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

TL;DR

This paper uses ALMA-ATOMS observations to quantify methanol excitation and chemical richness in hot molecular cores by separately analyzing CH$_3$OH A-type and E-type lines (v=0) and the E-type vibrationally excited vt$=1$ transitions. It finds that E-type v=0 lines are strongly affected by non-LTE effects and anomalously bright sub-thermally excited transitions, while vt$=1$ lines trace warm, dense gas with near-LTE-like behavior, making vt$=1 a robust tracer of hot-core conditions. The authors introduce two quantitative richness metrics, $F_{vt=1}$ and CDR$_{norm}$, and show that both hot-core molecular emission and COM emission scale with these metrics, indicating that molecular richness is governed by the overall warm, inner core environment rather than a few bright species. Their results provide a practical framework for identifying chemically rich hot cores in large ALMA surveys and highlight the importance of non-LTE modelling when interpreting methanol emission in star-forming regions.

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.

Figures (11)

• Figure 1: Line Images. Background: 3-mm continuum (100 GHz), bottom left circle shows the beam. Contours from left to right: Red:CH$_{3}$OH-A v=0 at 97582.8 MHz, CH$_{3}$OH-A v=0 at 97678.3 MHz; Green: CH$_{3}$OH-E v$_{t}$=1 at 99730.9 MHz, CH$_{3}$OH-E v$_{t}$=1 at 99772 MHz and 99776 MHz; Blue: CH$_{3}$OH-E v=0 at 100638.9 MHz, CH$_{3}$OH-E v=0 at 101293.3 MHz. The E$_u$ of the transitions are indicated in the first figure, see Table \ref{['Table 1']}. The lowest contour level is set at 3$\sigma$, with each subsequent contour increasing by a factor of 1.4 times. The complete figure set (94 images) is available in the online journal.
• Figure 2: Examples of the best-fit results from the myXCLASS function for spectra. The black line represents the observed values; the red, yellow, and blue lines represent the fitted values of CH$_{3}$OH-A v=0, CH$_{3}$OH-E v$_{t}$=1, and CH$_{3}$OH-E v=0, respectively. Notes: Hot Core I11298-6155 has no H${40\alpha}$ line and weak other molecular lines; hot core I16272-4837c1 has no H${40\alpha}$ line but shows rich emission lines of COMs; hot core/UC H II region I18507+0110 has a H${40\alpha}$ line and exhibits rich emission lines of COMs. The complete figure set (94 images) is available in the online journal.
• Figure 3: Top: Integrated intensity of CH$_{3}$OH-E v$_{t}$=1 vs. Integrated intensity of COMs in SPW7 and 8. Bottom: Integrated intensity vs. channel detection ratio of COMs. Note:In calculating the integrated intensities, we excluded the following frequencies and their adjacent channels: 97981.0 MHz (CS); 99299.9 MHz (SO); 100076.4 MHz (HC$_3$N); 97714.9 MHz, 97702.3 MHz (SO$_2$); 99023.0 MHz (H${40\alpha}$); and other ionized emission lines at 99225 MHz, 98199 MHz, 98671 MHz, and 100540 MHz. We did not specifically exclude the H$_2$CCO, H$_2$CO, NH$_2$D, and high-vibration transitions of HC$_3$N2025AA...694A.166C, although these molecules are not COMs, their emission features are similar to those of COMs.
• Figure 4: Top: The rotational temperature of CH$_{3}$OH-A v=0 (red) and CH$_{3}$OH-E v=0 (blue) versus CH$_{3}$OH-E v$_{t}$=1. Bottom: The column density comparison for CH$_{3}$OH-A v=0 (red), CH$_{3}$OH-E v=0 (blue) with respect to CH$_{3}$OH-E v$_{t}$=1.
• Figure 5: RADEX-calculated methanol line intensities are shown in blue, while the black spectra display the observed lines toward I16272-4837c1 (already divided by the beam filling factor).Both models adopt a column density of $2.7\times10^{18}~\mathrm{cm^{-2}}$ and a temperature of $230$ K. The left panel uses a hydrogen density of $3\times10^{9}~\mathrm{cm^{-3}}$, and the right panel $6\times10^{8}~\mathrm{cm^{-3}}$.