Mass accretion to young stars triggered by flaring activity in circumstellar discs

Abstract

Young low-mass stars are characterized by ejection of collimated outflows and by circumstellar discs which they interact with through accretion of mass. The accretion builds up the star to its final mass and is also believed to power the mass outflows, which may in turn remove the excess angular momentum from the star-disc system. However, although the process of mass accretion is a critical aspect of star formation, some of its mechanisms are still to be fully understood. A point not considered to date and relevant for the accretion process is the evidence of very energetic and frequent flaring events in these stars. Flares may easily perturb the stability of the discs, thus influencing the transport of mass and angular momentum. Here we report on 3D magnetohydrodynamic modelling of the evolution of a flare with an idealized non-equilibrium initial condition occurring near the disc around a rotating magnetized star. The model takes into account the stellar magnetic field, the gravitational force, the viscosity of the disc, the magnetic-field-oriented thermal conduction (including the effects of heat flux saturation), the radiative losses from optically thin plasma and the coronal heating. We show that, during its first stage of evolution, the flare gives rise to a hot magnetic loop linking the disc to the star. The disc is strongly perturbed by the flare: disc material evaporates under the effect of the thermal conduction and an overpressure wave propagates through the disc. When the overpressure reaches the opposite side of the disc, a funnel flow starts to develop there, accreting substantial disc material on to the young star from the side of the disc opposite to the flare.