JCMT Spectral and Continuum Imaging of Comet 252P/LINEAR

Comet 252P/LINEAR passed the Earth at a distance of 0.035 au on 2016 March 21, presenting a rare opportunity to study a comet at high spatial resolution. Even with a single dish facility such as JCMT, the chemical structure of the coma could be observed on scales of 500-1000 km, which are smaller than the scale lengths of known distributed cometary molecules. Our week-long observing campaign at JCMT started on March 27 (UT), 12 days after perihelion, and ended on April 3, during which time the comet's distance from Earth increased from 0.045 to 0.078 au. Our observations of the J = 4 - 3 transition of HCN showed generally uniform levels of activity. Expansion velocities were ∼0.6 km s^-1 (±10%), and the derived mean HCN production rate during the week was 6.4 ×10²⁴ mol s^-1. Comparison with independent estimates of the water production rate during the same period yields a mixing ratio of 0.12% with respect to water. Methanol emissions appear to arise from an extended source - probably in the form of an ice halo - suggesting that all the gases from 252P may originate in large part from the sublimation of icy grains in the coma. Adopting a mean dust particle size of 1 mm, the mass of dust in the coma at the same time is estimated at 4 ×10⁷ kg, implying a total dust production rate of 4 kg s^-1. The dust-to-gas mass ratio of ∼0.025 is one of the lowest values ever observed for a comet.