Magnetically Driven Jets from Accretion Disks. I. Steady Solutions and Application to Jets/Winds in Young Stellar Objects

Abstract

We solve one-dimensional steady and axisymmetric magnetohydrodynamic (MHD) equations to study basic properties of astrophysical jets from accretion disks. Assuming the configuration of the poloidal magnetic field, we solve for a wide range of parameters of the poloidal magnetic field strength in the disk. We include a thermal energy in the solution, although the jet is mainly accelerated by the magnetic force, so that we are able to obtain the mass flux of the jet and physical quantities, such as temperature, in the disk. We find that the mass flux () depends on the poloidal magnetic field strength of the disk (Bp0) when the toroidal component of the magnetic field (B0) is dominant near the disk surface, although it is independent of the magnetic field when the poloidal component is dominant there: Since Michel's minimum energy solution [v${i}$ -->(B -->2p0/) -->1/3] is almost satisfied in the magnetically driven jets, the terminal velocity (v∞) depends on Bp0 as v${i}$ -->B -->1/3p0 when | B/Bp |0 1, and as v${i}$ -->B -->2/3p0 when | B/Bp |0 1. When the toroidal component of the magnetic field is dominant near the disk surface (| B/Bp |0 1), the acceleration mainly takes place after the flow speed exceeds the Alfven speed. This means that the magnetic pressure largely contributes to the acceleration of these jets. We also study the dependence of mass flux on the other parameters, such as inclination angle of the poloidal field, the rotational velocity of the disk, and the r-dependence of the poloidal magnetic field strength along the field line, where r is the distance from the axis. We discuss the application of these models, i.e., the MHD jets from accretion disks, to jets/winds observed in young stellar objects (such as optical jets, T Tauri winds, and fast neutral winds). The mass-loss rates observed in these jets/winds will constrain the physical quantities in the disks. When the mass-loss rate is ~10 -->−8 M☉ yr-1 and the velocity is vjet ~ 100 km s-1, the physical quantities in the disk are n0 ~ 1013 cm-3, Bp0 ~ 10 G, and T0 ~ 1000 K for a jet/wind that is produced around r0 ~ 15 R☉, and the mass of the central star is 1 M☉, where n0 is the number density, T0 is the temperature in the disk, and r0 is the radial distance from the central star.