A multi-transition molecular line study of candidate massive young stellar objects associated with methanol masers

Aims. We characterize the molecular environment of candidate massive young stellar objects (MYSOs) signposted by methanol masers. Methods. Single pixel observations of 10 transitions of HCO, CO and CS isotopomers were carried out, using the IRAM 30 m telescope. We studied a sample of 28 targets for which the 6.7 GHz maser emission positions are known with a sub-arcsecond accuracy. Results. The systemic velocity inferred from the optically thin lines agrees within ±3 km s with the central velocity of the maser emission for most of the sources. About 64% of the sources show line wings in one or more transitions of CO, HCO and CS species, indicating the presence of molecular outflows. Comparison of the widths of line wings and methanol maser emission suggests that the 6.7 GHz maser line traces the environment of MYSO of various kinematic regimes. Therefore conditions conducive for the methanol maser can exist in the inner parts of molecular clouds or circumstellar discs as well as in the outer parts associated with molecular outflows. Calculations of the physical conditions based on the CO and HCO lines and the CS line intensity ratios refine the input parameters for maser models. Specifically, a gas number density of < 10 cm is sufficient for strong maser emission and a high methanol fractional abundance (> 5 × 10) is required.