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

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 between 07 August 2025 and 14 September 2025 (UT) using the ^\, Ū\, ^\, ū heterodyne receiver and ACSIS spectroscopic backend. The HCN line was detected at >6σ on 14 Sep 2025 (UT) and a production rate of Q(HCN) = (4.0 ± 1.7) × 1025 s−1 was derived by non-LTE radiative transfer modelling. Preliminary estimates of the HCN/H2O and CN/HCN abundance ratios suggest values similar to Solar System comets.