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Compositional measurements as comet C/2017 K2 (Pan-STARRS) transitioned from hypervolatile to water-dominated activity

Presentation #322.03 in the session Comets (Poster)

Published onOct 23, 2023
Compositional measurements as comet C/2017 K2 (Pan-STARRS) transitioned from hypervolatile to water-dominated activity

One unanswered question in comet science is what drives cometary volatile activity at large heliocentric distances. Beyond about 3 au, water is not expected to be fully activated, and hence the activity is likely driven by hypervolatile molecules like CO and CO2. Where the transition to water activation occurs remains uncertain because few comets have been observed at near-infrared (NIR) wavelengths (~1– 5 μm) over this transitional heliocentric distance (rH ~2.5 – 3.5 au). Thus, measurements in this critical range are crucial for understanding the evolution of outgassing activity. However, most comets are quite faint at these distances, or their activity levels are too low to accurately measure CO and water production. Comet C/2017 K2 (Pan-STARRS) (hereafter K2) provided a rare opportunity to measure these molecules in this critical range of rH, as it exhibited activity at rH = 23.7 au (based on pre-discovery imaging) [1, 2, 3], and a CO detection was reported at rH = 6.72 au [4]. We present observations of long-period comet K2 over rH ranging from 3.24 – 2.35 au (from 2022 May - August), spanning 11 pre-perihelion observing blocks. Our observations were acquired with both iSHELL and NIRSPEC, high-resolution (λ/Δλ > 20,000), cross-dispersed, NIR echelle spectrographs at the NASA Infrared Telescope Facility (IRTF) and Keck 2, respectively. Our study emphasized the hypervolatiles CO, C2H6, and CH4, while also simultaneously testing other volatile species (CH3OH, H2O, HCN, C2H2, NH3, OCS, CN, plus OH*, a proxy for H2O production). We report gas rotational temperatures (Trot), molecular production rates (Q), and mixing ratios (or stringent upper limits) of the targeted species with respect to CO, C2H6, and (when it was active) H2O. To put our K2 results in context, we compare our production rates and mixing ratios to other comets from the Oort cloud (the source of long-period comets, including K2) observed at NIR wavelengths, particularly those observed at larger heliocentric distances.

This work was supported by the NSF AARG program (award 2009910). We also thank the IRTF and Keck staffs for helping to make these observations successful.


[1] Hui et al., 2018, AJ 155: 25

[2] Meech et al., 2017, APJL 849:L8

[3] Jewitt et al., 2017, APJL 847:L19

[4] Yang et al., 2021, APJL 914:L17

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