Submillimeter Wave Astronomy SatelliteMonitoring of the Postperihelion Water Production Rate of Comet C/1999 T1 (M c Naught‐Hartley)

We present observations of the pure rotational transition 1 10 → 1 01 of ortho-H 2 O 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, r h , ranging from 1.41 to 2.05 AU. The water production rate, Q , derived from the observations depends on the assumptions regarding the electron abundance in the coma. We obtain Q ~ 4 × 10 28 s -1 for the observations made in the first week of February ( r h ~ 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 Q decreases for larger r h , and we derive ~1.5 × 10 28 s -1 for the observations made at r h ~ 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 Q obtained from observations of H 2 O photodissociation products. A χ 2 fit to the variation of Q with the heliocentric distance gives a power-law index of -3.0 ± 0.5 (statistical) ±0.1 (systematic).