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

TL;DR

This study presents high spectral resolution observations of comet C/2017 K2, revealing the onset of gas emissions at specific distances from the Sun and providing insights into cometary activity mechanisms.

Contribution

First high-resolution spectral observations of comet C/2017 K2 tracking gas emission onset at various heliocentric distances.

Findings

Detected CN emission at 3.39 au from the Sun.

Observed the appearance of different species as the comet approached the Sun.

Provided insights into the activity control by water in comets.

Abstract

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{\'e} 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.