  The fragmentation process in massive star-forming regions is one of the
contemporary problems in astrophysics, and several physical processes have been
proposed to control the fragmentation including turbulence, magnetic field,
rotation, stellar feedback, and gravity. However, the fragmentation process has
been poorly studied at small spatial scales well below 1000 AU. We aim to use
ALMA (Atacama Large Millimeter and Submillimeter Array) high angular resolution
data to identify the fragments in W51 IRS2 and to study the fragmentation
properties on a spatial scale of 200 AU. We used ALMA data of W51 IRS2 from
three projects, which give an angular resolution of 0.028$^{\prime\prime}$ (144
AU) at millimeter wavelengths. We identified compact fragments by using {\it
uv}-range constrained 1.3 mm continuum data. A Mean Surface Density of
Companions (MSDC) analysis has been performed to study the separations between
fragments. A total number of 33 continuum sources are identified and 29 out of
them are defined as fragments in the surveyed region.The MSDC analysis reveals
two breaks corresponding to spatial sales of 1845 AU and 7346 AU, indicative of
a two-level clustering phenomenon, along with a linear regime below 1845 AU,
mostly associated with W51 North, whose slope is consistent with the slope for
the clustering regime of other cluster-like regions in the Galaxy. The typical
masses and separations of the fragments as well as the relation between density
and number of fragments can be explained through a thermal Jeans process
operating at high temperatures of 200--400 K, consistent with previous
measurements of the temperature in the region, and produced by the nearby
massive stars. Therefore, although W51 IRS2 seems to be undergoing a thermally
inhibited fragmentation phase, this does not seem to prevent the formation of a
protocluster associated with W51 North.

Astrophysics of Galaxies