We present observations of the 36-GHz (4₋₁→3₀E) and 84-GHz (5₋₁→4₀E) class I methanol maser transitions towards a sample of 94 known class I sites. These observations resulted in 93 and 92 detections in the 84- and 36-GHz transitions. While the majority of the 36-GHz sources have been previously reported, many of the sites are observed in the 84-GHz transition for the first time. The near-simultaneous observations of the two transitions revealed strikingly similar spectral profiles and a mean and median 36- to 84-GHz integrated flux density ratio of 2.6 and 1.4. Alongside the 36- and 84-GHz observations, we included rare class II methanol masers at 37.7, 38.3, 38.5, 86.6, and 86.9 GHz, a number of recombination lines, and thermal molecular transitions. We detect one new site of 86.6- and 86.9-GHz methanol masers, as well as six maser candidates in one or more of 37.7-, 38.3-, 38.5-, 86.6-, and 86.9-GHz methanol maser transitions. We detect a relatively higher rate of HC 3 N compared to that reported by MALT90 (once the respective detection limits were taken into account), who targeted dense dust clumps, suggesting that the class I methanol maser targets incorporate a relatively higher number of warm protostellar sources. We further find that there are similar relationships between the integrated flux density of both class I transitions with the integrated intensity of HC₃N, HNC, HCO⁺, HNC, SiO, and H¹³CO⁺. We suggest that this indicates that the integrated flux densities of the 36- and 84-GHz transitions are closely linked to the available gas volume.
Bibliographical noteThis article has been accepted for publication in Monthly Notices of the Royal Astronomical Society, Volume 484, Issue 4, 21 April 2019, Pages 5072–5093, https://doi.org/10.1093/mnras/stz192. Copyright 2019 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.
- ISM: molecules
- radio lines: ISM
- stars: formation