MAGNETOCENTRIFUGALLY DRIVEN FLOWS FROM YOUNG STARS AND DISKS .1. 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 fr om young stellar objects. We consider the steady state dynamics of acc retion of matter from a viscous and imperfectly conducting disk onto a young star with a strong magnetic field. For an aligned stellar magne tosphere, 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 omegaBAR = R(x), where the Keplerian angular spe ed of rotation OMEGA(x) equals the angular speed of the star OMEGA. F or the low disk accretion rates and high magnetic fields associated wi th 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 radi us R(t) somewhat smaller than R(x). Where the closed field lines betwe en R(t) and R(x) bow sufficiently inward, the accreting gas attaches i tself to the field and is funneled dynamically down the effective pote ntial (gravitational plus centrifugal) onto the star. Contrary to comm on belief, the accompanying magnetic torques associated with this accr eting 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 s ignificantly greater than R. Exterior to R(x) field lines threading t he disk bow outward, which makes the gas off the midplane rotate at su per-Keplerian velocities. This combination drives a magnetocentrifugal wind with a mass-loss rate M(w) equal to a definite fraction f of the disk accretion rate M(D). For high disk accretion rates, R(x) is forc ed down to the stellar surface, the star is spun to breakup, and the w ind 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 the ory of the magnetocentrifugal acceleration process.