Sub-millimeter observation of water vapor at 557 GHz in Comet C/2002 T7 (LINEAR)

Abstract

We present sub-millimeter observations of the ground-state rotational transition (110–101) of water vapor from Comet C/2002 T7 (LINEAR) obtained with the MIRO instrument on the ESA Rosetta spacecraft (s/c) orbiter on April 30, 2004. At the time of the observations, the comet was at a distance of 0.63AU from the Sun, 0.68AU from the MIRO telescope, and about 7.5days after its perihelion. The ground state rotation transition of ortho-water at 556.936GHz was observed and integrated for ∼8h using a frequency switched radiometer to provide short and long term stability. The MIRO beam size is 7.5arcmin in terms of full width half maximum, corresponding to a radius of 1.1×105km at the comet location. The observed signal line area of the water line spectrum is 4.3±0.8Kkm/s. Using a molecular excitation and radiation transfer model and assuming the spherically symmetric and constant radial expansion of gas in the coma, we estimate that the production rate of water is (1.0±0.2)×1030molecules/s and the expansion velocity is 1.1±0.2km/s at the time of the MIRO observation. The present estimation of the water outgassing rate of the comet is in good agreement with other observation-based estimations when the outgassing rates with respect to the time after perihelion are compared. The Doppler-corrected center velocity of the observed line was red-shifted by 0.67±0.13km/s, of which only 0.18km/s shift is explained by the model and attributed to a self-absorption effect. The potential sources of the additional red shift are discussed.