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

## TL;DR

This study uses 3D magnetohydrodynamic simulations to show that protostellar flares can occur without a magnetosphere, driven by accretion of magnetic fields and reconnection, aligning with observed flare energies and hot ejecta.

## Contribution

It demonstrates that protostellar flares can be generated through accretion-driven magnetic flux accumulation and reconnection, even without a magnetosphere, which is a novel insight.

## Key findings

- Protostellar flares occur without a magnetosphere in simulations.
- Magnetic energy is accumulated via accretion of large-scale fields.
- Flares produce hot ejecta and influence disk magnetic field evolution.

## Abstract

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