Title: Thermal physics of the inner coma: ALMA studies of the methanol distribution and excitation in comet C/2012 K1 (PanSTARRS),
Abstract: We present spatially and spectrally-resolved observations of CH$_3$OH
emission from comet C/2012 K1 (PanSTARRS) using The Atacama Large
Millimeter/submillimeter Array (ALMA) 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
$\rho\sim500$-2500 km). The CH$_3$OH column density distributions are
consistent with isotropic, uniform outflow from the nucleus, with no evidence
for extended sources of CH$_3$OH in the coma. The $T_{rot}(\rho)$ radial
profiles show a significant drop within a few thousand kilometers of the
nucleus, falling from about 60 K to 20 K between $\rho=0$ and 2500 km on June
28, whereas on June 29, $T_{rot}$ fell from about 120 K to 40 K between $\rho=$
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 CH$_3$OH rotational
levels. Our excitation model shows that radiative cooling is more important for
the $J=7-6$ transitions (at 338 GHz) than for the $K=3-2$ transitions (at 252
GHz), resulting in a strongly sub-thermal distribution of levels in the $J=7-6$
band at $\rho\gtrsim1000$ km. 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.