The warm-up phase in massive star-forming cores around RCW 120

Abstract

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 (Eu > 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.