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Characterizing the Native Ice Composition of Comet C/2022 E3 (ZTF) with NIRSpec at Keck 2

Presentation #322.05 in the session Comets (Poster + Lightning Talk)

Published onOct 23, 2023
Characterizing the Native Ice Composition of Comet C/2022 E3 (ZTF) with NIRSpec at Keck 2

Overview. We present results of native ice composition for C/2022 E3 (hereafter E3), a long-period comet from the Oort cloud, based on near-IR (λ ~ 2.8 – 3.7 µm) observations using NIRSPEC, the long-slit (24"), cross-dispersed, high-resolution (λ/Δλ ~ 25,000) spectrograph at Keck 2. NIRSPEC was upgraded in 20181, resulting in substantially improved sensitivity. We present production rates and abundance ratios for H2O and seven trace molecules (C2H6, CH3OH, HCN, CH4, C2H2, NH3, and H2CO). Compared with their mean abundances among Oort cloud comets,2 our study reveals an overall depleted volatile composition for E3. We also compare spatial profiles of emissions in the coma, as these provide information regarding associations among ices in the nucleus.

Spectral Studies. Post-perihelion spectra of E3 were obtained on UT 2023 January 20 & 21. To characterize the eight targeted parent volatiles, two L-band echelle/cross-disperser settings were used (KL1 & KL2), each one including many lines of H2O plus multiple trace species. High signal-to-noise spectra were obtained in both settings on each date. KL1 includes strong emissions from H2O, C2H6 and CH3OH, thus relatively little time was spent on this setting (~20 minutes on-source/day). KL2 includes difficult-to-detect species having weaker emissions (C2H2, H2CO, NH3); therefore, ~80 minutes on-source/day were accrued. Both KL1 and KL2 also cover lines of CH4 that require sufficient geocentric Doppler shift (Δdot) to displace cometary lines from their opaque telluric counterpart absorptions (for E3, Δdot ~ -45 km s-1). Post-upgrade, the combined (KL1 + KL2) spectral coverage is more nearly complete from ~2.8 – 3.7 µm. This complements our concurrent study of E3 with iSHELL3 that features both L-band and M-band (~4.4 – 5.2 µm) spectra, the latter regime co-measuring CO and OCS with H2O.

Spatial Studies. The long slit permits studies of outgassing in the coma. One interesting question is whether differences exist for polar-dominated versus apolar-dominated ices, as has been observed in some comets (e.g., C/2007 W1 Boattini4, 103P/Hartley 25). We will assess this for E3, comparing outgassing patterns and production rates/abundances from observations separated by approximately 24 hours.

Acknowledgments: Support is recognized from NSF AARG and NASA SSO and EW programs, and from Keck Observatory staff to ensure the success of this study.

References: 1Martin E. C. et al. (2018) Proc. SPIE 10702, 107020A-1; 2Dello Russo, N. et al. (2016) Icar 278, 301; 3Rai et. al., this conference; 4Villanueva G. L. et al. (2011) Icar, 216, 227; 5Mumma, M. J. et al. (2011) ApJ, 734, L7

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