THE EVOLUTION OF VOLATILE PRODUCTION IN COMET C/2009 P1 (GARRADD) DURING ITS 2011-2012 APPARITION

Abstract

We report observations at millimeter and submillimeter wavelengths of comet C/2009 P1 (Garradd) from 2011 December 28 to 2012 April 24, using the Arizona Radio Observatory submillimeter telescope (SMT) and the James Clerk Maxwell Telescope (JCMT). Garradd is a dynamically young long-period comet from the Oort Cloud, with a periodicity of 127,000 yr, that reached perihelion on 2011 December 23 (at Rh = 1.55 AU and Δ = 20.1 AU) and made its closest approach to the Earth on 2012 March 05 (at Rh = 1.84 AU and Δ = 1.26 AU). We obtained gas production rates, and molecular abundances relative to water for HCN, ortho-H2CO, CS, CO and CH3OH. A rotational temperature, Trot ≈ 50 K, was determined by observing multiple methanol lines with the JCMT. By averaging the abundance ratio relative to water from the SMT and the JCMT we derive: CO: 7.03% ± 1.84%, HCN: 0.04% ± 0.01%, o-H2CO: 0.14% ± 0.03% as a parent molecule (and 0.28% ± 0.06% as an extended source), CS: 0.03% ± 0.01% and CH3OH: 3.11-0.51+1.86%. We concluded that Garradd is normal in CH3OH, depleted in HCN, o-H2CO and CS and slightly enriched in CO with respect to typically observed cometary mixing ratios. We also studied the temporal evolution of HCN and CO and find that the production of HCN has a trend similar to water (but with short-term variation), with a decrease after perihelion, while that of CO shows contrary behavior: remaining constant or increasing after perihelion.