Ionic emission from and activity evolution in comet C/2020 F3 (NEOWISE): Insights from long-slit spectroscopy and photometry

¹ Space sciences, Technologies & Astrophysics Research (STAR) Institute, University of Liège, Liège, Belgium
² Physical Research Laboratory, Ahmedabad-380009, India
³ Cadi Ayyad University (UCA), Oukaimeden Observatory (OUCA), Faculté des Sciences Semlalia (FSSM), High Energy Physics, Astrophysics and Geoscience Laboratory (LPHEAG), Marrakech, Morocco
⁴ Université Marie et Louis Pasteur, CNRS, Institut UTINAM (UMR 6213), OSU THETA, BP 1615, 25010 Besançon Cedex, France
⁵ Université Bourgogne Europe, CNRS, Laboratoire Interdisciplinaire Carnot de Bourgogne ICB UMR 6303, 21000 Dijon, France
⁶ Indian Institute of Astronomy, Bangalore-56034, India

^★ Corresponding author: aravind.krishnakumar@uliege.be

Received: 28 March 2025
Accepted: 28 July 2025

Abstract

Aims. The long-period comet C/2020 F3 (NEOWISE) was the brightest comet in the northern hemisphere since C/1995 O1 (Hale-Bopp). These comets offer a unique opportunity to study their composition and the spatial variation in the different emission in detail. We conducted long-slit low-resolution spectroscopy and narrow-band photometry to track the evolution of its activity and composition during several weeks after perihelion. The images were used to compute the production rates of neutral molecular species and dust, and the spectrum was used to analyse the variation in the emission along the spatial axis in the sunward and anti-sunward directions to detect ionic emission.

Methods. Narrow-band (OH[3090 Å], NH[3362 Å], CN[3870 Å], C₂[5140 Å], C₃[4062 Å], BC[4450 Å], GC[5260 Å], and RC[7128 Å]) and broad-band (Johnson-Cousins B, V, Rc, Ic) images of comet C/2020 F3 were taken with TRAPPIST-North from 22 July to 10 September 2022 to track the production rates, the evolution of the chemical mixing ratios with solar distance, and the proxy to the dust production (A(0)fρ). A long-slit low-resolution spectrum was obtained on 24 July 2020 using HFOSC on the 2 m HCT at IAO, Hanle. Spectra extracted along the spatial axis in the sunward and anti-sunward directions enabled a comparative analysis of the emission in both directions.

Results. We report production rates and mixing ratios of OH, NH, CN, C₂, C₃, and NH₂ and used the flux density of the forbidden oxygen line to derive the water-production rate. Ionic emission from N₂⁺, CO⁺, CO₂⁺, and H₂O⁺ was detected at 4 × 10⁴ km to 1 × 10⁵ km from the photocentre in the tail direction. The average N₂⁺/CO⁺ ratio for the CO⁺ (3-0) and (2-0) bands measured from the spectrum was (3.0 ± 1.0) × 10^–2, which we further refined to (4.8 ± 2.4) × 10^–2 using fluorescence modelling techniques. We measured the CO₂⁺/CO⁺ ratio to be 1.34 ± 0.21. Combining the N₂⁺/CO⁺ and CO₂⁺/CO⁺ ratios, we suggest the comet to have formed in the cold mid-outer nebula (~50–70 K). Furthermore, the average rotation period of the comet was calculated to be 7.28 ± 0.79 hours with a CN gas outflow velocity of 2.40 ± 0.25 km/s.