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% confidence. However low signal-to-noise on long baselines, which sample emission on small spatial scales closest to the nucleus, prevented definitively ruling out CH${}_3$OH as purely a parent species. In contrast, HCN production was indistinguishable from direct nucleus sublimation. The CH${}_3$OH production rate increased sharply from August through October, including an uptick near the inner edge of the H${}_2$O sublimation zone at $r_H$ = 2 au. Compared to comets measured to date at radio wavelengths, the derived CH${}_3$OH/HCN ratios in 3I/ATLAS of $124_{-34}^{+30}$ and $79_{-14}^{+11}$ on September 12 and 15, respectively, are among the most enriched values measured in any comet, surpassed only by anomalous solar system comet C/2016 R2 (PanSTARRS).