The role of cosmic rays on magnetic field diffusion and the formation of protostellar discs

Abstract

Context. The formation of protostellar discs is severely hampered by magnetic braking, as long as magnetic fields remain frozen in the gas. The latter condition depends on the levels of ionisation that characterise the innermost regions of a collapsing cloud. Aims. The chemistry of dense cloud cores and, in particular, the ionisation fraction is largely controlled by cosmic rays. The aim of this paper is to evaluate whether the attenuation of the flux of cosmic rays expected in the regions around a forming protostar is sufficient to decouple the field from the gas, thereby influencing the formation of centrifugally supported disc. Methods. We adopted the method developed in a former study to compute the attenuation of the cosmic-ray flux as a function of the column density and the field strength in clouds threaded by poloidal and toroidal magnetic fields. We applied this formalism to models of low-and high-mass star formation extracted from numerical simulations of gravitational collapse that include rotation and turbulence.Results. For each model we determine the size of the magnetic decoupling zone, where collapse or rotation motion becomes unaffected by the local magnetic field. In general, we find that decoupling only occurs when the attenuation of cosmic rays is taken into account with respect to a calculation in which the cosmic-ray ionisation rate is kept constant. The extent of the decoupling zone also depends on the dust grain size distribution and is larger if large grains (of radius ∼10 −5 cm) are formed by compression and coagulation during cloud collapse. The decoupling region disappears for the high-mass case. This is due to magnetic field diffusion caused by turbulence that is not included in the low-mass models. Conclusions. We conclude that a realistic treatment of cosmic-ray propagation and attenuation during cloud collapse may lead to a value of the resistivity of the gas in the innermost few hundred AU around a forming protostar that is higher than generally assumed. Forthcoming self-consistent calculations should investigate whether this effect is strong enough to effectively decouple the gas from the field and to compute the amount of angular momentum lost by infalling fluid particles when they enter the decoupling zone.

Interstellar Dust Charging in Dense Molecular Clouds: Cosmic Ray Effects

“…The resulting flux of UV photons F UV (in the energy range between 11.2 and 13.6 eV) 4 More advanced models should take into account the fact that molecular clouds are magnetized and CRs gyrate along magnetic field lines in addition to being scattered by magnetic fluctuations on the scale of the particle gyroradius. For a detailed treatment of these effects, see Padovani & Galli (2011), Padovani et al (2013Padovani et al ( , 2014, and Morlino & Gabici (2015). 5 We note that the results presented in Section 3 practically do not depend on the precise form of the formula for x e .…”

Chemical solver to compute molecule and grain abundances and non-ideal MHD resistivities in prestellar core-collapse calculations

“…Padovani & Galli (2011);Padovani et al (2013Padovani et al ( , 2014a found that mirroring always dominates focusing for a field topology obtained by ideal-MHD simulations of collapsing rotating clouds and that the ionisation rate could vary by up to a factor 50, depending on the mass-to-flux ratio and the magnetic flux tube considered. In addition, CR are partly absorbed by the dense medium, which lowers the CR ionisation rate (Padovani et al 2009).…”

Magnetic diffusivities in 3D radiative chemo-hydrodynamic simulations of protostellar collapse

“…Padovani et al (2013Padovani et al ( , 2014 studied the propagation of CR along magnetic fields lines in collapsing cores, accounting for the effect of CR energy loss and magnetic mirroring. They show how CR ionisation rate is efficiently reduced in the central region because of the complex magnetic field lines configurations found in numerical models of protostellar collapse, making magnetic resistivities larger towards the cloud center.…”