Watching a Comet Turn On: High Spectral Resolving Power Observations of Comet C/2017 K2 (PanSTARRS)

We report high spectral resolving power optical observations of comet C/2017,K2 (PanSTARRS) as it approached the Sun. This comet was discovered when it was 16,{\sc au} from the Sun. At discovery, the comet had a large and relatively bright coma. However, the spectrum at discovery showed only signatures of dust. We used the coud{é} spectrograph on the McDonald Observatory 2.7,m telescope to obtain spectra, starting when the comet was at a heliocentric distance of 5.06,{\sc au} and following it until 2.47,{\sc au}, to determine what spectral features would appear at each heliocentric distance. The first heliocentric distance for which we detected any emission from the gas was 3.39,{\sc au}, when we first detected CN. As the comet continued inward towards the Sun, various other species were detected. We discuss the implications of the early turn-on of CN and of species first appearing at different heliocentric distances in the context of control of the activity by water.