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Coma Volatile Composition of Comet C/2017 K2 (PanSTARRS) at Heliocentric Distances 2.5–3.0 au

Presentation #411.07 in the session Cometary Volatiles (iPosters).

Published onOct 20, 2022
Coma Volatile Composition of Comet C/2017 K2 (PanSTARRS) at Heliocentric Distances 2.5–3.0 au

Comets are generally considered to be the most primitive bodies in the solar system, and hence are often used to constrain conditions that existed in the protoplanetary disk mid-plane where (and when) they formed, providing information about the formation and subsequent evolution of primitive material in the early solar system [1]. To fully understand the evolution of coma composition and outgassing in comets, observations near the critical region (~2.5–3.0 au from the Sun) where comets transition from the release of predominantly hypervolatiles, such as CO, to H2O-dominated activity are crucial. We present results from pre-perihelion high-resolution, near-infrared spectroscopic observations of Oort cloud comet C/2017 K2 (PanSTARRS; hereafter K2) spanning UT 08 May – 18 June 2022, during which the heliocentric distance (rH) decreased from ~3.24 to 2.88 au. This allowed investigating how its hypervolatile abundances evolved over this heliocentric range. Our observations were obtained using the iSHELL instrument, a high resolution spectrograph at the NASA Infrared Telescope Facility (IRTF) with sensitivity in the 1.2–5.4 μm wavelength range. The spectra were acquired using the 15″ x 0.″75 slit, providing a resolving power (λ/Δλ) ~ 45,000. We targeted CO, C2H6, CH4, and H2O to characterize the pre-perihelion hypervolatile composition of K2, and to test for the onset of H2O-dominated sublimation. As of writing this abstract, we have four additional blocks of observations scheduled at rH ~2.4 au and spanning UT 10 – 20 Aug 2022, and we plan to incorporate results from these into our presentation. K2 started to show activity at ~35 au from the Sun [2]. This is particularly interesting because, other than C/1995 O1 Hale-Bopp, comets showing activity at large heliocentric distances have not had their volatile composition characterized pre-perihelion beyond ~2 au. Our observations, performed at rH ≥ ~2.88 au, indicate that although the hypervolatiles CO, C2H6, and CH4 were definitively detected in K2, H2O was not yet fully activated at these heliocentric distances. We will report gas rotational temperatures (Trot), production rates (Qs), and mixing ratios (or meaningful upper limits) with respect to CO and (assuming it becomes active) H2O. We will discuss the composition of K2 and place it into context by comparing with compositional measurements of other comets performed at near-infrared wavelengths.

Various co-authors gratefully acknowledge support from NSF AARG and NASA SSO Programs.


[1] Dello Russo et al. 2016, Icar, 278, 301

[2] Jewitt et al. 2021, AJ, 161:188

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