Physical Properties of Main-Belt Comet 176P/LINEAR

We present a physical characterization of main-belt comet 176P/LINEAR. Observations show the object exhibiting a fan-shaped tail in late 2005, but then becoming inactive in early 2006. Using data from 2007 when the object was observed to be inactive, we derive best-fit IAU phase function parameters of H=15.10 mag and G=0.15, linear phase function parameters of m(1,1,0)=15.35 mag and Beta=0.038 mag/deg, and a rotation period of P_rot=22.23 hr and a photometric range of delta~0.7 mag. In 2005, the nucleus exhibits a significantly smaller photometric range that is attributed to viewing geometry effects. An analysis of these effects showed that 176P likely has an axis ratio of 1.8<b/a<2.1, an obliquity of epsilon~60 deg, and a solstice position at a true anomaly of ~20 deg. Numerical modeling found that 176P's activity can only be reproduced by asymmetric dust emission, such as a cometary jet. We find plausible fits to our observations using models assuming ~10 micron dust particles continuously emitted over 176P's active period, and a jet direction of 180 < alpha_jet < 120 deg and delta_jet ~ -60 deg. We do not find good fits to our observations using models of impulsive dust emission, i.e., what would be expected from an impact. Since for a rotating body, the time-averaged direction of a non-equatorial jet is equivalent to the direction of the nearest rotation pole, we find an equivalent orbital obliquity of 50 < epsilon < 75 deg, consistent with the results of our lightcurve analysis. The results of our lightcurve analysis and dust modeling analysis are furthermore both consistent with the seasonal heating hypothesis used to explain the modulation of 176P's activity. Additional observations are highly encouraged to further characterize 176P's active behavior as the object approaches perihelion on 2011 July 01.