The ALMA-QUARKS Survey: Detection of Two Extremely Dense Substructures in a Massive Prestellar Core

Only a handful of massive starless core candidates have been discovered so far, but none of them have been fully confirmed. Within the MM1 clump in the filamentary infrared dark cloud G34.43+0.24 that was covered by the Atacama Large Millimeter/submillimeter Array (ALMA) ATOMS survey at Band 3 (similar to 2 '', 6000 au) and the ALMA-QUARKS survey at Band 6 (similar to 0.'' 3, 900 au), two prestellar core candidates MM1-C and E1 with masses of 71 and 20 M circle dot and radii of 2100-4400 au were discovered. The two cores show no obvious sign of star formation activities. In particular, MM1-C is a very promising massive prestellar core candidate with a total gas mass of 71 M circle dot. Within MM1-C, we detected two extremely dense substructures, C1 and C2, as characterized by their high densities of nH2 similar to 108-9cm-3 . Moreover, evidence of further fragmentation in C2 was also revealed. We have detected the primordial fragmentation in the earliest stage of massive star formation, and we speculate that MM1-C would be the birthplace of a massive multiple system. However, we cannot fully rule out the possibility that the massive prestellar core MM1-C will just form a cluster of low-mass stars if it undergoes further fragmentation.