| 研究実績の概要 |
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.
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