What determines the formation and characteristics of protoplanetary discs?

Abstract

Context. Planets form in protoplanetary discs. Their masses, distribution, and orbits sensitively depend on the structure of the protoplanetary discs. However, what sets the initial structure of the discs in terms of mass, radius and accretion rate is still unknown. Aims. It is therefore of great importance to understand exactly how protoplanetary discs form and what determine their physical properties. We aim at quantifying the role of the initial dense core magnetisation, rotation, turbulence and misalignment between rotation and magnetic field axis as well as the role of the accretion scheme onto the central object.Methods. We perform non-ideal MHD numerical simulations using the adaptive mesh refinement code Ramses, of a collapsing, one solar mass, molecular core to study the disc formation and early, up to 100 kyr, evolution, paying great attention to the impact of numerical resolution and accretion scheme.Results. We found that while the mass of the central object is almost independent of the numerical parameters such as the resolution and the accretion scheme onto the sink particle, the disc mass, and to a lower extent its size, heavily depend on the accretion scheme, which we found, is itself resolution dependent. This implies that the accretion onto the star and through the disc are largely decoupled. For a relatively large domain of initial conditions (except at low magnetisation), we found that the properties of the disc do not change too significantly. In particular both the level of initial rotation and turbulence do not influence the disc properties provide the core is sufficiently magnetized. After a short relaxation phase, the disc settles in a stationary state. It then slowly grows in size but not in mass. The disc itself is weakly magnetized but its immediate surrounding is on the contrary highly magnetized. Conclusions. Our results show that the disc properties directly depend on the inner boundary condition, i.e. the accretion scheme onto the central object, suggesting that the disc mass is eventually controlled by the small scale accretion process, possibly the stardisc interaction. Because of ambipolar diffusion and its significant resistivity, the disc diversity remains limited and except for low magnetisation, their properties are weakly sensitive to initial conditions such as rotation and turbulence.

Formation and Evolution of Disks Around Young Stellar Objects

“…On the other-hand they infer that when a density is prescribed as a threshold above which accretion occurs, a disk generally forms but its mass and size vary with the chosen density threshold. Similar conclusion is reached by Hennebelle et al (2020) who also pointed out that the density threshold is resolution dependent. That is to say, the same threshold does not has the same influence at different spatial resolution.…”

“…The influence of turbulence has also been studied for non-ideal MHD collapse by Hennebelle et al (2016) and more recently by Hennebelle et al (2020). They inferred that the disks which form in the runs with turbulence are not very different from the ones that formed in the runs without turbulence initially.…”

“…While the density profile is essentially independent of the density threshold applied to the sink, its value determine the largest density value and therefore the mass of the whole disk. Hennebelle et al (2020) speculated that in reality this condition would be determined by the star-disk connection and inferred a simple model for the accretion density threshold that should be used. They indeed found that it is resolution dependent.…”

An observational correlation between magnetic field, angular momentum and fragmentation in the envelopes of Class 0 protostars?

“…Numerical simulations taking into account non-ideal MHD effects (such as ambipolar diffusion or Hall effect) were able to overcome the magnetic braking catastrophe, leading to the formation of disks similar to those observed (Hennebelle et al 2016;Zhao et al 2018;Wurster & Bate 2019). Studies by Hennebelle et al (2020) or Wurster & Lewis (2020) seem to also predict that the misalignment of the magnetic field with the envelope rotation axis directly affects the protostellar disk formation, for instance leading to the formation of larger planet-forming disks in the misaligned cases investigated compared to the smaller disks observed in aligned cases. This is particularly clear if the field intensity is such that the mass-to-flux ratio is on the order of 10.…”

Chemical evolution during the formation of a protoplanetary disk

“…First, the barotropic EOS is a crude approximation of the radiative transfer modeling of the star-disk system, but remains efficient as long as no central object with an internal luminosity (the protostar) has formed. Second, we do not employ sink particles either to describe the central collapsing part, as is done in Hennebelle et al (2020) for instance, because we restrict our study to the first Larson core stage. Sink particles and a more accurate radiation transport scheme (e.g., Mignon-Risse et al 2020) will be used in future works investigating a longer time evolution.…”