The origin of magnetism on the upper main sequence

We consider the incidence of magnetism in main-sequence stars with mainly radiative en-velopes. We propose that the small fraction, which increases with mass, of stars which are magnetic can be explained if towards the end of the formation process, after the stars have developed a substantial radiative envelope, a correspondingly small fraction of stars merge. Such late mergers would produce a brief period of strong differential rotation and give rise to large-scale fields in the radiative envelopes. Such late mergers can also account for the lack of close binaries among these stars