Extinction in the Coma of Comet 17P/Holmes

TL;DR

This study measures the optical depth and dust properties of comet 17P/Holmes's coma during its 2007 outburst, providing the most stringent constraints on coma extinction to date.

Contribution

It presents the first detailed optical depth measurements of the coma during the outburst, revealing its optical thickness and dust albedo with unprecedented precision.

Findings

Optical depth tau~0.04 at 1.5 arcsec from nucleus

Optically thick only within ~0.01 arcsec from nucleus

Dust geometric albedo p_d=0.006±0.002

Abstract

On 2007 October 29 the outbursting comet 17P/Holmes passed within 0.79 arcsec of a background star. We recorded the event using optical, narrowband photometry and detect a 3% to 4% dip in stellar brightness bracketing the time of closest approach to the comet nucleus. The detected dimming implies an optical depth tau~0.04 at 1.5 arcsec from the nucleus and an optical depth towards the nucleus center tau_n<13.3. At the time of our observations, the coma was optically thick only within rho<~0.01 arcsec from the nucleus. By combining the measured extinction and the scattered light from the coma we estimate a dust red geometric albedo p_d=0.006+/-0.002 at 16 deg phase angle. Our measurements place the most stringent constraints on the extinction optical depth of any cometary coma.