ALMA observations of two massive and dense MALT90 clumps

Abstract: We report Atacama Large Millimeter Array observations of 3 mm dust continuum emission and line emission, in HCO$^{+}$, H$^{13}$CO$^{+}$, N${2}$H$^{+}$ and CH${3}$CN, towards 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 $\sim$1 $\arcsec$. Towards the prestellar clump we detected ten compact structures (cores), with radius from 1200 to 4500 AU and masses from 1.6 to 20~M$\odot$. Half of these cores exhibit inverse P Cygni profiles in HCO$^{+}$ and are subvirialized indicating that they are undergoing collapse. Towards the protostellar clump we detected a massive (119~M$\odot$) central core, with a strong mass infall rate, and nine less massive cores, with masses from 1.7 to 27~M$\odot$ and radius from 1000 to 4300 AU. CH${3}$CN rotational temperatures were derived for 8 cores in the protostellar clump and 3 cores in the prestellar clump. Cores within the prestellar clump have smaller linewidths 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 ($\sim$6\%) than in the protostellar clump ($\sim$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.