ALMA observations of two massive and dense MALT90 clumps
  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.
