Observations of the 3.4 mm line of HCN in C/2020 T2 (Palomar)

ABSTRACT The observations of comet C/2020 T2 (Palomar) were carried out using the 3.4 mm wavelength microwave band before perihelion from 2021 January 22 to July 5 and after perihelion on 2021 July 13. During this period, the comet was located at a heliocentric distance of between $r_\text{h}$ = 2.840 and 2.055 au. The consecutive long-term monitoring of outgassing of C/2020 T2 was conducted with Purple Mountain Observatory (PMO) 13.7 m telescope and the Institut de RadioAstronomie Millimétrique (IRAM) 30 m telescope in the atmospheric radio window. The hyperfine triplet components of hydrogen cyanide (HCN (1−0) F = 0−1, F = 2−1, and F = 1−1) of the J = 1−0 vibrational ground-state transitions, as the primary tracer of molecular gas, were unambiguously identified in this comet C/2020 T2. Combining all data, we derived the positive signal of line width corresponds to the coma expansion velocity $v_{\rm exp}$ from $\sim$0.2 to $\sim$0.4 km s$^{-1}$. The mean gas molecular production rates of HCN were derived $Q_{\rm HCN}$ = (2.92 $\pm$ 0.51) $\times$ 10$^{25}$ molecules s$^{-1}$ at PMO 13.7 m, and $Q_{\rm HCN}$ = (6.26 $\pm$ 1.55) $\times$ 10$^{25}$ molecules s$^{-1}$ at IRAM 30 m, respectively. Overall, studying the gas composition of the long-period comet revealed abundant information about the missing link between interstellar molecular clouds and the outer regions of a distant protoplanetary accretion disc, and the relationship between amorphous ice sublimation mechanism and heliocentric distance.