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Revealing unexpected hot gas deeply embedded in 70 um 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. At about 5000 au resolution, 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). Compared to the 1.3 mm dust continuum and other molecular line emission, such as C18O and deuterated species, we find that H2CO emission is mainly sensitive to the outflow/jet components rather than the quiescent gas expected in the early phases of high-mass 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 > 100 K). With the supreme sensitivity and angular resolution provided by ALMA, we have discovered that a portion of the embedded cores in 70 um IRDCs have already unexpectedly entered the protostellar phase.