Cosmic-Ray Ionization Rate in Protoplanetary Disks with Sheared Magnetic Fields
Cosmic-Ray Ionization Rate in Protoplanetary Disks with Sheared Magnetic Fields
复制标题
DOI：
10.3847/2041-8213/ac86c2
复制
发表时间：
2022-08
期刊：
影响因子：
--
通讯作者：
Y. Fujii;S. Kimura
中科院分区：
文献类型：
--
作者：
Y. Fujii;S. Kimura
我们研究了磁场配置对原行星盘中宇宙射线电离率的影响。首先，我们考虑宇宙射线从星际介质（ISM）到原行星盘的传播，并表明盘周围的宇宙射线密度应比 ISM 值低 2 倍。然后，我们计算原行星盘中宇宙射线的衰减。圆盘中的磁场被拉伸到方位角方向，宇宙射线在传播到中平面时需要绕道。我们的结果表明，绕行有效地将柱密度提高了大约两个数量级。我们采用弥散ISM中宇宙射线的典型电离率，该电离率被认为太高，与原行星盘的观测结果不一致，并发现宇宙射线在中面被明显屏蔽。对于 IM Lup 周围的圆盘，对于 r ≲ 100 au，中面电离率非常低，而该值与外半径中的扩散 ISM 一样大。我们的结果与最近阿塔卡马大型毫米/亚毫米阵列观测结果一致，该观测表明宇宙射线电离率的径向梯度。圆盘外半径的高电离率可能会激活磁旋转不稳定性，而这种不稳定性被认为是由于双极扩散而被抑制的。这些结果将对原行星盘的动力学和化学演化产生重大影响。
We investigate the effects of magnetic-field configurations on the ionization rate by cosmic rays in protoplanetary disks. First, we consider cosmic-ray propagation from the interstellar medium (ISM) to the protoplanetary disks and showed that the cosmic-ray density around the disk should be 2 times lower than the ISM value. Then, we compute the attenuation of cosmic rays in protoplanetary disks. The magnetic fields in the disk are stretched to the azimuthal directions, and cosmic rays need to detour while propagating to the midplane. Our results show that the detouring effectively enhances the column density by about two orders of magnitude. We employ a typical ionization rate by cosmic rays in diffuse ISM, which is considered too high to be consistent with observations of protoplanetary disks, and find that the cosmic rays are significantly shielded at the midplane. In the case of the disk around IM Lup, the midplane ionization rate is very low for r ≲ 100 au, while the value is as large as a diffuse ISM in the outer radii. Our results are consistent with the recent Atacama Large Millimeter/submillimeter Array observation that indicates the radial gradient in the cosmic-ray ionization rate. The high ionization rate in the outer radii of disks may activate the magnetorotational instability that was thought to be suppressed due to ambipolar diffusion. These results will have a strong influence on the dynamical and chemical evolutions of protoplanetary disks.