HARPS-N spectroscopy of the comet C/2020 F3 (NEOWISE)

Comet C/2020 F3 (NEOWISE) is considered the brightest comet in the northern hemisphere since comet C/1995 O1 (Hale-Bopp) in 1997. The perihelion occurred on 3 July, 2020, at a distance of 0.29 AU, and its closest approach to Earth occurred on July 23, 2020, at a distance of 0.69 AU. The unique passage of comet NEOWISE, in combination with the high resolution of HARPS-N (High Accuracy Radial velocity Planet Searcher in North hemisphere), offered a unique opportunity to deeply investigate the emission spectrum of a comet at high resolution.

Two high-resolution spectra of comet NEOWISE were obtained on 26 July (heliocentric distance of 0.72 au) and 5 August (heliocentric distance of 0.93 au), 2020. The spectra cover the range between 383 and 693 nm, and the spectral extraction was performed automatically by the HARPS-N Data Reduction Pipeline. HARPS-N data allowed to identify more than 4000 comet emission lines, which represents the 87% of the total emission lines (5159). The identified cometary lines belong to C2, C3, CN, CH, NH2, NaI, and [OI]. In particular, the structure of the C2 Swan system and the 4050 Å C3 group are revealed. The P-branch of the definite structure of CN in the 3883 Å sequence is resolved into individual lines, and the (0-0) B2Σ-X2Σ band is resolved from CN thanks to the resolution of our spectra. Strong emission of atomic oxygen [OI] has been detected in both spectra, and the cometary green and red lines are well solved and separated from the corresponding telluric lines. This allows to calculate the intensity ratio between these lines, which is about 0.1, suggesting that H2O is the main source of the atomic oxygen. The NEOWISE spectrum of 26 July, 2020, reveals the presence of sodium, which is quite rare to find in a comet. The identified sodium emissions belong to the sodium doublet at 5889.95 (D2) and 5895.92 (D1) Å. The spectrum of 5 August, 2020, shows the telluric lines, but the cometary sodium emission is no longer evident. A comparison of the HARPS-N spectra with those obtained with the FLECHAS spectrograph (Bischoff & Mugrauer 2021) reveals that the NEOWISE sodium emission follows an exponential decrease. Another important result is that this comet, despite being very bright and close to the Sun, does not show any iron emission lines.

More details in the paper "A high spectral resolution catalog of emission lines in the visible spectrum of comet C/2020 F3 (NEOWISE)", Cambianica et al., 2021, A&A 656, A160

The video of the talk given by Pamela Cambianica at the event “I 25 ANNI DEL TNG” is available at Youtube.

Comet NEOWISE over Mount Hood, USA, on July 11, 2020. Credit: Kevin Morefield Getty Images.

Comet NEOWISE observed on 9 August, 2020 at the Osservatorio astrofisico e astronomico di Asiago.

Identification of emission lines in the spectrum of comet C/2020 F3 (NEOWISE). The spectrum refers to 26 July, 2020. Figure from Cambianica et al 2021b, A&A, in press.