Probing fragmentation with ALMA continuum and spectral line observations of the dense clumps in the ℓ = 224° region

Abstract

ABSTRACT We report observations, performed with the Atacama Large Millimeter/submillimeter Array (ALMA), of 1 mm dust continuum emission and molecular line emission in 13CO(2–1) and C18O(2–1), towards a sample of starless and protostellar clumps selected from a region, towards the ℓ = 224° field, of the Herschel Infrared GALactic Plane Survey (Hi-GAL). Using the ALMA images and a source extraction algorithm we have analysed the small-scale (∼1000 AU) structure of the clumps and their population of cores (or fragments). We find in general multiple cores in each Hi-GAL clump, both in the continuum and spectral lines, but we do not find a dominant fragmentation mode and the morphologies are very different among the various sources. Our results suggest that during the transition phase from clump to core, those sources with a higher core formation efficiency are also associated with parent clumps that are more likely to convert a higher fraction of their initial mass into a single or a few cores. We were able to obtain a core mass function, or CoMF, covering masses in the range ∼2 × 10−3 to ∼1 M⊙ for the C18O cores, and ∼4 × 10−2 to ∼10 M⊙ for the continuum cores. We find that the CoMF in our sample is much shallower than the higher mass ($\gtrsim 1$ M⊙) IMF, thus indicating that while approaching the final phase of fragmentation the mass function does not resemble the IMF more closely.