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Atomic and molecular emissions of bright comet C/2020 F3 (NEOWISE) at 0.5 au

Presentation #111.02 in the session “Comet Comae: C/2020 F3 (NEOWISE)”.

Published onOct 26, 2020
Atomic and molecular emissions of bright comet C/2020 F3 (NEOWISE) at 0.5 au

We conduct a search for various atomic, molecular and ionic emissions from bright comet C/2020 F3 (NEOWISE) via ultra-high resolution spectroscopy with the EXtreme PREcision Spectrograph (EXPRES) on the 4.3 m Lowell Discovery Telescope. Observations were taken at around 12 hours UTC on 2020 July 15 and 16 with an on-sky aperture of 0.9" in diameter (500 km at the distance of the comet) centered at the nucleus. Observations were acquired during nautical twilight at an airmass of ~10. The comet was then at heliocentric distances of 0.46 and 0.48 au in its outbound leg. The spectrum covers a wavelength of 3800 to 8200 Angstrom at a resolution of R~140,000. We detect numerous lines from major cometary species such as CN, C2, C3, and NH2. Strong sodium and potassium lines are also present. We also detect H2O+ lines in the visible-red wavelength (5800 to 7400 Ang), though they are much weaker than the sodium line, suggesting that the red straight tail reported by numerous observers in mid-July was likely dominated by sodium atoms rather than H2O+ ions. The forbidden oxygen doublet at 6300 and 6364 Ang is also detected and consistent with the known branching ratio of ~3. During preliminary analyses, we did not detect any S2 lines (shortward of 4500 Ang), nor any metallic lines other than sodium and potassium, such as Li I (6708 Ang) and various Fe I lines. We also did not detect any N2+, CO+ and CO2+ lines in the near-ultraviolet, though this is likely in part due to the reduced sensitivity of EXPRES at these wavelengths as well as considerable atmospheric extinction.


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