Abstract

We report Atacama Large Millimeter Array observations of 3 mm dust continuum emission and line emission, in HCO+, (HCO+)-C-13, N2H+, and CH3CN, toward two massive and dense clumps (MDCs) in early but distinct evolutionary phases (prestellar and protostellar), made with the goal of investigating their fragmentation characteristics at angular scales of similar to 1 ''. Toward the prestellar clump we detected 10 compact structures (cores), with radii from 1200-4500 au and masses from 1.6-20 M-circle dot. Half of these cores exhibit inverse P Cygni profiles in HCO+ and are subvirialized indicating that they are undergoing collapse. Toward the protostellar clump we detected a massive (119 M-circle dot) central core, with a strong mass infall rate, and nine less massive cores, with masses from 1.7-27 M-circle dot and radii from 1000-4300 au. CH3CN rotational temperatures were derived for eight cores in the protostellar clump and three cores in the prestellar clump. Cores within the prestellar clump have smaller line widths and lower temperatures than cores within the protostellar clump. The fraction of total mass in cores to clump mass is smaller in the prestellar clump (similar to 6%) than in the protostellar clump (similar to 23%). We conclude that we are witnessing the evolution of the dense gas in globally collapsing MDCs; the prestellar clump illustrating the initial stage of fragmentation, harboring cores that are individually collapsing, and the protostellar clump reflecting a later stage in which a considerable fraction of the gas has been gravitationally focused into the central region.