2023MNRAS.526.1165S

Infrared dark clouds (IRDCs) represent the earliest stage of high-mass star formation and host molecular cores at different states of activity from quiet state without any IR-signature of star formation to active state with IR-signatures and pronounced maser activity. Many IRDCs show indications of shocked gas associated with protostar outflows and can contain class I methanol masers (cIMMs). The aim of this study is to probe different types of cores in IRDCs with cIMMs and thermal molecular emission and to investigate the relationship between presence of cIMMs and physical conditions in IRDCs. For a sample of 37 molecular cores, using the 20-m Onsala radio telescope, we collected molecular line data at 44, 85, and 97 GHz for more than 15 species including CH₃OH, CH₃CCH, and CS. Kinetic temperature of the gas and molecular column densities were obtained. Methanol emission at 44 GHz was detected in 29 sources, with 4 sources are being new discoveries. None of the cores in quiescent state show emission at 44 GHz. Our results testify that cIMM emission is a reliable marker of advanced state of molecular cores. The higher detection rate for intermediate and IR-quiet sources suggests that cIMMs most readily trace the early stages of star formation characterized by moderate IR-signatures. We found that masers with higher flux densities tend to be associated with emission in the CH₃CCH lines with higher integrated intensities. Sources undetected at 44 GHz have 4 times lower integrated intensities of CH₃CCH and exhibit poorer molecular spectra than the most sources with cIMMs.