JCMT Constraints on the Early-Time HCN and CO Emission and HCN Temporal Evolution of 3I/ATLAS

Interstellar objects (ISOs), particularly those with cometary activity, provide unique insight into the primordial physical and chemical conditions present during the formation of the planetary system in which they originated. Observations in the sub-mm regime allow for direct measurements of several parent molecules released from the comet nucleus into the coma. Here we present observations of the third ISO, 3I/ATLAS, with the heterodyne receiver on the James Clerk Maxwell Telescope (JCMT), which targeted emission from HCN( J = 3 − 2 ) and CO( J = 2 − 1 ). Our observations, taken between 16 July 2025 and 21 July 2025 (UT), when 3I/ATLAS was at a heliocentric distance between 4.01 and 3.84 au, provide the earliest sub-mm constraints on its activity. We do not detect HCN or CO in these epochs, with 3 σ upper-limits on the production rates of Q ( H C N ) < 1.7 × 10 24 s at r h = 4.01 − 3.97 au and Q ( C O ) < 1.1 × 10 27 s at r h = 3.94 − 3.84 au, respectively. We combine this HCN limit with later JCMT observations of HCN to constrain its temporal evolution. Fitting the HCN detections with a Q ( H C N ) ∝ r h − n model and accounting for the upper-limits yields n = 12.7 − 2.5 + 6.9 . This slope is steeper than those of typical Solar System comets, but consistent with the production rate slopes measured for other species in the coma of 3I/ATLAS.