Fragmentation in the massive G31.41+0.31 protocluster
  Context. ALMA observations at 1.4 mm and 0.2'' (750au) angular resolution of
the Main core in the high-mass star forming region G31.41+0.31 have revealed a
puzzling scenario: on the one hand, the continuum emission looks very
homogeneous and the core appears to undergo solid-body rotation, suggesting a
monolithic core stabilized by the magnetic field; on the other hand, rotation
and infall speed up toward the core center, where two massive embedded
free-free continuum sources have been detected, pointing to an unstable core
having undergone fragmentation. Aims. To establish whether the Main core is
indeed monolithic or its homogeneous appearance is due to a combination of
large dust opacity and low angular resolution, we carried out millimeter
observations at higher angular resolution and different wavelengths. Methods.
We carried out ALMA observations at 1.4 mm and 3.5 mm that achieved angular
resolutions of 0.1''(375 au) and 0.075'' (280 au), respectively. VLA
observations at 7 mm and 1.3 cm at even higher angular resolution, 0.05'' (190
au) and 0.07'' (260 au), respectively, were also carried out to better study
the nature of the free-free continuum sources detected in the core. Results.
The millimeter continuum emission of the Main core has been clearly resolved
into at least four sources, A, B, C, and D, within 1'', indicating that the
core is not monolithic. The deconvolved radii of the dust emission of the
sources, estimated at 3.5 mm, are 400-500au, their masses range from 15 to 26
Msun, and their number densities are several 1E9 cm-3. Sources A and B, located
closer to the center of the core and separated by 750 au, are clearly
associated with two free-free continuum sources, likely thermal radio jets, and
are the brightest in the core. The spectral energy distribution of these two
sources and their masses and sizes are similar and suggest a common origin.
