Neutron stars (NSs) obtain kicks, typically of several 100 km s?1, at birth. The gravitational tugboat mechanism can explain these kicks as consequences of asymmetric mass ejection during the supernova (SN) explosion. Support for this hydrodynamic explanation is provided by observations of SN remnants with associated NSs, which confirm the prediction that the bulk of the explosion ejecta, particularly the chemical elements between silicon and the iron group, are dominantly expelled in the hemisphere opposite to the direction of the NS kick. Here, we present a large set of two- and three-dimensional explosion simulations of electron-capture SNe, considering explosion energies between ~3?×?1049 erg and ~1.6?×?1050 erg. We find that the fast acceleration of the SN shock in the steep density gradient delimiting the O–Ne–Mg core of the progenitor enables such a rapid expansion of neutrino-heated matter that the growth of neutrino-driven convection freezes out quickly in a high-mode spherical harmonics pattern. Because the corresponding momentum asymmetry of the ejecta is very small and the gravitational acceleration by the fast-expanding ejecta abates rapidly, the NS kick velocities are a few km s?1, at most. The extremely low core compactness of O–Ne–Mg-core progenitors therefore favors hydrodynamic NS kicks much below the ~160 km s?1 measured for the Crab pulsar. This suggests either that the Crab Nebula is not the remnant of an electron-capture SN, but rather of a low-mass iron-core progenitor; or that the Crab pulsar was not accelerated by the gravitational tugboat mechanism, but instead received its kick by a non-hydrodynamic mechanism such as, e.g., anisotropic neutrino emission.
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机译：中子星（NSs）在出生时会获得踢力，通常为几百公里s？1。引力拖船机制可以解释这些踢动是超新星（SN）爆炸过程中不对称质量抛射的结果。 SN残留物和相关NS的观测结果为这种流体动力学解释提供了支持，这证实了以下预测：爆炸喷出物的大部分，特别是硅和铁基团之间的化学元素，主要在与反方向相反的半球中被排出。 NS踢。在这里，我们考虑到约3？×？1049 erg和〜1.6？×？1050 erg之间的爆炸能量，给出了电子俘获SNe的二维和三维大爆炸模拟。我们发现，在陡峭的密度梯度中，SN激波的快速加速界定了祖先的O–Ne–Mg核，从而使中微子加热物质得以迅速膨胀，以致中微子驱动对流的生长在高温度下迅速冻结。模式球谐模式。由于相应的射流动量不对称性非常小，并且由于快速膨胀的射流引起的重力加速度迅速减弱，因此，NS踢球速度最多为几千米s？1。因此，O-Ne-Mg核祖细胞的极低的核致密性有利于流体动力学的NS突跳，远低于对蟹状脉冲星测量的〜160 km s？1。这表明，蟹状星云不是电子捕获SN的残余，而是低质量铁芯祖细胞。或说蟹状脉冲星不是由重力拖船机制加速的，而是由非流体动力机制（例如各向异性中微子发射）引起的。
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