Revealing deeply embedded star formation in 70-micron dark IRDCs

ABSTRACT :
Infrared dark clouds (IRDCs) are a suitable target for studying the earliest stages of high-mass star formation. Those that are 70 um dark are of special interest because they are apparently the coldest more quiescent clouds. In our previous studies, we have investigated the kinetic temperature of dense cores determined from formaldehyde (H2CO) emission in 12 IRDCs obtained from the pilot ALMA Survey of 70 um dark High-mass clumps in Early Stages (ASHES) at about 5000 AU resolution (Izumi et al. 2024). Compared to the 1.3 mm dust continuum and other molecular line emission, such as C18O and deuterated species, we found that H2CO emission is mainly sensitive to low-velocity outflow components rather than to quiescent gas expected in the early phases of star formation. These components show warm-hot gas with temperatures ranging from ~50 to >200 K. In addition, in some cores we detect compact emission of HC3N J=24-23 and OCS J=18-17, which require high temperatures to be excited (Eu/k > 100 K).  With the supreme sensitivity and angular resolution provided by ALMA, we have discovered that some of the embedded cores in the ASHES fields are in an advanced evolutionary stage, previously unexpected for 70 um IRDCs. In addition to the above, we report on the follow-up of 39 IRDCs, a full sample of ASHES in this workshop.