The Nucleus of Main-Belt Comet P/2010 R2 (La Sagra)

We present recent observations of main-belt comet P/2010 R2 (La Sagra) obtained using the Gemini North telescope on five nights in late 2011 and early 2013 during portions of the object's orbit when dust emission was expected to be minimal or absent. We find that P/La Sagra continues to exhibit a faint dust trail aligned with its orbit plane as late as 2011 December 31, while no activity is observed by the time of our next observations on 2013 March 3, shortly before aphelion. Using only photometry measured when the comet was observed to be inactive, we find best-fit IAU phase function parameters of H_R=18.4+/-0.2 mag and G=0.17+/-0.10, corresponding to an effective nucleus radius of r_N=0.55+/-0.05 km (assuming p_R=0.05). We revisit photometry obtained when P/La Sagra was observed to be active in 2010 using our revised determination of the object's nucleus size, finding a peak dust-to-nucleus mass ratio of M_d/M_N = (5.8+/-1.6)x10^(-4), corresponding to an estimated total peak dust mass of M_d = (5.3+/-1.5)x10^8 kg. We also compute the inferred peak total active surface area and active surface fraction for P/La Sagra, finding A_act ~ 5x10^4 m^2 and f_act ~ 0.01, respectively. Finally, we discuss P/La Sagra's upcoming perihelion passage, particularly focusing on the available opportunities to conduct follow-up observations in order to search for recurrent activity and, if recurrent activity is present, to search for changes in P/La Sagra's activity strength on successive orbit passages that should provide insights into the evolution of MBC activity over time.Comment: 20 pages, 8 figures; accepted for publication in Icaru