Mapping the carbon monoxide coma of comet 29P/Schwassmann-Wachmann 1

Context. CO is assumed to be the main driver behind the activity of comet 29P/Schwassmann-Wachmann 1, which resides in a near-circular orbit at 6 AU from the Sun. Several properties of the outgassing of CO can be deduced from its millimetre-wave emission. Earlier studies have indicated CO production from the nucleus as well as an extended source. Aims. We have sought to further investigate the nature of the CO production in comet 29P/Schwassmann-Wachmann 1, through the use of newly available instrumentation. Methods. We used the HERA receiver array on the 30-m IRAM telescope to map the 230 GHz CO(J = 2-1) line in the comet with an unprecedented sensitivity and spatial coverage, and a high spectral resolution (20 kHz, i.e., 25 m s-1). A 36-point map, 60 by 60'', was obtained in June 2003, and a 25-point map, 96 by 96'', in January 2004. Results. The CO emission line has a characteristic asymmetric profile. Our analysis is based on a coma model, where the outgassing pattern is derived from the shape of this line at the central position of the map. When comparing to the observations, both maps show a line intensity at offset positions that is 2-3 times stronger than the model prediction. Different explanations for this are evaluated, and it is found that for the global coma character, an extremely low gas temperature in the inner coma reproduces the observed radial profile. A cold inner coma depletes the population of the CO $J=2$ rotational level in the region closest to the nucleus, making spectra observed at offset positions relatively stronger. From the global appearance of the maps, the coma was found to be largely axisymmetric, and the presence of a strong extended source of CO, as indicated from earlier observations using the SEST telescope, was not seen. When examining the maps in more detail, a possible exception to this was seen in an area ~30'' south of the comet, where an excess in emission is present in both maps. Model fits to the spectra based on the cold inner coma that we find, with an intital kinetic temperature $T_{\rm kin}=4$ K, give a measure of QCO, the CO production rate. QCO was found to be (3.9 ± 0.2) $\times$ 1028 mol s-1 in June 2003, and (3.7 ± 0.2) $\times$ 1028 mol s-1 in January 2004. These values are a factor ~1.5 higher than that derived using only the information available from non-mapped data, and this adjustment applies also to previously published production rates