Wind-driving protostellar accretion discs. II. Numerical method and illustrative solutions

(abridged) We continue our study of weakly ionized protostellar discs that are threaded by a large-scale magnetic field and power a centrifugally driven wind. It has been argued that in several protostellar systems such a wind transports a significant fraction of the angular momentum from at least some part of the disc. We model this case by considering a radially localized disc model in which the matter is well coupled to the field and the wind is the main repository of excess angular momentum. We consider stationary solutions in which magnetic diffusion counters the shearing and advection of the field lines. In Wardle & K\"onigl we analysed the disc structure in the hydrostatic approximation and presented disc/wind solutions for the ambipolar diffusivity regime. In K\"onigl, Salmeron & Wardle (Paper I) we generalized the hydrostatic analysis to the Hall and Ohm diffusivity domains and identified the parameter sub-regimes in which viable solutions occur. In this paper we test these results by deriving numerical solutions (integrated through the sonic critical surface) of the disc equations in the Hall domain. We confirm the predictions of the hydrostatic analysis and demonstrate its usefulness for clarifying the behaviour of the derived solutions. We show that the solutions can be extended to larger scales (so that they also cross the Alfv\'en critical surface) by matching the localized disc solutions to global wind solutions of the type introduced by Blandford & Payne. To facilitate this matching, we construct a library of wind solutions, which is made available to the community. The results presented in Wardle & K\"onigl, Paper I and this work form a comprehensive framework for the study of wind-driving accretion discs in protostellar and other astrophysical environments. This tool could be useful for interpreting observations and for guiding numerical simulations of such systems.Comment: 20 pages, 13 figures, 3 tables; submitted for publication in MNRA

The Effects of Large-Scale Magnetic Fields on Disk Formation and Evolution

This is a draft chapter for a book, entitled Physical Processes in Circumstellar Disks around Young Stars, which is scheduled for publication by the University of Chicago Press as one of its Theoretical Astrophysics Series volumes. Sect. 1 presents the motivation for considering the effects of a large-scale, ordered magnetic field on the formation and evolution of protostellar disks. Sect. 2 outlines the physical principles that underlie the magnetohydrodynamics of disks that are threaded by such a field. Sect. 3 discusses the formation and early evolution of disks that result from the collapse of a rotating molecular cloud core that is coupled to the insterstellar magnetic field. Sect. 4 reviews the observational evidence for the disk--wind connection and describes the structure of magnetically accelerated disk outflows, focusing on centrifugally driven winds; it then goes on to discuss the equilibrium and stability properties of weakly ionized protostellar accretion disks in which the transport of angular momentum is dominated by a wind of this type. Sect. 5 considers the coupling between the central protostar and the surrounding disk through the protostellar magnetic field, covering, in turn, the phenomenology, basic concepts, and results of numerical simulations. The chapter is summarized in Sect. 6, which also contains a discussion of future research directions.Comment: 68 pages, 8 figures, to appear in Physical Processes in Circumstellar Disks around Young Stars, ed. P. J. V. Garcia (Chicago: University of Chicago Press), uses svmult.cl

Classification of magnetized star--planet interactions: bow shocks, tails, and inspiraling flows

Close-in exoplanets interact with their host stars gravitationally as well as via their magnetized plasma outflows. The rich dynamics that arises may result in distinct observable features. Our objective is to study and classify the morphology of the different types of interaction that can take place between a giant close-in planet (a Hot Jupiter) and its host star, based on the physical parameters that characterize the system. We perform 3D magnetohydrodynamic numerical simulations to model the star--planet interaction, incorporating a star, a Hot Jupiter, and realistic stellar and planetary outflows. We explore a wide range of parameters and analyze the flow structures and magnetic topologies that develop. Our study suggests the classification of star--planet interactions into four general types, based on the relative magnitudes of three characteristic length scales that quantify the effects of the planetary magnetic field, the planetary outflow, and the stellar gravitational field in the interaction region. We describe the dynamics of these interactions and the flow structures that they give rise to, which include bow shocks, cometary-type tails, and inspiraling accretion streams. We point out the distinguishing features of each of the classified cases and discuss some of their observationally relevant properties. The magnetized interactions of star--planet systems can be categorized, and their general morphologies predicted, based on a set of basic stellar, planetary, and orbital parameters.Comment: Accepted for publication in A&