Letter to the Editor

First detection of the J₋₁ → (J − 1)₀ − E methanol maser transitions at J = 7 and 10

¹ Departamento de Astronomia, Instituto de Astronomia, Geofísica e Ciências Atmosféricas da USP, Cidade Universitária, 05508-090 São Paulo, SP, Brazil
e-mail: pedrokhumire@usp.br
² Max-Planck-Institut für Radioastronomie, Auf-dem-Hügel 69, 53121 Bonn, Germany
³ Joint ALMA Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile
⁴ Departamento de Física, Universidad Técnica Federico Santa María, Casilla 110-V, Avda. España 1680, Valparaíso, Chile
⁵ Instituto Nacional de Astrofísica, Óptica y Electrónica, Apartado Postal 51 y 216, 72000 Puebla, Mexico
⁶ Tecnologico de Monterrey, Escuela de Ingeniería y Ciencias, Avenida Eugenio Garza Sada 2501, Monterrey 64849, Mexico
⁷ School of Astronomy & Space Science, Nanjing University, 163 Xianlin Avenue, Nanjing 210023, PR China
⁸ Xinjiang Astronomical Observatory, Chinese Academy of Sciences, 830011 Urumqi, PR China
⁹ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Santiago 763-0355, Chile

Received: 5 June 2024
Accepted: 28 June 2024

Abstract

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.