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

Fragmentation in the massive G31.41+0.31 protocluster