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M. C. Festou (Midi-Pyrénées Obser., Toulouse, France), M. Gunnarsson, H. Rickman (Astron. Observatory, Uppsala, Sweden), A. Winnberg (Onsala Rymdobservatorium, Onsala, Sweden), G. Tancredi (Fac. Ciencias, Montevideo - Uruguay)
Observations of CO in comet 29P/SW1 were obtained with the SEST antenna in December 1996 when the comet was in its quiescent state. The shape of the lines remained remarkably constant for all positions of the telescope beam in the coma. Day-to-day nuclear output variations were small, probably less than half the monthly average production rate. The on-nucleus line presented the following characteristics: the line area is 0.04 K km /sec; there is a sunward-displaced velocity peak near -0.45 km /sec and a velocity skirt extends towards positive values, which indicates CO production on the night side. Assuming a spherical nucleus, we find that the velocity, an indicator of the surface temperature, decreases monotonically from the subsolar point to the terminator and is essentially constant over the night side. The CO production in the comet quiescent state is (1.8 ±0.1)e28 molecules /sec. Roughly 75% of the CO is released on the sunlit side of the nucleus. The outgassing pattern of SW1's nucleus in the quiescent state can be viewed as folllows: (1) the entire surface produces CO molecules at a low intensity level and, (2) the CO production is the highest in the subsolar region. These features imply that the surface is homogeneous at a small scale relative to the radius of the comet. Although, the nucleus can be viewed as a single source of coma materials, with an enhanced production where solar energy is deposited, the possible role of Halley-like active regions, if they exist, is unclear.
The Swedish-ESO Sub-millimetre Telescope is jointly operated by ESO and the Swedish National Facility for Radio Astronomy, Onsala Space Observatory, Chalmers University of Technology. MCF was consultant for the Southwest Research Institute, Boulder extension office (CO), when a significant fraction of this work was performed.
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