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至关重要，因为它直接影响行星的形成，以及可能导致双星形成的原行星盘的碎裂。磁场通过减缓气体的旋转，在这一过程中发挥了主要作用。这种“刹车”的效率取决于磁场和气体之间的耦合，而这与化学环境有关。有三种影响会影响联轴器，从而影响刹车。它们是“非理想磁流体效应”，即欧姆扩散、霍尔效应和双极扩散。磁场还可以启动原恒星外流，从原恒星系统中移除质量和角动量。在他担任研究员的第二年，马钱德博士使用欧拉拉美西斯程序对恒星形成进行了数值模拟，其中包括自重和两极扩散。他研究了在不同的化学环境和不同的初始条件下，双极扩散对磁制动的影响。他确定，在每种情况下，磁制动从磁盘中移除的角动量比流出的更多，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一个中等大小的圆盘，但会显著减弱磁刹车和原恒星外流。在这种情况下，他还发现了原恒星流出的离子前驱物质，这一现象以前从未报道过。

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.