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

TL;DR

This study uses ALMA observations to analyze methanol emission in hot molecular cores, revealing its role as a tracer of chemical complexity and demonstrating the significance of methanol symmetry types in temperature diagnostics.

Contribution

It provides the first comprehensive analysis of methanol nuclear spin isomers and their relation to chemical richness in massive star-forming regions.

Findings

Methanol lines are reliable tracers of hot molecular cores.

Channel detection ratio correlates with molecular richness.

Temperature differences are influenced by methanol symmetry types.

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.