Physical Characterization of Main-belt Comet (248370) 2005 QN 173

We report results from new and archival observations of the newly discovered active asteroid (248370) 2005 QN173 (also now designated Comet 433P), which has been determined to be a likely main-belt comet based on a subsequent discovery that it is recurrently active near perihelion. From archival data analysis, we estimate $g^{\prime} $-, $r^{\prime} $-, $i^{\prime} $-, and $z^{\prime} $-band absolute magnitudes for the nucleus of Hg = 16.62 ± 0.13, Hr = 16.12 ± 0.10, Hi = 16.05 ± 0.11, and Hz = 15.93 ± 0.08, corresponding to nucleus colors of $g^{\prime} -r^{\prime} =0.50\pm 0.16$, $r^{\prime} -i^{\prime} =0.07\pm 0.15$, and $i^{\prime} -z^{\prime} =0.12\pm 0.14$; an equivalent V-band absolute magnitude of HV = 16.32 ± 0.08; and a nucleus radius of rn = 1.6 ± 0.2 km (using a V-band albedo of pV = 0.054 ± 0.012). Meanwhile, we find mean near-nucleus coma colors when 248370 is active of $g^{\prime} -r^{\prime} =0.47\pm 0.03$, $r^{\prime} -i^{\prime} =0.10\pm 0.04$, and $i^{\prime} -z^{\prime} =0.05\pm 0.05$ and similar mean dust tail colors, suggesting that no significant gas coma is present. We find approximate ratios between the scattering cross sections of near-nucleus dust (within 5000 km of the nucleus) and the nucleus of Ad/An = 0.7 ± 0.3 on 2016 July 22 and 1.8 < Ad/An < 2.9 in 2021 July and August. During the 2021 observation period, the coma declined in intrinsic brightness by ∼0.35 mag (or ∼25%) in 37 days, while the surface brightness of the dust tail remained effectively constant over the same period. Constraints derived from the sunward extent of the coma and width of the tail as measured perpendicular to the orbit plane suggest that the terminal velocities of ejected dust grains are extremely slow (∼1 m s−1 for 1 μm particles), suggesting that the observed dust emission may be aided by rapid rotation of the nucleus lowering the effective escape velocity.