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