Are the outflows in FU Orionis systems driven by the stellar magnetic field?

Abstract

FU Orionis (FUOR) outbursts are major optical brightening episodes in low-mass protostars that correspond to rapid mass-accretion events in the innermost region of a protostellar disc. The outbursts are accompanied by strong outflows, with the inferred mass outflow rates reaching ~10% of the mass inflow rates. Shu et al. proposed that the outflows represent accreted disc material that is driven centrifugally from the spun-up surface layers of the protostar by the stellar magnetic field. This model was critiqued by Calvet et al., who argued that it cannot reproduce the photospheric absorption-line shifts observed in the prototype object FU Ori. Calvet et al. proposed that the wind is launched, instead, from the surface of the disc on scales of a few stellar radii by a non-stellar magnetic field. In this paper we present results from numerical simulations of disc accretion on to a slowly rotating star with an aligned magnetic dipole moment that gives rise to a kilogauss-strength surface field. We demonstrate that, for parameters appropriate to FU Ori, such a system can develop a strong, collimated disc outflow of the type previously identified by Romanova et al. in simulations of protostars with low and moderate accretion rates. At the high accretion rate that characterizes FUOR outbursts, the radius at which the disc is truncated by the stellar magnetic field moves much closer to the stellar surface, but the basic properties of the outflow, which is launched from the vicinity of the truncation radius along opened-up stellar magnetic field lines, remain the same, and are distinct from those of the mechanism proposed by Shu et al. We show that the simulated outflow can in principle account for the main observed characteristics of FUOR winds, including the photospheric line shifts measured in FU Ori.

11 pages, 4 figures, 1 table, to appear in MNRAS

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