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Protostellar flares are rapid magnetic energy release events associated withformation of hot plasma in protostars. In the previous models of protostellarflares, the interaction between a protostellar magnetosphere with thesurrounding disk plays crucial roles in building-up and releasing the magneticenergy. However, it remains unclear if protostars indeed have magnetospheresbecause vigorous disk accretion and strong disk magnetic fields in theprotostellar phase may destroy the magnetosphere. Considering this possibility,we investigate the energy accumulation and release processes in the absence ofa magnetosphere using a three-dimensional magnetohydrodynamic simulation. Oursimulation reveals that protostellar flares are repeatedly produced even insuch a case. Unlike in the magnetospheric models, the protostar accumulatesmagnetic energy by acquiring large-scale magnetic fields from the disk byaccretion. Protostellar flares occur when a portion of the large-scale magneticfields are removed from the protostar as a result of magnetic reconnection.Protostellar flares in the simulation are consistent with observations; thereleased magnetic energy (up to $\sim 3\times 10^{38}$ erg) is large enough todrive observed flares, and the flares produce hot ejecta. The expelled magneticfields enhance accretion, and the energy build-up and release processes arerepeated as a result. The magnetic flux removal via reconnection leads toredistribution of magnetic fields in the inner disk. We therefore consider thatprotostellar flares will play an important role in the evolution of the diskmagnetic fields in the vicinity of protostars.

Giant protostellar flares: accretion-driven accumulation and reconnection-driven ejection of magnetic flux in protostars