Evolution of H$_2$O, CO, and CO$_2$ Production in Comet C/2009 P1 Garradd During the 2011-2012 Apparition

We present analysis of high spectral resolution NIR spectra of CO and H$_2$O in comet C/2009 P1 (Garradd) taken during its 2011-2012 apparition with the CSHELL instrument on NASA's Infrared Telescope Facility (IRTF). We also present analysis of observations of atomic oxygen in comet Garradd obtained with the ARCES echelle spectrometer mounted on the ARC 3.5-meter telescope at Apache Point Observatory and the Tull Coude spectrograph on the Harlan J. Smith 2.7-meter telescope at McDonald Observatory. The observations of atomic oxygen serve as a proxy for H$_2$O and CO$_2$. We confirm the high CO abundance in comet Garradd and the asymmetry in the CO/H$_2$O ratio with respect to perihelion reported by previous studies. From the oxygen observations, we infer that the CO$_2$/H$_2$O ratio decreased as the comet moved towards the Sun, which is expected based on current sublimation models. We also infer that the CO$_2$/H$_2$O ratio was higher pre-perihelion than post-perihelion. We observe evidence for the icy grain source of H$_2$O reported by several studies pre-perihelion, and argue that this source is significantly less abundant post-perihelion. Since H$_2$O, CO$_2$, and CO are the primary ices in comets, they drive the activity. We use our measurements of these important volatiles in an attempt to explain the evolution of Garradd's activity over the apparition.
Comment: 35 pages, 10 figures, 9 tables, accepted for publication in Icarus