2019AJ....158...18H

We present 247 GHz molecular line observations of methanol (CH₃OH) toward 16 massive star-forming regions using the APEX telescope with an angular resolution of 25''. The sample covers a range of evolutionary states, including warm molecular cores (WMCs), hot molecular cores, and ultracompact H II regions. The hot cores, all of which include UC H II regions, show rich molecular line spectra, although the strength of different species and transitions varies from source to source. In contrast, the warm cores do not show significant molecular line emission. Multiple methanol transitions are detected toward nine of the hot cores; eight of these had enough transitions to use the rotation diagram method to estimate rotational temperatures and column densities. The temperatures lie in the range of 104-190 K and column densities from 3 x 10¹⁶ to 7 x 10¹⁸ cm^–2. Using the average methanol line parameters, we estimate virial masses, which fall in the range from 145 to 720 M_☉ and proved to be significantly higher than the measured gas masses. We discuss possible scenarios to explain the chemical differences between hot cores and WMCs. One of the observed methanol lines, 4₂-5₁A⁺ at 247.228 GHz, is predicted to show Class II maser emission, similar in intensity to previously reported J₀-J_–1E masers at 157 GHz. We did not find any clear evidence for maser emission among the observed sources, however, a weak maser in this line may exist in G345.01+1.79.