Simultaneous Gas and Dust Monitoring of Active Centaur 29P/Schwassmann-Wachmann 1 by NEOWISE

We report monitoring observations of the active Centaur 29P/Schwassmann-Wachmann 1 by the NEOWISE mission. This imaging from NEOWISE simultaneously samples both 29P’s dust and its gas production. 29P is a continuously-active cometary Centaur orbiting just beyond Jupiter and known for its wide and frequent swings in gas and dust production rates. 29P’s distance from the Sun suggests that its activity is likely due to CO sublimation and possibly the conversion of amorphous water ice to its crystalline form. As with all comets, simultaneous measurements of the gas and dust over time can give insight into the nature of the activity and to the structure of the interior, but are often difficult to do for comets more than a few au away.

NEOWISE is an ongoing NASA mission currently making a multiband, multiepoch, infrared all-sky survey at its “W1” and “W2” bands centered at 3.4 and 4.6 microns [1]. We have twelve epochs of 29P NEOWISE imaging from Feb. 2014 to Dec. 2019 at about six-month intervals, spanning about 150° in true anomaly. We also have two additional epochs from May and Dec. 2010 at the tail end of the primary WISE mission. The W1 passband samples reflected sunlight off of the dust grains in 29P’s coma (the comet is too far from the Sun for thermal emission to be significant). The W2 passband likewise samples dust but also samples CO and CO2 gas owing to the ro-vibrational bands near 4.67 and 4.26 microns, respectively. Thus our NEOWISE imaging provides a rare opportunity to sample both the gas and dust being emitted by 29P.

In this presentation we will describe our preliminary analysis of the imaging and photometry. The comet is detected at all epochs but two, and as expected the gas and dust production rates are highly variable. Photometry of the W1 detections varies by a factor of about ~5, whereas that of W2 varies less, by about a factor of ~2. The W1-W2 color of the coma is thus also variable, indicating that the dust-to-gas ratio of the comet is not constant. There is some indication that the dust-to-gas ratio is higher when the comet is relatively bright, possibly due to an outburst. If true, this suggests that the physical mechanics of the outburst and the way it liberates dust is different from 29P’s regular background activity. This variability of the dust-to-gas ratio is consistent with earlier measurements by [2] that indicate that 29P has gassy and dusty outbursts that are not necessarily coincident.

This work makes use of data products from NEOWISE, which is a project of JPL/Caltech, funded by the Planetary Science Division of NASA.

1. Mainzer A. et al. (2011) ApJ, 731, 53.

2. Wierzchos K. and Womack M. (2020) AJ, 159, 136.