Magnetically Channeled Accretion in T Tauri Stars

Abstract

We review observational evidence and open issues related to the process of magnetospheric accretion in T Tauri stars. Emphasis is put on recent numerical simulations and observational results which suggest that the interaction between the stellar magnetosphere and the inner accretion disk is a highly time dependent process on timescales ranging from hours to months.

Magnetic Interaction Between Stars and Accretion Disks

“…The spin-up magnetic torque due to the field lines connecting to the disk inside r co is balanced by the spin-down torque by the lines connecting to the disk outside r co . Königl has proposed that a magnetic field of 10 3 G at the stellar surface (a value consistent with observations, see Bouvier et al, 2003) can disrupt the disk at a few stellar radii (but well inside the corotation radius) and channel the accretion flow to higher stellar latitudes. He also has estimated the typical time needed to bring the star into the spin equilibrium with the disk to be ∼ 10 5 years, much shorter than the typical accretion time for CTTSs.…”

“…This picture has been suggested by van Ballegooijen (1994) and has subsequently been studied in extensive numerical simulations by a number of authors (Goodson et al, 1997(Goodson et al, , 1999Goodson and Winglee, 1999;Hayashi et al, 2000;Matt et al, 2002). Some recent observational results also seem to favor this point of view (e.g., Bouvier et al, 2003). Let us consider this most interesting scenario in more detail.…”

“…Unfortunately, the value of this diffusivity is not very well known (e.g., Bouvier et al, 2003). Nevertheless, one can set a reasonable upper limit by assuming that it is caused by the same turbulence that facilitates angular momentum transport across the disk.…”

“…The situation depends critically on the disk's effective magnetic diffusivity (which we shall sometimes call the resistivity). Unfortunately, the value of this diffusivity is not very well known (e.g., Bouvier et al, 2003). Nevertheless, one can set a reasonable upper limit by assuming that it is caused by the same turbulence that facilitates angular momentum transport across the disk.…”