Submillimeter Wave Astronomy Satellite Monitoring of the Postperihelion Water Production Rate of Comet C/1999 T1 (MCNaught-Hartley)
Abstract
We present observations of the pure rotational transition 110-->101 of ortho-H2O vapor at 556.936 GHz made with the Submillimeter Wave Astronomy Satellite (SWAS) for comet C/1999 T1 (McNaught-Hartley). The emission was monitored during the postperihelion phase from (UT) 2001 February 2.00 to 11.07 and February 23.01 to April 5.95 for a Sun-comet distance, rh, ranging from 1.41 to 2.05 AU. The water production rate, QH2O, derived from the observations depends on the assumptions regarding the electron abundance in the coma. We obtain QH2O~4×1028 s-1 for the observations made in the first week of February (rh~1.4 AU) using a model in which the electron abundance has been normalized to the in situ measurements obtained in 1P/Halley. The value of QH2O decreases for larger rh, and we derive ~1.5×1028 s-1 for the observations made at rh~2 AU in the last week of 2001 March. The inferred water production rate is larger by ~40% for a radiative transfer model with the electron abundance reduced by 80% because of the reduced excitation of water by electron collisions in this model. A comparison to literature data suggests that the SWAS results derived with the smaller electron abundance are in better agreement with independent measurements of QH2O obtained from observations of H2O photodissociation products. A χ2 fit to the variation of QH2O with the heliocentric distance gives a power-law index of -3.0+/-0.5 (statistical) +/-0.1 (systematic).
- Publication:
-
The Astrophysical Journal
- Pub Date:
- July 2004
- DOI:
- 10.1086/421287
- Bibcode:
- 2004ApJ...609.1164B
- Keywords:
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- Comets: General;
- Comets: Individual: Alphanumeric: C/1999 T1;
- Radio Lines: Solar System;
- Submillimeter