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.
- Comets: individual (C/2012 K1, PanSTARRS)
- molecular processes
- techniques: imaging spectroscopy
- techniques: interferometric
ASJC Scopus subject areas
- Astronomy and Astrophysics
- Space and Planetary Science