EVIDENCE OF FRAGMENTING DUST PARTICLES FROM NEAR-SIMULTANEOUS OPTICAL AND NEAR-INFRARED PHOTOMETRY AND POLARIMETRY OF COMET 73P/SCHWASSMANN-WACHMANN 3

Abstract

We report imaging polarimetry of segments B and C of the Jupiter-family comet 73P/Schwassmann–Wachmann 3i n theI and H bandpasses at solar phase angles of approximately 35 ◦ and 85 ◦ . The level of polarization was typical for active comets, but larger than expected for a Jupiter-family comet. The polarimetric color was slightly red (δP /δλ = +1.2 ± 0.4) at a phase angle of ∼35 ◦ and either neutral or slightly blue at a phase angle of ∼85 ◦ . Observations during the closest approach from 2006 May 11–13 achieved a resolution of 35 km at the nucleus. Both segments clearly depart from a 1/ρ surface brightness for the first 50–200 km from the nucleus. Simulations of radiation-driven dust dynamics can reproduce some of the observed coma morphology, but only with a wide distribution of initial dust velocities (at least a factor of 10) for a given grain radius. Grain aggregate breakup and fragmentation are able to reproduce the observed profile perpendicular to the Sun–comet axis, but fit the observations less well along this axis (into the tail). The required fragmentation is significant, with a reduction in the mean grain aggregate size by about a factor of 10. A combination of the two processes could possibly explain the surface brightness profile of the comet.