The ALMA Survey of 70 m Dark High-mass Clumps in Early Stages (ASHES). X. Hot Gas Reveals Deeply Embedded Star Formation

Massive infrared dark clouds (IRDCs) are considered to host the earliest stages of high-mass star formation. In particular, 70 mu m dark IRDCs are the colder and more quiescent clouds. At a scale of about 5000 au using formaldehyde (H2CO) emission, we investigate the kinetic temperature of dense cores in 12 IRDCs obtained from the pilot Atacama Large Millimeter/submillimeter Array Survey of 70 mu m dark High-mass clumps in Early Stages (ASHES). Compared to the 1.3 mm dust continuum and other molecular lines, such as (CO)-O-18 and deuterated species, we find that H2CO is mainly sensitive to low-velocity outflow components rather than to quiescent gas expected in the early phases of star formation. The kinetic temperatures of these components range from 26 to 300 K. The Mach number reaches about 15 with an average value of about 4, suggesting that the velocity distribution of gas traced by H2CO is significantly influenced by a supersonic nonthermal component. In addition, we detect warm line emission from HC3N and OCS in 14 protostellar cores, which requires high excitation temperatures (E-u/k similar to 100 K). These results show that some of the embedded cores in the ASHES fields are in an advanced evolutionary stage, previously unexpected for 70 mu m dark IRDCs.