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

TL;DR

Using high-resolution optical spectroscopy from McDonald Observatory and Keck, the study tracks the onset of gas-phase emission in comet C/2017 K2 (PanSTARRS) as it approaches perihelion. CN emission appears first at Rh=3.39 au, followed by detections of C3, CH, C2, and NH2 at progressively smaller distances, while forbidden oxygen lines indicate H$_2$O as a primary parent with modest distance-dependent variation. JWST observations near 2.35 au reveal CO and CO$_2$ contributions alongside dominant H$_2$O, and ground-based data suggest elevated volatile-to-water ratios, pointing to a more complex, possibly heterogeneous volatile inventory. The outbound detections of CN beyond 4 au imply clumpy, volatile-driven activity on an Oort Cloud comet, challenging the notion that water is the sole driver of early activity and highlighting the value of far-distance, high-resolution spectroscopy for understanding cometary volatiles.

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{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.

Figures (9)

• Figure 1: An example of a spectrum from 25 Jun 2022 in the region of the C$_{2}$ (0,0) bandhead is shown. The top panel is the spectrum with the molecular emission spectrum plus the absorption and continuum from dust reflected sunlight. The bottom panel shows that spectrum with the solar contribution removed. In the lower panel, many emission lines are quite obvious that cannot be seen in the upper panel, including the C$_{2}$ (0,0) bandhead at $\sim$5165Å.
• Figure 2: The CN B$^2\Sigma^+$ -- X$^2\Sigma^+$ order of the spectra for each pre-perihelion night except 15 Jan 2022 are shown. There are clearly no CN lines in the spectrum from 18 Oct 2021 when the comet was at a heliocentric distance of 5.06 au. By the time the comet is at 3.39 au on 22 Apr 2022, the CN band is well developed. The dashed lines indicate the lowest j-levels of the P branch (right 2) and of the R branch (left 2).
• Figure 3: The C$_{3}$ A$^1\Pi_u$ -- X$^1\Sigma_g^1$ (0,0,0) - (0,0,0) order of the spectra for each pre-perihelion night except 15 Jan 2022 are shown. There are clearly no C$_{3}$ lines in the spectrum from 18 Oct 2021 at a heliocentric distance of 5.06 au nor at R$_h$=3.39 au on 22 Apr 2022. By 9 May 2022 at 3.24 au, the Q-branch bandhead at 4050Å is clearly visible.
• Figure 4: The CH A$^2\Delta$ -- X$^2\Pi$ band does not have many lines. No CH lines are visible until 10 Jun 2022 at R$_h$=2.95 au. The spectrum from 9 May 2022 is ambiguous for a line detection.
• Figure 5: The C$_{2}$ d$^3\Pi_g$ -- a$^3\Pi_u$$\Delta v=0$ order of the spectra for each pre-perihelion night except 15 Jan 2022 are shown. There are clearly no C$_{2}$ lines in the spectrum from 18 Oct 2021 at a heliocentric distance of 5.06 au. The spectrum from 22 April 2022 probably shows no features, while the (0,0) bandhead is possibly visible on 9 May 2022. The (0,0) bandhead is obviously present on 10 Jun 2022 while the (1,1) bandhead begins to be believable on 31 Jul 2022. The two bandheads are marked with dashed lines.