Astronomy and Astrophysics – Astronomy
Scientific paper
Sep 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004apj...613..615l&link_type=abstract
The Astrophysical Journal, Volume 613, Issue 1, pp. 615-621.
Astronomy and Astrophysics
Astronomy
7
Comets: General, Molecular Processes
Scientific paper
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 σe-HCN can be as high as 1.3×10-12 cm2, more than 2 orders of magnitude greater than the commonly assumed HCN-H2O cross section. For the ground rotational transitions of HCN, the electron-HCN collision rate is found to exceed the HCN-H2O 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.
Combi Michael R.
Gombosi Tamas I.
Hansen Kenneth Calvin
Kallivayalil Nitya
Lovell Amy Jean
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