JCMT detection of HCN emission from 3I/ATLAS at 2.1 AU

Abstract

We report the detection of HCN ($J=3-2$) rotational emission from comet 3I/ATLAS at a heliocentric distance of 2.13 AU with the James Clerk Maxwell Telescope (JCMT). Observations were conducted from 07 August 2025 and 14 September 2025 (UT) using the ${}^{\prime }{\overline{U}}^{\prime }\overline{u}$ heterodyne receiver and ACSIS spectroscopic backend. The HCN line was detected at $>6\ddot{I}ƒ$ on 14 Sep 2025 (UT) and a production rate of $Q(\mathrm{HCN})=(4.0\pm 1.7)\times 10^{25}{\mathrm{s}}^{-1}$ was derived by non-LTE radiative transfer modelling. Preliminary estimates of the HCN/H${}_2$O and CN/HCN abundance ratios suggest values similar to Solar System comets.