Giant Protostellar Flares: Accretion-driven Accumulation and Reconnection-driven Ejection of Magnetic Flux in Protostars
Giant Protostellar Flares: Accretion-driven Accumulation and Reconnection-driven Ejection of Magnetic Flux in Protostars
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DOI：
10.3847/2041-8213/ab22bb
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发表时间：
2019-02
期刊：
影响因子：
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通讯作者：
S. Takasao;K. Tomida;K. Iwasaki;T. Suzuki
中科院分区：
文献类型：
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作者：
S. Takasao;K. Tomida;K. Iwasaki;T. Suzuki
原恒星耀斑是与原恒星热等离子体形成有关的快速磁能释放事件。在以往的原恒星耀斑模型中，原恒星磁层与周围盘的相互作用对磁能的产生和释放起着至关重要的作用。然而，目前还不清楚原恒星是否真的有磁层，因为在原恒星阶段，剧烈的盘吸积和强大的盘磁场可能会破坏磁层。考虑到这种可能性，我们调查的能量积累和释放过程中的磁层使用三维磁流体动力学模拟。我们的模拟表明，原恒星耀斑重复产生，即使在这种情况下。与磁层模型不同的是，原恒星通过吸积从盘中获得大规模磁场来积累磁能。原恒星耀斑是由于原恒星的磁场重联而导致一部分大规模磁场被移除。模拟中的原恒星耀斑与观测结果一致;释放的磁能（高达103 × 1038 erg）足以驱动观测到的耀斑，耀斑产生热喷出物。被驱逐的磁场增强了吸积，因此重复了能量积累和释放过程。通过重联去除磁通量导致内部磁盘中的磁场重新分布。因此，我们认为，原恒星耀斑将在原恒星附近的磁盘磁场的演化中发挥重要作用。
Protostellar flares are rapid magnetic energy release events associated with the 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 role 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 ∼3 × 1038 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.