Analysis of HST WFPC2 Observations of Centaur 29P/Schwassmann–Wachmann 1 while in Outburst to Place Constraints on the Nucleus’ Rotation State

Abstract

Abstract We present analysis of Hubble Space Telescope observations of Centaur 29P/Schwassmann–Wachmann 1 (SW1) while in outburst to characterize the outburst coma and place constraints on the nucleus’ spin state. The observations consist of Wide Field and Planetary Camera 2 (WFPC2) images from Cycle 5, GO-5829 acquired on UT 1996 March 11.3 and 12.1, which serendipitously imaged the Centaur shortly after a major outburst. A multi-component coma was detected consisting of an expanding outburst dust coma with complex morphology possessing an east–west asymmetry and north–south symmetry contained within 5″ (∼19,000 km) of the nucleus, the residual dust shell of an earlier UT 1996 February outburst, and a nearly circular coma with underlying quiescent activity level detectable to ∼70″ (∼267,000 km) away from the nucleus. Photometry of the calibrated WFPC2 images resulted in an equivalent R-band magnitude of 12.86 ± 0.02 for a measured 5″ radius aperture and an estimated (2.79 ± 0.05) × 108 kg for the lower limit of dust material emitted during the outburst. No appreciable evolution of morphological features, indicating signatures of nuclear rotation, was detected between the two imaging epochs. The observations were modeled using a 3D Monte Carlo coma model to place constraints on the nucleus’ rotation state. Modeling indicated that the morphology is representative of a non-isotropic ejection of dust emitted during a single outburst event with a duration of the order of hours from a single source region corresponding to ∼1% of the surface area. A spin period with lower limit of the order of days is suggested to reproduce the coma morphology seen in the observations.