An Extended CO Source around Comet 29P/Schwassmann–Wachmann 1

Comet 29P/Schwassmann–Wachmann 1 has been studied during seven days in August 1998 with the SEST submillimeter telescope at ESO, La Silla, Chile. The CO (J=2−1) emission at 230 GHz was mapped by directing the telescope beam at the nucleus and six off-nucleus positions. The CO line profiles exhibit the blue- and redshifted components previously observed by various observers. The strength of the observed lines does not decrease with projected distance to the nucleus as expected if CO molecules were coming from the nucleus only. Instead, the line area is nearly constant throughout the map. This can be explained if CO molecules are being released from both the sunlit side of the nucleus and CO-bearing particles distributed in a shell-like cloud. The extended source must consist of icy grains globally moving toward the Sun at ∼50 m s−1 released ∼30 days before the observations were made. The nuclear and extended sources produce (7±1)×1027 and 2.4×1028 molecules s−1, respectively. Our 1996 observations of the comet (Festou, M., M. Gunnarsson, A. Winnberg, H. Rickman, and G. Tancredi 2001. Icarus150, 140–150) were reexamined using this new two-source model. In this case, the nuclear and extended CO sources produced 10±1×1027 and 2.9×1028 CO molecules s−1, respectively. It is not necessary to postulate night side outgassing, but a large quantity of solid grains has to be expelled into the coma.