Fragmentation, disks, and outflows in collapsing low and high-mass dense cores
Most stars form in multiple systems. This indicates that a fragmentation process occurs during star formation, which can be explained by several mechanisms. The first picture is to consider the interplay between turbulence and gravity within molecular clouds, which can lead to an initial fragmentation prior to the collapse. In this picture, the stellar initial mass function (IMF) is mainly determined at the dense-core formation stage, and the dense cores undergo collapse without fragmenting into individual objects. On the other hand, fragmentation may also occur during the collapse of molecular clouds or within disks that are formed because of the conservation of angular momentum. The fragmentation process thus remains a matter of intense debate; in particular, disk formation, outflows and early fragmentation (i.e., during the early phase of the collapse) appear to be critical for better constraining the star formation mechanism.
Disks and outflows formation is regulated by magnetic fields and rotational motions, but the details of angular momentum transport in collapsing dense cores remains partly understood.
I will present 3D radiation-magneto-hydrodynamics models of collapsing low-mass and high-mass dense cores using the adaptive-mesh-refinement code RAMSES (Teyssier 2002) which includes resistive MHD (Fromang et al. 2006, Masson et al. 2012) and radiative transfer (Commerçon et al. 2011, 2014, Gonzalez et al. 2015). The numerical models account for a wide range of initial masses, from one to hundred solar masses, as well as the effect of initial rotation and/or turbulence. I will show that the interplay between magnetic fields (via magnetic braking) and radiative transfer (via accretion shock) is of prime importance for the fragmentation of collapsing dense cores. Second, I will show how ambipolar diffusion acts in the formation of protostellar disks, which can subsequently launch collimated outflows depending on the initial conditions. I will discuss the properties of the disks and outflows (plasma beta, magnetic field topology, mass and extent) and the similarities between low and high-mass star formation. The numerical results will be compared to an analytical model of disk formation regulated by ambipolar diffusion (Hennebelle et al. 2016). Last, I will present comparison between synthetic observations derived from the numerical models and recent observations of low-mass and massive star forming regions.