The Reactivation and Nucleus Characterization of Main-belt Comet 358P/PANSTARRS (P/2012 T1)

We present observations of main-belt comet (MBC) 358P/PANSTARRS (P/2012 T1) obtained using the Gemini South telescope from 2017 July to December, as the object approached perihelion for the first time since its discovery. We find best-fit IAU phase function parameters of H _R = 19.5 ± 0.2 mag and G _R = -0.22 ± 0.13 for the nucleus, corresponding to an effective radius of r _N = 0.32 ± 0.03 km (assuming an albedo of p _R = 0.05). The object appears significantly brighter (by ≥1 mag) than expected starting in 2017 November, while a faint dust tail oriented approximately in the antisolar direction is also observed on 2017 December 18. We conclude that 358P has become active again for the first time since its previously observed active period in 2012-2013. These observations make 358P the seventh MBC candidate confirmed to exhibit recurrent activity near perihelion with intervening inactivity away from perihelion, strongly indicating that its activity is sublimation-driven. Fitting a linear function to the ejected dust masses inferred for 358P in 2017 when it is apparently active, we find an average net dust production rate of kg s^-1 (assuming a mean effective particle radius of mm) and an estimated activity start date of 2017 November 8 ± 4 when the object was at a true anomaly of ν = 316° ± 1° and a heliocentric distance of R = 2.54 au. Insufficient data is currently available to ascertain whether activity strength has changed between the object's 2012-2013 and 2017 active periods. Further observations are therefore highly encouraged during the object's upcoming observing window (2018 August through 2019 May).