Magnetocentrifugally driven flows from young stars and disks. I. A generalized model

We propose a generalized model for stellar spin-down, disk accretion, and truncation, and the origin of winds, jets, and bipolar outflows from young stellar objects. We consider the steady state dynamics of accretion of matter from a viscous and imperfectly conducting disk onto a young star with a strong magnetic field. For an aligned stellar magnetosphere, shielding currents in the surface layers of the disk prevent stellar field lines from penetrating the disk everywhere except for a range of radii about ω R_x, where the Keplerian angular speed of rotation Ω_x quals the angular speed of the star Ω. For the low disk accretion rates and high magnetic fields associated with typical T Tauri stars, R_x exceeds the radius of the star R by a factor of a few, and the inner disk is effectively truncated at a radius R_t somewhat smaller than R_x. Where the closed field lines between R_t and R_x bow sufficiently inward, the accreting gas attaches itself to the field and is funneled dynamically down the effective potential (gravitational plus centrifugal) onto the star. Contrary to common belief, the accompanying magnetic torques associated with this accreting gas may transfer angular momentum mostly to the disk rather than to the star. Thus, the star can spin slowly as long as R_x remains significantly greater than R. Exterior to R_x field lines threading the disk bow outward, which makes the gas off the midplane rotate at super-Keplerian velocities. This combination drives a magnetocentrifugal wind with a massloss rate Ṁ_w equal to a definite fraction f of the disk accretion rate M_D. For high disk accretion rates, R_x is forced down to the stellar surface, the star is spun to breakup, and the wind is generated in a manner identical to that proposed by Shu, Lizano, Ruden, & Najita in a previous communication to this journal. In two companion papers (II and III), we develop a detailed but idealized theory of the magnetocentrifugal acceleration process.