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Massive infrared dark clouds (IRDCs) are considered to host the earlieststages of high-mass star formation. In particular, 70 $\mu$m dark IRDCs are thecolder and more quiescent clouds. At a scale of about 5000 au usingformaldehyde (H2CO) emission, we investigate the kinetic temperature of densecores in 12 IRDCs obtained from the pilot ALMA Survey of 70 $\mu$m darkHigh-mass clumps in Early Stages (ASHES). Compared to 1.3 mm dust continuum andother molecular lines, such as C18O and deuterated species, we find that H2COis mainly sensitive to low-velocity outflow components rather than to quiescentgas expected in the early phases of star formation. The kinetic temperatures ofthese components range from 26 to 300 K. The Mach number reaches about 15 withan average value of about 4, suggesting that the velocity distribution of gastraced by H2CO is significantly influenced by a supersonic non-thermalcomponent. In addition, we detect warm line emission from HC3N and OCS in 14protostellar cores, which requires high excitation temperatures (Eu/k ~ 100 K).These results show that some of the embedded cores in the ASHES fields are inan advanced evolutionary stage, previously unexpected for 70 $\mu$m dark IRDCs.

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