2021MNRAS.503..633K

We study molecular emission in a massive condensation at the border of the H II region RCW 120, paying particular attention to the Core 1 and 2 objects, the most massive fragments of the condensation found previously by ALMA. The latter fragment was previously suggested to host a high-mass analogue of Class 0 young stellar object. We present spectra of molecular emission in the 1 mm range made with the APEX telescope. We detect CH₃OH and C³⁴S lines in Cores 1 and 2. The CH₃CN series and the SO₂ lines are only found in Core 2. We estimate gas physical parameters using methanol lines and obtain gas temperature less than 100 K in both regions. Molecular hydrogen number density in Core 2 is in the range of 10⁵-10⁷ cm^–3 and is more uncertain in Core 1. However, the detection of the CH₃CN lines corresponding to highly excited transitions (E_u > 400 K) in Core 2 indicates that the region contains hot gas, while the abundances of CH₃OH, CS, SO₂, and CH₃CN are quite low for a hot core stage. We propose that Core 2 is in the warm-up phase prior to the establishing of the hot gas chemistry. We suggest that Core 2 is in the beginning of the hot core stage. There are no detected CH₃CN lines in Core 1; therefore, it might be on an even less evolved evolutionary stage.