could allow local thermodynamic equilibrium ( LTE) to be maintained at a lower density than that required in optically thin cases (e.g., Crovisier 1984;Bockelée-Morvan et al 1987). Lovell et al (2004) determined that for the J ¼ 1-0 transition of HCN for comet Hyakutake (C/1996 B2), the transition between collisionally dominated and fluorescence-dominated emission is between 26,000 and 38,000 km from the nucleus. Since both comet LINEAR and comet NEAT have similar water production rates relative to comet Hyakutake (D. G. Schleicher 2005, private communication; see x 4.2) at comparable heliocentric distances, we assume that their transition radii are also comparable.…”

“…In order to do this, we modeled the HCN emission as concentric shells (of thickness 100 km out to a radius of 10,000 km and a thickness of 2000 km for radii of 10,000-30,000 km) around the nucleus and calculated the total column density and production rate. Since we consider both comets to be similar to comet Hyakutake, we used the velocity profiles from Figure 3 of Combi et al (1999a) or the fit to those data from Lovell et al (2004) to give velocities to each shell. The April 9 profile was used for comet LINEAR, and the March 30 profile and fit were used for comet NEAT, since on these dates comet Hyakutake was at similar heliocentric distances.…”

Electron impact excitation of carbon monoxide in comet Hale‐Bopp

“…As the lifetimes of the O I and CO excited states are very short, it is not expected that collisional quenching is significant [ McPhate et al , 1999] but quenching was included to be sure. To calculate quenching rates, a density of 10 4 cm −3 for H 2 O was taken from a previous model [ Lovell et al , 2004], then densities of CO and O atoms were set at 10 3 cm −3 and 10 2 cm −3 , based on ratios (to H 2 O) calculated by Bhardwaj et al [1990]. Quenching rates for excited CO and O I are not known to us, so as the point of the calculation was to show that quenching is insignificant, an improbably large value of 10 −9 cm 3 s −1 was used.…”