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

Abstract

We present analysis of Hubble Space Telescope (HST) 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 (Feldman (1995)) 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 underlying quiescent activity level coma detectable to ∼70 (∼267,000 km) away from the nucleus. Photometry of the calibrated WFPC2 images resulted in a measured 5 radius aperture equivalent R-band magnitude of 12.86 ± 0.02 and an estimated (2.79 ± 0.05)×10 8 kg for the lower limit of dust material emitted during the outburst. No appreciable evolution of morphologic features, indicating signatures of nucleus rotation, were detected between the two imaging epochs. The observations were modeled using a 3-D Monte Carlo coma model (Samarasinha (2000)) to place constraints on the nucleus' rotation state. Modeling indicated the morphology is repre-2 Schambeau et al.sentative of a non-isotropic ejection of dust emitted during a single outburst event with a duration on the order of hours from a single source region corresponding to ∼1% of the surface area. A spin period with lower limit on the order of days is suggested to reproduce the coma morphology seen in the observations.

CO Gas and Dust Outbursts from Centaur 29P/Schwassmann–Wachmann

“…We consider a dust outburst to be a brightening event of at least 1 magnitude of the nuclear magnitude (typically the central 5-10 arcsec part of the coma) that takes place within a few hours to a day. Brightness variations occur on smaller scales, but this threshold definition for an outburst is consistent with the analysis of many other published datasets (Richter 1941;Roemer 1958;Trigo-Rodríguez et al 2008, 2010Miles et al 2016;Schambeau et al 2019). The dust coma can also present morphological changes as an outburst progresses (Schambeau et al 2017) and outbursts are documented at longer wavelengths (e.g., Hosek et al (2013).…”

“…Thus, 29P/SW1's coma must be generated by a different mechanism, as with other distantly active comets (Womack et al 2017). Many alternatives have been considered for distant activity, including sublimation of a cosmogonically abundant low-condensation temperature ice, an amorphous-to-crystalline phase change of water-ice, HCN polymerization, cryovolcanism or meteoroid impacts (Prialnik & Bar-Nun 1990;Senay & Jewitt 1994;Enzian et al 1997;Gronkowski 2004;Miles 2016;Schambeau et al 2019). A strong candidate for involvement with the activity is CO outgassing, since its emission was detected in 29P/SW1 over 25 years ago and has been measured on a Gaseous mass loss rates were calculated using production rates and appropriate atomic mass units.…”

“…There are many studies documenting 29P/SW1 quiescent and outbursting dust activity (Trigo-Rodríguez et al 2008;Hosek et al 2013;Miles et al 2016;Schambeau et al 2019), but very few include both dust and gas components measured at or about the same time. Analysis of optical spectra of 29P/SW1 obtained in 1990 indicated that the CO + and dust production were not strongly coupled during an outburst.…”

Characterization of Thermal Infrared Dust Emission and Refinements to the Nucleus Properties of Centaur 29P/Schwassmann-Wachmann 1

“…Taking into consideration the great similarity in the 16 µm and 24 µm image morphology taken 1.3 days apart, and the relationship between the projected nucleus-Sun vector and the curved wing's structure suggests that this feature is possibly not the result of nucleus rotation, but is instead due to solar radiation pressure effects on micron sized dust grains emitted in the sunward direction being turned back to form the dust tail in the north-east direction (Farnham & Schleicher 2005;Li et al 2014;Mueller et al 2013). While the ∼ 60 day rotation period derived by the earlier work may in fact coincidentally be reflective of SW1 potentially possessing a long rotation period (Miles et al 2016;Schambeau et al 2017Schambeau et al , 2019, we propose that this curved wing feature is not the result of a slowly rotating nucleus.…”

“…Whipple (1980) presents a detailed analysis of SW1's outburst coma morphology as detected over a 50 year baseline, resulting in the descriptive term of "ringtailed snorter" for this often seen curved shape feature. While it may at first seem appropriate to compare the outburst and quiescent coma morphologies, detailed analyses of SW1's dust coma while in both phases of activity (Hosek et al 2013;Miles et al 2016;Schambeau et al 2017Schambeau et al , 2019 have provided descriptions of the underlying processes ongoing in both phases of activity and that the two are different. The morphology of the 24 µm quiescent coma's wing may resemble that of SW1's outburst coma; however, it was produced by different mechanisms (i.e., slow, sustained dust lofting with expansion velocities in the range of 10-50 m/s while quiescent (Jewitt 1990) vs. impulsive short lived dust emission at high velocities in the 100-300 m/s range during major outbursts (Feldman 1995;Schambeau et al 2017Schambeau et al , 2019Trigo-Rodríguez et al 2010).…”