A new analysis of Spitzer observations of Comet 29P/Schwassmann–Wachmann 1

Abstract

We present a new analysis of Spitzer observations of comet 29P/Schwassmann-Wachmann 1 taken on UT 2003 November 21, 23, and 24, similar to a previous investigation of the observations (Stansberry et al. 2004), but using the most recent Spitzer data pipeline products and intensive image processing techniques. Analysis of images from the IRAC 5.8 & 8.0 µm bands and the MIPS 24.0 & 70.0 µm bands resulted in photometry measurements of the nucleus after a suite of coma modeling and removal processes were implemented. SW1 was not identified in the 5.8 µm image from the previous work so its incorporation into this analysis is entirely new. Using the Near Earth Asteroid Thermal Model (Harris 1998) resulted in a nucleus radius measurement of R = 30.2 +3.7 −2.9 km and an infrared beaming parameter value of η = 0.99−0.19 . We also measured an infrared geometric albedo, p 5.8 = 0.5 ± 0.5. Extrapolating a 0.04 V-band albedo and using a normalized reflectivity gradient S = 14.94 ± 1.09 [% (1000Å)−1 ] (Duffard et al. 2014) we recover an infrared albedo of p 5.8 = 0.31 in the near infrared consistent with the value recovered from thermal modeling. The dust composition extracted from IRS spectra are very comet-like, containing mainly amorphous ferromagnesian silicates (but with a minority of crystalline silicates as well), water ice, and metal sulfides.

Mid-infrared spectra of comet nuclei

“…In contrast to this discussion, analysis of the nucleus of comet Schwassmann-Wachmann 1 by Schambeau et al (2015), using photometry spanning a broad wavelength range (5 to 70 µm), results in a beaming parameter of 0.99 +0.26 −0.19 . The best-fit range is large, but it is centered near 1.0 and biased towards larger values.…”

“…The Schwassmann-Wachmann 1 spectrum is based on our own processing and spectral extraction, following the methods of Kelley et al (2006). This includes the removal of a model nucleus, based on the nucleus parameters of Schambeau et al (2015). The properties of the spectra of the comet comae are summarized in Table 5.…”

“…We have ignored the best-fit temperature for comet Schwassmann-Wachmann 1 as it does not agree with the longer wavelengths (λ > 15 µm). The 10-µm spectrum is strongly dominated by the nucleus (Stansberry et al 2004, Schambeau et al 2015, and its high apparent temperature is likely due to uncertainties in the nucleus subtraction.…”

“…We examined the mineral decompositions of the spectra of comets Holmes, Hale-Bopp, Schwassmann-Wachmann 1, Schwassmann-Wachmann 3, and Lulin (Harker et al 2002, Reach et al 2010, Sitko et al 2011, Woodward et al 2011, Schambeau et al 2015 Wooden et al (1999) indicates the shape of the short-wavelength shoulder is controlled by the relative amounts of amorphous and crystalline pyroxene (see their Figs. 5, 6, and 7), but in other comets the coma grain size distribution must also play a role.…”

Carbon Monoxide in the Distantly Active Centaur (60558) 174P/Echeclus at 6 au

“…Echeclus is approximately the same diameter (D Echeclus ∼ 65 km) as Hale-Bopp (D HB ∼ 60 km, Fernández (2000)) and 29P (D 29P ∼ 60 km, Schambeau et al (2015)). This fortuitous alignment of size and heliocentric distance allows us to eliminate some key parameters when comparing their CO production rates.…”