Submillimeter Wave Astronomy Satellite monitoring of the postperihelion water production rate of comet C/1999 T1 (McNaught-Hartley)

We present observations of the pure rotational transition 1(10) --> 1(01) 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, r(h), ranging from 1.41 to 2.05 AU. The water production rate, Q(H2O), derived from the observations depends on the assumptions regarding the electron abundance in the coma. We obtain Q(H2O) similar to 4 x 10(28) s(-1) for the observations made in the first week of February (r(h) similar to 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(H2O) decreases for larger r(h), and we derive similar to1.5 x 10(28) s(-1) for the observations made at r(h) similar to 2 AU in the last week of 2001 March. The inferred water production rate is larger by similar to40% 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(H2O) obtained from observations of H2O photodissociation products. A chi(2) fit to the variation of Q(H2O) with the heliocentric distance gives a power-law index of similar to3.0 +/- 0.5 (statistical) +/-0.1 (systematic).