ALMA Observations of a Massive and Dense Cold Clump: G305.137+0.069

We report observations, made with the Atacama Large Millimeter/submillimeter Array, of 3 mm dust continuum emission and molecular line emission in HCO+, CS, and N2H+, toward G305.137+0.069, a massive, dense, and cold clump, in order to characterize its small-scale (∼2000 au) structure. The dust continuum observations reveal the presence of twelve compact structures (cores) with masses ranging from 3.3 to 50.6 M , radii from 1800 to 5300 au, and densities from 3.1 × 106 to 3.1 × 107 cm−3. The line observations show that the molecular emission arises from a bright central region, with an angular size of ∼12″, and an extended, weaker envelope best seen in the HCO+ line. The N2H+ emission is best correlated with the continuum emission and hence with the cores. For cores with single line profiles, the line widths range from 1.9 to 3.1 km s−1 with an average value of 2.6 km s−1, indicating that they are dominated by nonthermal motions, either due to random turbulence or core-scale motions. The virial parameter of the three most massive cores are smaller than one, suggesting that they are undergoing collapse. We find that in the regime of masses probed by our observations (M > 3 M ) the shape of the core-mass function is notably different from the initial mass function, showing an overpopulation of high-mass cores. We suggest that the formation and mass distribution of the dense cores within G305.137+0.069 can be explained as the result of hierarchical or turbulent fragmentation in a gravitationally collapsing clump.