The regulation of angular momentum in star formation: the secret behind planet formation

角动量的调节在恒星形成过程中是最重要的，因为它直接影响行星的形成，以及导致双星形成的原行星盘的可能碎片。磁场在这个过程中起着重要作用，它减缓了气体的旋转。这种“制动”的效率取决于磁场和气体之间的耦合，这与化学环境有关。三种效应影响联轴器，从而影响制动。它们是“非理想磁流体动力学效应”，即欧姆扩散效应、霍尔效应和双极扩散效应。磁场还可以发射原恒星外流物，从原恒星系统中移除质量和角动量。在他研究的第二年，Marchand博士使用欧拉代码RAMSES进行了恒星形成的数值模拟，其中包括自引力和双极扩散。他研究了在不同化学环境和不同初始条件下双极性扩散对磁制动的影响。他确定，在任何情况下，磁制动从圆盘上带走的角动量都比流出的角动量多，这仍然是一个有争议的问题。他发现，减少小尘埃颗粒的数量会导致圆盘大小适中，但磁性制动和原恒星流出明显减弱。在这种情况下，他还发现了原恒星流出物的离子前体，这是以前从未报道过的现象。

The regulation of angular momentum is of prime importance during star formation, as it directly impacts the formation of planets, and the possible fragmentation of the protoplanetary disk leading to the creation of binary stars. The magnetic field plays a major role in this process by slowing down the rotation of the gas. The efficiency of this "braking" depends on the coupling between the magnetic field and the gas, which is related to the chemical environment. Three effects impact the coupling, and therefore the braking. They are "non-ideal magnetohydrodynamics (MHD) effects", namely the Ohmic diffusion, the Hall effect and the ambipolar diffusion. The magnetic field can also launch protostellar outflows, that remove mass and angular momentum from the proto-stellar system.During the second year of his fellowship, Dr. Marchand have performed numerical simulations of star formation using the eulerian code RAMSES, that includes self-gravity and the ambipolar diffusion. He has studied how the magnetic braking was impacted by the ambipolar diffusion in different chemical environments and with different initial conditions. He determined that in every situation, the magnetic braking removes more angular momentum from the disk than the outflows, which is still a debated question. He found that reducing the number of small dust grains leads to a moderately larger disk, but significantly weaker magnetic braking and protostellar outflows. In this situation, he also found an ionic precursor to the protostellar outflow, a phenomenon never reported before.