Distribution and properties of fragments and debris from the split Comet 73P/Schwassmann-Wachmann 3 as revealed by Spitzer Space Telescope

Abstract

During 2006 March–2007 January, we used the IRAC and MIPS instruments on the Spitzer Space Telescope to study the infrared emission from the ensemble of fragments, meteoroids, and dust tails in the more than 3° wide 73P/Schwassmann-Wachmann 3 debris field. We also investigated contemporaneous ground-based and HST observations. In 2006 May, 55 fragments were detected in the Spitzer image. The wide spread of fragments along the comet's orbit indicates they were formed from the 1995 splitting event. While the number of major fragments in the Spitzer image is similar to that seen from the ground by optical observers, the correspondence between the fragments with optical astrometry and those seen in the Spitzer images cannot be readily established, due either to strong non-gravitational terms, astrometric uncertainties, or transience of the fragments' outgassing. The Spitzer data resolve the structure of the dust comae at a resolution of ~1000 km, and they reveal the infrared emission due to large (mm to cm size) particles in a continuous dust trail that closely follows the projected orbit. We detect fluorescence from outflowing CO_2 gas from the largest fragments (B and C), and we measure the CO_2:H_2O proportion (1:10 and 1:20, respectively). We use three dimensionless parameters to explain dynamics of the solid particles: the rocket parameter α is the reaction force from day-side sublimation divided by solar gravity, the radiation pressure parameter β is the force due to solar radiation pressure divided by solar gravity, and the ejection velocity parameter ν is the particle ejection speed divided by the orbital speed of the comet at the time of ejection. The major fragments have ν>α>β and are dominated by the kinetic energy imparted to them by the fragmentation process. The small, ephemeral fragments seen by HST in the tails of the major fragments have α>ν>β and are dominated by rocket forces (until they become devolatilized). The meteoroids along the projected orbit seen by Spitzer have β~ν≫α and are dominated by radiation pressure and ejection velocity, though both influences are much less than gravity. Dust in the fragments' tails has β≫(ν+α) and is dominated by radiation pressure.

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