Submitted, 2011-02-28; Accepted, 2011-05-03 Physical Properties of Main-Belt Comet 176P/LINEAR 1

We present a physical characterization of comet 176P/LINEAR, the third member of the new class of main-belt comets, which exhibit cometary activity but are dynamically indistinguishable from main-belt asteroids, to be discovered. Observations show the object exhibiting a fan-shaped tail for at least one month in late 2005, but then becoming inactive in early 2006. During this active period, we measure broadband colors of B − V = 0.63 ± 0.02, V − R = 0.35 ± 0.02, and R − I = 0.31 ± 0.04. Using data from when the object was observed to be inactive, we derive best-fit IAU phase function parameters of H = 15.10 ± 0.05 mag and G = 0.15 ± 0.10, and best-fit linear phase function parameters of m(1, 1, 0) = 15.35 ± 0.05 mag and β = 0.038 ± 0.005 mag deg−1. From this baseline phase function, we find that 176P exhibits a mean photometric excess of ∼30 % during its active period, implying an approximate total coma dust mass of Md ∼ (7.2 ± 3.6) × 104 kg. From inactive data obtained in early 2007, we find a rotation period of Prot = 22.23 ± 0.01 hr and a peak-to-trough photometric range of ∆m ∼ 0.7 mag. Phasing our photometric data from 176P’s 2005 active period to thi