Retrieving the coma gas expansion velocity in P/Halley, Wilson (1987 VII) and several other comets from the 18-cm OH line shapes

Details Retrieving the coma gas expansion velocity in P/Halley, Wilson (1987 VII) and several other comets from the 18-cm OH line shapes Retrieving the coma gas expansion velocity in P/Halley, Wilson (1987 VII) and several other comets from the 18-cm OH line shapes

Nov 1990

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1990a%26a...238..382b&link_type=abstract

Astronomy and Astrophysics (ISSN 0004-6361), vol. 238, no. 1-2, Nov. 1990, p. 382-400.

Astronomy and Astrophysics

Astrophysics

68

Cometary Atmospheres, Comets, Hydroxyl Radicals, Line Spectra, Radio Spectra, Giotto Mission, Halley'S Comet, Photodissociation

Scientific paper

A method for retrieving the H2O expansion velocity in the case of photodissociation in comets using the 18-cm OH line profiles is proposed. The method consists in fitting trapezia to the observed spectra, and its validity is checked by Monte Carlo simulations. The method is applied to nine different comets observed using the Nancay radio telescope over a large range of heliocentric distances and a large range of OH production rates. The OH data obtained for comets at large heliocentric distances allow evaluation of the OH ejection velocity. For the OH ejection velocity upon photodissociation of H2O the value of 0.9 km/s is adopted. For large heliocentric distances and small OH production rates, the H2O velocity has small values throughout the coma (0.5-0.7 km/s). It is found that H2O velocity increases within the field of view, confirming the acceleration of gas expansion expected from the models and measured by the Giotto NMS experiment. The density model can reconcile all UV and radio determinations of OH production rates in Comet P/Halley.

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