The Activity of Comet 29P/Schwassmann-Wachmann 1 Monitored through Its CO J=2-->1 Radio Line

The 230-GHz emission from CO was observed on 18 different days between 4 December 1996 and 1 January 1997 with the 15-m SEST antenna pointed toward Comet 29P/Schwassmann-Wachmann 1. The CO emission was measured at four different locations within the coma. During the entire month, the shape of the lines remained remarkably constant for all positions of the telescope beam. Under these conditions, the properties of the daily spectra can be compared almost independent of any coma and excitation model. We conclude that the physical conditions for both the nucleus and the coma were stable over one month. Day-to-day nuclear output variations were small, probably less than half the monthly average production rate. The spectra could therefore be combined to produce line profiles with an unprecedented SNR. Our data and previous millimeter observations of the comet allow us to describe the properties of the comet in its quiescent state. In that state, the on-nucleus line presents the following characteristics: The line area is 0.04 K km s ^-1; there is a sunward-displaced velocity peak near -0.45 km s ^-1; a velocity skirt extends toward positive values, which might indicate CO production on the night side. We have derived simple expressions for both the CO emission rate per unit surface and time and the gas radial outflow velocity in terms of the cosine of the angle between the flow direction and the comet-Sun line. We find that the velocity decreases monotonically from the subsolar region to the terminator and is essentially constant over the anti-sunward hemisphere. Roughly 75% of the outgassing occurs on the sunward hemisphere. The mean CO production rate in the quiescent state is 1.8±0.1×10 ²⁸ molecules s ^-1. This steady-state outgassing can be described as comprising two components: (1) a low-intensity, nearly isotropic source of CO and (2) a major source peaking near the subsolar point that exhibits the properties of a production region of small spatial extent. The former source may be of either nuclear or distributed nature. The appropriate surface and near-surface properties required to explain the latter source are maintained by the constant infall of solar energy on the rotating nucleus. While the nucleus may be viewed as a single source of coma materials, with enhanced production where the influx of solar energy is highest, the possible role of Halley-like active regions, if they exist, as inferred from these observations is very likely.