A CH₃CN and HCO⁺ survey towards southern methanol masers associated with star formation

We present the initial results of a 3-mm spectral-line survey towards 83 methanol maser-selected massive star-forming regions. Here, we report observations of the J = 5-4 and 6-5 rotational transitions of methyl cyanide (CH₃CN) and the J = 1-0 transition of HCO⁺ and H¹³CO⁺. CH ₃CN emission is detected in 58 sources (70 per cent of our sample). We estimate the temperature and column density for 37 of these using the rotational diagram (RD) method. The temperatures we derive range from 28-166 K, and are lower than previously reported temperatures, derived from higher J transitions. We find that CH₃CN is brighter and more commonly detected towards ultracompact H ii (UCH ii) regions than towards isolated maser sources. Detection of CH₃CN towards isolated maser sources strongly suggests that these objects are internally heated and that CH₃CN is excited prior to the UCH II phase of massive star formation. HCO⁺ is detected towards 82 sources (99 per cent of our sample), many of which exhibit asymmetric line profiles compared to H¹³CO⁺. Skewed profiles are indicative of inward or outward motions, however, we find approximately equal numbers of red- and blue-skewed profiles among all classes. Column densities are derived from an analysis of the HCO⁺ and H ¹³CO⁺ line profiles. 80 sources have mid-infrared (mid-IR) counterparts: 68 seen in emission and 12 seen in absorption as 'dark clouds'. Seven of the 12 dark clouds exhibit asymmetric HCO⁺ profiles, six of which are skewed to the blue, indicating infalling motions. CH₃CN is also common in dark clouds, where it has a 90 per cent detection rate.