High Spectral Resolution Observations of Comet C/2020 F3 (NEOWISE) From McDonald Observatory

We report on observations of comet C/2020 F3 (NEOWISE) obtained on 24 July and 11 August 2020 UT. The observations were obtained with the Tull Coudé Spectrograph (Tull et al. 1995 (PASP 107,251)) on the 2.7m Harlan J. Smith Telescope at McDonald Observatory. The spectra cover the spectral range from 3700 to over 10,000 Å, with complete coverage from 3700–5700 Å and increasing interorder gaps redward of 5700 Å. The spectra have a resolving power R = λ/Δλ = 60,000. The comet was at a heliocentric distance of 0.65 AU and a geocentric distance of 0.69 AU on 24 July and a heliocentric distance of 1.03 AU and geocentric distance of 1.05 AU on 11 August. These spectra have very high signal/noise. They show well developed emissions due to CN, C₃, C₂, CH, NH₂, Na and [O I]. With these spectra we can determine the rotational temperature of C₂, calculate isotopic abundances of carbon and nitrogen, determine the oxygen line ratios to continue our work on developing a proxy for measuring CO₂ from the ground, etc. With observations on two nights, one near 1 AU and one much closer to the Sun, we can measure how the spectra change with heliocentric distance.

The observations reported in this paper were obtained at The McDonald Observatory, operated by The University of Texas at Austin. This work was supported by NASA Grant NNX17A186G.