Observations of the Hydrogen Cyanide in Comet 46P/Wirtanen at a 3.4 mm Wavelength

We searched for rotational lines of H¹²C¹⁴N in comet 46P/Wirtanen, using the Purple Mountain Observatory 13.7 m radio telescope. The J = 1-0 lines of HCN (3.4 mm wavelength) were unambiguously detected in 46P during the 2018 December 14-15 period, with a beam resolution of 62.″2 and a velocity resolution of ∼0.21 km s^-1 per channel. The molecular production rates were computed. The mean HCN(1-0) production rate of 46P was (6.45 ± 1.31) × 10²⁴ molec s^-1 in the above two days. We found possible indications of time variation in the outgassing activity. The average HCN abundance relative to water Q _[HCN]/Q_[H2O] of 46P was (0.09 ± 0.01)%. The HCN line shapes gave primary information on the kinematics of the cometary atmosphere. The HCN linewidths provided a direct determination of the coma expansion velocity, which was about 0.5 ± 0.05 km s^-1 at post-perihelion (1.056 au).