论文标题
带有剪切磁场的原星盘的宇宙射线电离速率
Cosmic-ray ionization rate in protoplanetary disks with sheared magnetic fields
论文作者
论文摘要
我们研究了磁场构型对宇宙磁盘中宇宙射线电离速率的影响。首先，我们考虑从星际介质（ISM）到原球磁盘的宇宙射线传播，并表明磁盘周围的宇宙射线密度应比ISM值低2倍。然后，我们计算了原星磁盘中宇宙射线的衰减。磁盘中的磁场伸展到方位角方向，宇宙射线需要在传播到中平面时绕道。我们的结果表明，绕道可有效地增强了柱密度的大约两个大小。我们在弥漫性ISM中采用典型的电离速率，这被认为太高，无法与原星盘的观测值保持一致，并发现宇宙射线在平面平面上被显着屏蔽。对于IM LUP周围的磁盘，对于$ r \ lyssim \，100 $ au的中平面电离速率非常低，而该值与外部半径中的弥漫性ISM一样大。我们的结果与最近的Alma观察结果一致，该观察表明宇宙射线电离速率中的径向梯度。磁盘外部半径的高电离速率可能会激活磁化不稳定性，而磁化磁不稳定性被认为由于双极扩散而被抑制。这些结果将对原球磁盘的动力学进化有很大的影响。
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 magnitudes. 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 $r\lesssim\,100$ au, while the value is as large as a diffuse ISM in the outer radii. Our results are consistent with the recent ALMA 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.
