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

Abstract

We present analyses of Spitzer observations of 29P/Schwassmann-Wachmann 1 using 16 µm IRS "blue" peak-up (PU) and 24 µm and 70 µm MIPS images obtained on UT 2003 November 23 and 24 that characterize the Centaur's large-grain (10-100 µm) dust coma during a time of non-outbursting "quiescent" activity. Estimates of f ρ for each band (16 µm (2600 ± 43 cm), 24 µm (5800 ± 63 cm), and 70 µm (1800 ± 900 cm)) follow the trend between nucleus size vs. f ρ that was observed for the WISE/NEOWISE comet ensemble. A coma model was used to derive a dust production rate in the range of 50-100 kg/s. For the first time, a color temperature map of SW1's coma was constructed using the 16 µm and 24 µm imaging data. With peaks at ∼ 140K, this map implies that coma water ice grains should be slowly sublimating and producing water gas in the coma. We analyzed the persistent 24 µm "wing" (a curved southwestern coma) feature at 352,000 km (90 ) from the nucleus attributed by Stansberry et al. (2004) to nucleus rotation and instead propose that it is largely created by solar radiation pressure and gravity acting on micron sized grains. We performed coma removal to the 16 µm PU image in order to refine the nucleus' emitted thermal flux. A new application of the Near Earth Asteroid Thermal Model (NEATM; Harris (1998)) at five wavelengths (5.730 µm, 7.873 µm, 15.80 µm, 23.68 µm, and 71.42 µm) was then used to refine SW1's effective radius measurement to R = 32.3 ± 3.1 km and infrared beaming parameter to η = 1.1 ± 0.2, respectively.