Collapse, outflows and fragmentation of massive, turbulent and magnetized prestellar barotropic cores

Abstract

Context. Stars, and more particularly massive stars, have a drastic impact on galaxy evolution. Yet the conditions in which they form and collapse are still not fully understood. Aims. In particular, the influence of the magnetic field on the collapse of massive clumps is relatively unexplored, it is therefore of great relevance in the context of the formation of massive stars to investigate its impact. Methods. We perform high resolution, MHD simulations of the collapse of one hundred solar masses, turbulent and magnetized clouds, with the adaptive mesh refinement code RAMSES. We compute various quantities such as mass distribution, magnetic field, and angular momentum within the collapsing core and study the episodic outflows and the fragmentation that occurs during the collapse. Results. The magnetic field has a drastic impact on the cloud evolution. We find that magnetic braking is able to substantially reduce the angular momentum in the inner part of the collapsing cloud. Fast and episodic outflows are being launched with typical velocities of the order of 1−3 km s −1 , although the highest velocities can be as high as 20−40 km s −1 . The fragmentation in several objects is reduced in substantially magnetized clouds with respect to hydrodynamical ones by a factor of the order of 1.5−2. Conclusions. We conclude that magnetic fields have a significant impact on the evolution of massive clumps. In combination with radiation, magnetic fields largely determine the outcome of massive core collapse. We stress that numerical convergence of MHD collapse is a challenging issue. In particular, numerical diffusion appears to be important at high density and therefore could possibly lead to an overestimation of the number of fragments.

Protostellar disk formation and transport of angular momentum during magnetized core collapse

“…It is therefore unlikely that the Alfvén speed drops with radius as 1/r, and collapse calculations indeed show that the Alfvén speed remains roughly constant in the dense cores (e.g. Hennebelle et al 2011). In this case, the braking time is given by…”

EVN observations of 6.7 GHz methanol maser polarization in massive star-forming regions

“…Besides contributing to the formation of outflows, the simulations show that magnetic fields prevent fragmentation, reduce angular momentum via magnetic braking, and, marginally, influences the accretion rate (Banerjee & Pudritz 2007;Peters et al 2011;Hennebelle et al 2011;Seifried et al 2011). The magnetic fields also play a significant role in the evolution of the circumstellar disk.…”

“…By producing cavities through which radiation pressure can be released, these early outflows reduce the limitations on the final mass of massive stars imposed by simply considering the gravitational collapse. The outflows seem to be relatively fast and well-collimated for low and intermediate magnetic intensities (μ 1 = 30−120), and A&A 556, A73 (2013) more slowly and poorly collimated for stronger fields (μ ∼ 5; Hennebelle et al 2011;Seifried et al 2012a). Furthermore, Seifried et al (2012a) show that magneto-centrifugally driven outflows consist of two regimes.…”

Magnetically regulated fragmentation of a massive, dense, and turbulent clump

“…The main physical mechanisms that oppose gravity during collapse are intrinsic turbulence, radiation feedback, and magnetic pressure (e.g. Krumholz 2006;Hennebelle et al 2011). Feedback from nascent protostellar objects through outflows, winds, and/or expansion of ionised regions (especially from newly born massive objects) can be important in relatively evolved stages (Bate 2009), but even then seems to be of only secondary importance (Palau et al 2013).…”