2024A&A...688L...1H

Context. Class I methanol masers provide sensitive information about the shocked environment around star-forming regions. Among the brightest Class I methanol masers, we have those in the J_–1 → (J - 1)₀ - E line series, currently reported for the J = 4 - 9 transitions, with the only exception being the J = 7 one at 181.295 GHz, and never expanded to higher J transitions.
Aims. We aim to search for population inversion in the 7_–1 → 6₀ - E and 10_–1 → 9₀ - E methanol transition lines at 181.295 and 326.961 GHz, respectively, and also extend the number of known low-mass star-forming sources harboring Class I methanol masers.
Methods. We employed the Atacama Pathfinder Experiment (APEX) 12 m telescope to survey low-mass Galactic sources, focusing on methanol emission lines. We built rotation diagrams for all sources with detected J = 7 methanol line transitions, while employing radiative transfer modeling (both in and out of local thermodynamic equilibrium) to characterize methanol excitation conditions in detail for one specific source with detected masers.
Results. We detected the 7_–1 → 6₀ - E and 10_–1 → 9₀ - E methanol transitions in 6 out of 19 sources. Among them, we firmly determined the 10_–1 → 9₀ - E maser nature in CARMA 7, L1641N, NGC 2024, and Serpens FIRS, and we show evidence for the presence of inverted population emission in the 7_–1 → 6₀ - E line toward CARMA 7 and L1641N. This represents the first report of methanol maser emission in these particular transitions. Our study supports previous works indicating that conditions for Class I methanol maser emission are satisfied in low-mass star-forming regions and expands the range of detectable frequencies toward higher values.