NEAR-NUCLEUS PHOTOMETRY OF OUTBURSTING COMET 17P/HOLMES

Abstract

Comet 17P/Holmes underwent the largest cometary outburst in recorded history on UT 2007 Oct. 23, releasing massive quantities of dust and gas. We used the Canada-France-Hawaii telescope to obtain wide-field images of 17P/Holmes on 15 dates over a period of 3 months following the outburst and employ them here to examine the subsequent activity of the nucleus and the nature of the ejecta closest to the nucleus. Through aperture photometry we observed the in- ner coma (within 2500 km of the nucleus) to fade from an apparent magnitude of 11.7 mag to 17.6 mag, corresponding to absolute magnitudes of 8.1 mag and 12.4 mag, between UT 2007 Nov. 6 and 2008 Feb. 12. A second much smaller outburst occurred on UT 2007 Nov. 12, three weeks after the original outburst, suggesting that the nucleus remained unstable. The surface brightness profile of the inner coma was consistently shallow relative to the expected steady-state profile, and showed a persistent brightness enhancement within ~ 5000 km of the nucleus. We propose that sublimating ice grains created an ice grain halo around the nucleus, while fragmenting grains were responsible for the shallow surface brightness profile.