The fragmentation properties of massive protocluster gas clumps: an ALMA
  study
  Fragmentation of massive dense molecular clouds is the starting point in the
formation of rich clusters and massive stars. Theory and numerical simulations
indicate that the population of the fragments (number, mass, diameter,
separation) resulting from the gravitational collapse of such clumps is
probably regulated by the balance between the magnetic field and the other
competitors of self-gravity, in particular turbulence and protostellar
feedback. We have observed 11 massive, dense and young star-forming clumps with
the Atacama Large Millimeter Array (ALMA) in the thermal dust continuum
emission at $\sim 1$~mm with an angular resolution of 0.25 arcseconds with the
aim of determining their population of fragments. We find fragments on
sub-arcsecond scales in 8 out of the 11 sources. The ALMA images indicate two
different fragmentation modes: a dominant fragment surrounded by companions
with much smaller mass and size, and many ($\geq 8$) fragments with a gradual
change in masses and sizes. On average, the largest number of fragments is
found towards the warmer and more massive clumps. Also, the warmer clumps tend
to form fragments with larger mass and size. To understand the role of the
different physical parameters to regulate the final population of the
fragments, we have simulated the collapse of a massive clump of $100$ and $300$
M$_{\odot}$ having different magnetic support. The simulations indicate that:
(1) fragmentation is inhibited when the initial turbulence is low, independent
of the other physical parameters. (2) a filamentary distribution of the
fragments is favoured in a highly magnetised clump. We conclude that the clumps
that show many fragments distributed in a filamentary-like structure are likely
characterised by a strong magnetic field, while the others are possible also in
a weaker magnetic field.
