Torsionally excited methanol in massive young stellar objects: The role of the mid-IR radiation field
Abstract
Observing the earliest stages of high-mass star formation in the mid-infrared (mid-IR) is challenging because the extinction is high and the resolution limited. Millimeter vibrationally and torsionally excited transitions of several molecules have been proposed as indirect probes of local mid-IR radiation fields around young stellar objects (YSOs). We investigate the excitation and spatial distribution of torsionally excited methanol (CH3OH) in 12 high-mass star-forming clumps in different evolutionary stages, assessing their potential as tracers of the mid-IR luminosity of embedded protostars. We analyzed rotational transitions of methanol within the same torsionally excited states vt≥1 in the Atacama Large Millimeter/submillimeter Array (ALMA) 890 μm observations and compared them with Spitzer 24 μm data by plotting line luminosities normalized by methanol abundance against mid-IR luminosities. The torsionally excited methanol emission is generally compact and systematically coincident with ALMA dust continuum peaks and 24 μm emission, even in faint sources. The only clump without an IR counterpart also shows no vt≥1 emission, supporting radiative pumping as the dominant excitation mechanism. We found a promising power-law trend between vt≥1 methanol lines and 24 μm luminosities, largely independent of the methanol abundance. CH3OH vt≥1 transitions appear to be promising tracers of local mid-IR radiation fields of embedded protostars, providing core-scale information even in highly extincted regions. As a pilot study, our results suggest that extending our study to a larger statistical sample is crucial for testing and improving this method.
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BibTeX
@misc{sanna2026torsionally,
title = {{Torsionally excited methanol in massive young stellar objects: The role of the mid-IR radiation field}},
author = {C. Sanna and S. Leurini and A. Giannetti and E. Schisano and L. Testi and S. Molinari and J. S. Urquhart and T. Pillai and F. Massi and K. Immer and E. Molinari and S. Casu},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.37955},
eprint = {2609.37955},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.37955},
}
RIS
TY - UNPB
TI - Torsionally excited methanol in massive young stellar objects: The role of the mid-IR radiation field
AU - C. Sanna
AU - S. Leurini
AU - A. Giannetti
AU - E. Schisano
AU - L. Testi
AU - S. Molinari
AU - J. S. Urquhart
AU - T. Pillai
AU - F. Massi
AU - K. Immer
AU - E. Molinari
AU - S. Casu
PY - 2026
DA - 2026/09/29
PB - arXiv
DO - 10.48550/arxiv.2609.37955
UR - https://arxiv.org/abs/2609.37955
AB - Observing the earliest stages of high-mass star formation in the mid-infrared (mid-IR) is challenging because the extinction is high and the resolution limited. Millimeter vibrationally and torsionally excited transitions of several molecules have been proposed as indirect probes of local mid-IR radiation fields around young stellar objects (YSOs). We investigate the excitation and spatial distribution of torsionally excited methanol (CH_3OH) in 12 high-mass star-forming clumps in different evolutionary stages, assessing their potential as tracers of the mid-IR luminosity of embedded protostars. We analyzed rotational transitions of methanol within the same torsionally excited states v_t≥1 in the Atacama Large Millimeter/submillimeter Array (ALMA) 890 μm observations and compared them with Spitzer 24 μm data by plotting line luminosities normalized by methanol abundance against mid-IR luminosities. The torsionally excited methanol emission is generally compact and systematically coincident with ALMA dust continuum peaks and 24 μm emission, even in faint sources. The only clump without an IR counterpart also shows no v_t≥1 emission, supporting radiative pumping as the dominant excitation mechanism. We found a promising power-law trend between v_t≥1 methanol lines and 24 μm luminosities, largely independent of the methanol abundance. CH_3OH v_t≥1 transitions appear to be promising tracers of local mid-IR radiation fields of embedded protostars, providing core-scale information even in highly extincted regions. As a pilot study, our results suggest that extending our study to a larger statistical sample is crucial for testing and improving this method.
ER -