Complementary studies on the unexpected activity of comet Schwassmann-Wachmann 1

The orbit of comet P/Schwassmann-Wachmann 1 (P/SW 1) is confined to a range of heliocentric distances between 5.7 and 6.4 AU. Despite the large distance from the sun, this comet presents an unexpected and irregular activity. Its brightness can change up to six orders of magnitude during “outbursts”. Up to now, no generally accepted theory exists which explains this behaviour. In a previous paper (Cabot et al., 1996), it was shown that collisions of this comet with interplanetary objects were unlikely events and thus, were unable to account for the frequent outbursts. A data analysis of total magnitudes was performed and did not show any periodicity in the signal. Nevertheless an increase of the average level of activity was found after the recent orbit change, which was caused by a secular perturbation of Jupiter. The present work tries to shed light on these unexpected phenomena. The light curve analysis of P/SW 1 is completed with data recorded after 1986. This study tends to confirm earlier results. Although there are much more data during recent years, no characteristic frequency in the signal is detected. The peak of activity after perihelion is confirmed. The delay between the two events seems to increase through the last three revolutions. In order to explain this tendency, the propagation time of a heat wave in a cometary nucleus is estimated. The result is in good agreement with the observed lag. The erratic activity of P/SW 1 can be explained by the amorphous-crystalline water ice phase transition, which is triggered by the propagation of a heat wave into the nucleus.