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

As a comet approaches the Sun, its surface gets heated. At some point, the surface materials will become hot enough to sublime. Depending on what is on the surface, the observed activity will be controlled by the sublimation of CO, CO₂ or H₂O. During 2021 – 2022, we had the opportunity to observe comet C/2017 K2 (PanSTARRS) – hereafter K2 – approach the Sun, with observations starting at a heliocentric distance of 5.05 AU. Observations of K2 were obtained with the Tull coude spectrograph (Tull et al. PASP 107, 251, 1995) on the 2.7m telescope at McDonald Observatory as follows:

(Table omitted.)

Observations will be continued using the 2.7m telescope in early August and a Las Cumbres Observatory 1m telescope with an NRES spectrograph after that, but we will not know which nights were successful before the abstract deadline.

We first detected CN, with a possible very weak detection in January 2022 and a definite detection in April 2022. C₃ and possibly C₂ followed in subsequent runs. CH made an appearance in June. NH₂ is possibly detected in the later data. Forbidden oxygen was observed whenever there was sufficient Doppler shift to distinguish the cometary and telluric oxygen lines.

These observations will allow us to determine if the molecule driving activity changes with heliocentric distance (e.g as with C/2009 P1 (Garradd), McKay et al. Icarus 250, 504, 2015). We will show spectra and report on our findings, including spectra obtained after the abstract deadline.