Abstract
This paper examines the outflows associated with the interaction of a stellar magnetosphere with an accretion disk. In particular, we investigate the magnetospheric ejections (MEs) due to the expansion and reconnection of the field lines connecting the star with the disk. Our aim is to study the dynamical properties of the outflows and evaluate their impact on the angular momentum evolution of young protostars. Our models are based on axisymmetric time-dependent magneto-hydrodynamic simulations of the interaction of the dipolar magnetosphere of a rotating protostar with a viscous and resistive disk, using alpha prescriptions for the transport coefficients. Our simulations are designed in order to model: the accretion process and the formation of accretion funnels; the periodic inflation/reconnection of the magnetosphere and the associated MEs; the stellar wind. Similarly to a magnetic slingshot, MEs can be powered by the rotation of both the disk and the star so that they can efficiently remove angular momentum from both. Depending on the accretion rate, MEs can extract a relevant fraction of the accretion torque and, together with a weak but non-negligible stellar wind torque, can balance the spin-up due to accretion. When the disk truncation approaches the corotation radius, the system enters a "propeller" regime, where the torques exerted by the disk and the MEs can even balance the spin-up due to the stellar contraction. The MEs spin-down efficiency can be compared to other scenarios, such as the Ghosh & Lamb, X-wind or stellar wind models. Nevertheless, for all scenarios, an efficient spin-down torque requires a rather strong dipolar component, which has been seldom observed in classical T Tauri stars. A better analysis of the torques acting on the protostar must take into account non-axisymmetric and multipolar magnetic components consistent with observations.
Highlights
Classical T Tauri stars (CTTS) are pre-main sequence stars that show clear signatures of accretion from a surrounding accretion disk (Edwards et al 1994; Hartmann et al 1998) and ejection in the form of collimated jets (Cabrit et al 1990; Burrows et al 1996)
Four groups of field lines can be distinguished: (1) the field lines steadily connecting the disk with the star below the magnetic surface (b); (2) the open field lines anchored on the surface of star at latitudes higher than the position of the surface (a); (3) the open field lines attached to the accretion disk beyond the surface (c); (4) the field lines enclosed between surfaces (a), (b), and (c) connecting the disk with the star, periodically evolving through stages of inflation, reconnection, and contraction
The accretion spin-up torque can be parametrized as Jacc,s = Macc,s jacc,s, where jacc,s is the specific angular momentum transported by the accretion streams and along those magnetic surfaces connected to the disk that are not mass-loaded
Summary
Classical T Tauri stars (CTTS) are pre-main sequence stars that show clear signatures of accretion from a surrounding accretion disk (Edwards et al 1994; Hartmann et al 1998) and ejection in the form of collimated jets (Cabrit et al 1990; Burrows et al 1996). These stellar winds would carry the entire mass flux typically observed in T Tauri jets, which seems unlikely (Cabrit 2009) Such high ejection efficiency presents a serious energetic problem (Ferreira et al 2006): since CTTS are slow rotators and their centrifugal push is not strong enough to drive these outflows, an extra energy input is required. It is quite tricky to assume that some accretion energy would be missing (the fraction that would possibly feed the stellar wind), while still explaining the observed UV luminosity It would imply an even higher mass flux onto the star, a higher spinning up torque (Zanni & Ferreira 2011).
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