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Context. CO is assumed to be the main driver behind the activity of comet 29P/Schwassmann-Wachmann 1, which resides in a nearcircular 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 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 kin = 4 K, give a measure of Q CO , the CO production rate. Q CO was found to be (3.9 ± 0.2) × 10 28 mol s −1 in June 2003, and (3.7 ± 0.2) × 10 28 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.

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Abstract.Radio observations of different molecular species in comet C/1995 O1(Hale-Bopp) have been carried out regularly since August 1995. We present an analysis of carbon monoxide spectra observed at SEST. A detailed picture of the nature of the post-perihelion outgassing at large heliocentric distances is shown. The data starts at 2.9 AU, with active water sublimation, and stretch outside 10 AU, where CO-driven activity is still detected. The activity is studied using a coma model which separates the outgassing into two different sources, one insolation-dependent source, peaking at the subsolar point, and one isotropically outgassing source. Gas velocities and production rates of the two sources are derived for two different cases. In the first case the isotropic source is assumed to be inside the nucleus, and in the second case this source is assumed to be CO-rich material in the coma. Under both assumptions, the results indicate that the two component sources evolve independently. At heliocentric distances around 3 AU, acceleration of the CO molecules in the innermost region of the coma is required for the model to fit the observations. This acceleration must occur at a slower rate than that predicted by hydrodynamic simulations. From 6 to 10 AU, the derived parameters appear remarkably constant. The nuclear outgassing velocity is generally unchanged with heliocentric distance during this wide interval. Observations of molecules other than CO are included in the SEST dataset, and a comparison is made to the modelling results. HCN and CH 3 OH show signs of being produced in the same way as the CO up to a heliocentric distance of 6 AU.