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

TL;DR

This study reports the first detection of HCN in the interstellar comet 3I/ATLAS observed with JCMT at a heliocentric distance of 2.1 AU. Using a 1D non-LTE SUBLIME radiative transfer model, the authors derive a production rate $Q({\rm HCN})=(4.0\pm1.7)\times10^{25}$ s$^{-1}$ and an HCN/H2O abundance of $(2.0\pm0.8)\times10^{-3}$. The line fit also yields an HCN outflow velocity of $0.46\pm0.14$ km s$^{-1}$ and a CN/HCN ratio suggesting CN largely arises from HCN photolysis. The results indicate Solar System–like volatile chemistry in an interstellar object and establish a baseline for ISO–Solar System comparisons, enabling future monitoring of chemistry and coma structure.

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 (UT) using the $^{\prime}\overline U^{\prime}\overline u$ heterodyne receiver and ACSIS spectroscopic backend. The HCN line was detected at $>5σ$ on 14 Sep 2025 (UT) and a production rate of $Q({\rm HCN})=(4.0\pm1.7)\times10^{25}\ {\rm s}^{-1}$ 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.

Figures (2)

• Figure 1: HCN($J=3-2$) spectra of comet 3I/ATLAS obtained at JCMT. on (a) (top) UT 07 Sep 2025 (b) (bottom) UT 14 Sep 2025. Data are shown in the cometocentric velocity frame at 1.0 km s$^{-1}$ resolution.
• Figure 2: SUBLIME model fit to the HCN($J=3-2$) line observed on September 14. Here, the spectrum is shown at a 0.24 km/s channel binning, with the strongest 5 HCN hyperfine components labeled by tick marks. Details of the line profile fitting are given in the text.