Astronomy and Astrophysics – Astrophysics
Scientific paper
Apr 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996a%26a...308...77p&link_type=abstract
Astronomy and Astrophysics, v.308, p.77-90
Astronomy and Astrophysics
Astrophysics
58
Magnetohydrodynamics (Mhd), Stars: Pre-Main Sequence, Stars: Mass-Loss, Accretion
Scientific paper
Low-mass protostellar objects are often found to be surrounded by circumstellar disks. Due to a strong magnetization of the protostellar object and its accretion disk, accretion onto the central object occurs along magnetic surfaces. Similarly, winds are driven away by the magnetic properties of the underlying disk and star. We solve the stationary axisymmetric equations of motion for a polytropic flow in a given magnetosphere in a Newtonian approximation. Wind solutions are uniquely determined by the requirement that the flow successively passes through the slow magnetosonic, the Alfven and the fast magnetosonic points. Accretion flows, on the other hand, can never reach super-Alfvenic speeds. We consider several topologies for the acceleration of a wind: a magnetosphere which is built up by magnetic fields of the accretion disk only and a magnetosphere which results from the interaction of the stellar magnetic field with the surrounding accretion disk. In the latter case the wind can originate from the disk as well as from the star. The stellar magnetosphere is assumed to be of dipolar structure. In contrast to purely hydrodynamic winds, the form of the magnetic flux-tube is essential for the conversion of magnetic Poynting flux into kinetic energy of the wind. Initial conditions appropriate for T Tauri stars lead to outflow velocities of a few hundred kilometers per second, as observed in these systems. Winds driven by the magnetosphere of a disk are only slightly supermagnetosonic, while those driven away by a star-disk magnetosphere are found to be highly supermagnetosonic, provided the magnetic flux surfaces are collimated into a conical shape at a distance of a few thousand stellar radii. Magnetized winds carry away angular momentum and in this way exert a torque on the star-disk system. This leads to a braking of the stellar rotation. The corresponding time scale depends on the position of the Alfven point. Our results imply that rapid protostellar rotation can be spun down on a time-scale of a few million years. In this way, a protostar accreting from a circumstellar disk reaches an equilibrium rotation period at the level of about ten percent of breakup, which agrees with observed rotation periods of T Tauri stars. The strong stellar magnetisation leads to a gap between the star and the surrounding disk. For gaps of the order of a few stellar radii accretion achieves a sub-Alfvenic velocity of 300 km/s at the stellar surface. Flow topologies are discussed for parameters characteristic of T Tauri systems.
Camenzind Max
Paatz G.
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