CH 3 OH and HCN in Interstellar Comet 3I/ATLAS Mapped with the ALMA Atacama Compact Array: Distinct Outgassing Behaviors and a Remarkably High CH 3 OH/HCN Production Rate Ratio

Abstract We report the detection of methanol (CH 3 OH) toward interstellar comet 3I/ATLAS using the Atacama Compact Array of the Atacama Large Millimeter/submillimeter Array (ALMA) on UT 2025 August 28, September 18 and 22, and October 1 and of hydrogen cyanide (HCN) on September 12 and 15. These observations spanned pre-perihelion heliocentric distances ( r H ) of 2.6–1.7 au. The molecules showed outgassing patterns distinct from one another, with HCN production being depleted in the sunward hemisphere of the coma, whereas CH 3 OH was enhanced in that direction. Statistical analysis of molecular scale lengths in 3I/ATLAS indicated that CH 3 OH included production from coma sources at L p > 258 km at 99%…