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

TL;DR

This study investigates molecular emissions in massive star-forming cores near RCW 120, revealing that Core 2 is in an early warm-up phase before hot core chemistry develops, while Core 1 appears less evolved.

Contribution

It provides new observational evidence of the warm-up phase in massive star-forming cores, highlighting differences in evolutionary stages between Core 1 and Core 2.

Findings

Core 2 shows signs of hot gas with highly excited CH₃CN lines.

Core 2 is likely in the warm-up phase prior to hot core chemistry.

Core 1 appears to be in an even earlier evolutionary stage.

Abstract

We study molecular emission in a massive condensation at the border of the HII region RCW 120, paying particular attention to the Core 1 and Core 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 CH$_3$OH and C$^{34}$S lines in Core 1 and Core 2. The CH$_3$CN series and the SO$_2$ 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 $10^5-10^7$ cm$^{-3}$ and is more uncertain in Core 1. However, the detection of the CH$_3$CN lines corresponding to highly excited transitions ($E_{\rm u}> 400$~K) in Core~2 indicates that the region contains hot gas, while the abundances of CH$_3$OH, CS, SO$_2$ and CH$_3$CN 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 CH$_3$CN lines in Core 1, therefore, it might be on an even less evolved evolutionary stage.