Activity-Induced Near-Infrared Variability at 29P/Schwassmann-Wachmann 1, 2017-2022

Abstract

29P/Schwassmann-Wachmann 1 (SW1) is both the first-discovered active Centaur and the most outburst-prone comet known. The nature of SW1's many outbursts, which regularly brighten the comet by five magnitudes or more, and what processes power them has been of particular interest since SW1's discovery in the 1920s. In this paper, we present and model four epochs of low-resolution near-infrared spectroscopy of SW1 taken with the NASA Infrared Telescope Facility and Lowell Discovery Telescope between 2017 and 2022. This dataset includes one large outburst, two periods of low activity ("quiescence" or "quiescent activity"), and one mid-sized outburst a few days after one of the quiescent observations. The two quiescent epochs appear similar in both spectral slope and modeled grain size distributions, but the two outbursts are significantly different. We propose that the two can be reconciled if smaller dust grains are accelerated more than larger ones, such that observations closer to the onset of an outburst are more sensitive to the finer-grained dust on the outside of the expanding cloud of material. These outbursts can thus appear very rapid but there is still a period in which the dust and gas are well-coupled. We find no strong evidence of water ice absorption in any of our spectra, suggesting that the areal abundance of ice-dominated grains is less than one percent. We conclude with a discussion of future modeling and monitoring efforts which might be able to further advance our understanding of this object's complicated activity patterns.