A Physical Model for the Ice Coma of the Interstellar, Hyperactive Comet 3I/ATLAS

Abstract

A previous study suggests that the observed exponential character of the surface brightness profiles in the coma around the interstellar comet 3I/ATLAS at 4 au can be explained as a consequence of the destruction of the icy scattering particles by sublimation. Here we follow the evolution of the ice coma as a function of heliocentric distance. We describe the evolution of the space and time distribution of the albedo within the coma by a Haser model for the fading of the grain albedo from a higher, icy value to a lower, refractory value. The competing effects of increasing rates of production and sublimation produce a peak in the total scattering cross-section due to ice at a heliocentric distance of 3 - 4 au. The modeled apparent visual magnitudes match the observed photometry for a range of initial conditions. The conventional, anti-solar tail observed at 3 au may be present at 4 au but suppressed by 2.6 magnitude in surface brightness by a combination of a decreased production rate and phase angle. The ice coma of 3I/ATLAS at 4 au resembles a hyperactive coma but with different rates of sublimation and Haser length scales.