Presentation #412.03 in the session Dusty and Icy Cometary Comae (iPosters).
Comets afford a window into the chemistry and physics of planet formation. Through remote sensing of coma gases, the composition of nucleus ices can be inferred and placed into the context of the protoplanetary disk midplane and solar system formation. At large heliocentric distances comet activity is driven by the most volatile species, with CO sublimation beginning at heliocentric distances (rH) of up to tens of au and CO2 sublimation activating near 6 – 8 au. Although H2O is the dominant ice in most comets, vigorous H2O sublimation does not begin until comets reach rH ~ 2 – 3 au in the inner solar system. The interplay between these three major drivers of cometary activity (H2O, CO, and CO2) across an orbit is not well understood, and the behavior of the coma as it crosses the H2O sublimation region and transitions from CO/CO2-driven activity to H2O-dominated outgassing requires further study. Here we report ALMA Cycle 8 observations of distantly active Oort cloud comet C/2017 K2 (PanSTARRS). C/2017 K2 was already active upon its discovery at 16 au, with activity consistent with CO sublimation. Its large discovery distance and high intrinsic brightness afforded an opportunity to characterize the evolution in molecular abundances, spatial distributions, and kinematics in the coma as the comet crossed the H2O sublimation region inbound to perihelion. We monitored C/2017 K2 using ALMA beginning near rH = 4 au in 2021 January with follow-up observations in 0.5 au increments, sampling molecular emission from CO, CH3OH, H2CO, CS, HCN, and HNC and continuum emission from dust/icy grains as the comet approached and crossed the H2O sublimation region in advance of its perihelion at rH = 1.79 au on UT 2022 December 19. We report molecular production rates and spectrally integrated flux maps on each date, and place our results into the context of the larger comet population. This work makes use of the following ALMA data: ADS/JAO.ALMA #2021.1.00862.S.