Giant protostellar flares: accretion-driven accumulation and reconnection-driven ejection of magnetic flux in protostars. (arXiv:1902.02007v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Takasao_S/0/1/0/all/0/1">Shinsuke Takasao</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Tomida_K/0/1/0/all/0/1">Kengo Tomida</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Iwasaki_K/0/1/0/all/0/1">Kazunari Iwasaki</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Suzuki_T/0/1/0/all/0/1">Takeru K. Suzuki</a>
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, we point out that the magnetosphere will not form in
protostars because of a vigorous accretion through the disk, and the
magnetospheric models may not be applicable. We investigate the energy
accumulation and release processes in the absence of a magnetosphere using a
three-dimensional magnetohydrodynamics 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. A protostellar flare
occurs 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.
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, we point out that the magnetosphere will not form in
protostars because of a vigorous accretion through the disk, and the
magnetospheric models may not be applicable. We investigate the energy
accumulation and release processes in the absence of a magnetosphere using a
three-dimensional magnetohydrodynamics 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. A protostellar flare
occurs 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.
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