The ALMA-ATOMS survey: Methanol emission in a large sample of hot molecular cores

Abstract

Methanol (CH${}_3$OH) is a key complex organic molecule (COM) in the interstellar medium, widely used as a tracer of dense gas and hot molecular cores (HMCs). Using high-resolution ALMA observations from the ATOMS survey, we investigate the excitation and abundance of methanol nuclear spin isomers and their relationship to chemical complexity in massive star-forming cores. We identify 20 methanol transitions, including A- and E-type lines in the v=0 state and E-type lines in the v${}_t$=1 state, and detect 94 HMC candidates. Rotational temperature analysis under the LTE assumption yields average values of 194 $\pm$ 33 K for CH${}_3$OH-E v${}_t$=1, 178 $\pm$ 33 K for CH${}_3$OH-A v=0, and 75 $\pm$ 33K for CH${}_3$OH-E v=0. Emission from COMs other than methanol is detected in 87 of the 94 cores, with the CH${}_3$OH-E v${}_t$=1 line intensity showing a strong correlation with the channel detection ratio (CDR). These results demonstrate that CH${}_3$OH-E v${}_t$=1 lines are reliable tracers of HMCs and chemical complexity, and that the CDR provides a robust indicator of molecular richness. The temperature difference between A- and E-type methanol transitions is driven by anomalously strong J(2,J-2)$-$J(-1,J-1) lines, highlighting the importance of analyzing methanol symmetry types separately.

Accepted for publication in The Astrophysical Journal on January 24, 2026