Yamaguchi interferometer survey of protostellar outflows embedded in 70-μm dark infrared dark cloud

Recent ALMA observations detected protostellar outflows in 70-μm dark infrared dark clouds (IRDCs). These sources are candidates for the initial stages of high-mass star formation. We launched a new survey for free-free emission from outflow shocks using the Yamaguchi Interferometer (YI) at 8 GHz. We aim to catalog "proto-high-mass protostar" candidates that are still in the low to intermediate-mass phase. We selected starless-like clumps without any 70-μm point source from Traficante et al. (2015). We currently detected 82 sources from 167 clumps. 37 of them are fainter than 20 mJy (down to a few mJy). They tend to associate with colder and denser clumps that are suitable for star formation. This fact suggests that, at least, some of them trace star-formation activities. The highest-density clumps are, in fact, associated with several masers and molecular outflows. Furthermore, some of them have already shown a signature of ongoing cluster formation.