Thermal Physics of the Inner Coma: ALMA Studies of the Methanol Distribution and Excitation in Comet C/2012 K1 (PanSTARRS)

We present spatially and spectrally resolved observations of CH₃OH emission from comet C/2012 K1 (PanSTARRS), using the Atacama Large Millimeter/submillimeter Array on 2014 June 28-29. Two-dimensional maps of the line-of-sight average rotational temperature (T _rot) were derived, covering spatial scales 0.″3-1.″8 (corresponding to sky-projected distances ρ ∼ 500-2500 km). The CH₃OH column density distributions are consistent with isotropic, uniform outflow from the nucleus, with no evidence for extended sources of CH₃OH in the coma. The radial profiles show a significant drop within a few thousand kilometers of the nucleus, falling from about 60 to 20 K between and 2500 km on June 28, whereas on June 29, T _rot fell from about 120 to 40 K between ρ = 0 km and 1000 km. The observed T _rot behavior is interpreted primarily as a result of variations in the coma kinetic temperature due to adiabatic cooling of the outflowing gas, as well as radiative cooling of the CH₃OH rotational levels. Our excitation model shows that radiative cooling is more important for the transitions (at 338 GHz) than for the transitions (at 252 GHz), resulting in a strongly sub-thermal distribution of levels in the band at . For both bands, the observed temperature drop with distance is less steep than predicted by standard coma theoretical models, which suggests the presence of a significant source of heating in addition to the photolytic heat sources usually considered.