Abstract:

Cool magnetohydrodynamics (MHD) disc wind physics is reviewed by means of a self-similar analytical model, putting special emphasis on the mathematical aspects of the solution. It is found that the key parameter of the theory (μ.) measures the relation between magnetic and tidal forces. The generation of MHD winds from accretion discs requires a subtle tuning between both stresses because only a narrow range of μ values is allowed; this range is, indeed, close to the cut-off of the magnetic turbulence induced by the development of the Balbus-Hawley instability. The space of solutions can be separated into two quite distinct classes: low-μ solutions generate magnetically dominated outflows and display a characteristic density change from horizontal to vertical stratification, while in high-μ solutions the density decreases without any intermediate enhancement as the rotation axis is approached. These theoretical (dynamical) results have been used to study the properties of the base of the wind. Density and velocity laws have been derived directly from the dynamics. The effect of the propagation of the stellar X-ray radiation through the wind has been analysed to determine the temperature law at the base of the wind (polar angles θ > 45°). It is shown that a cocoon of photoionized gas is generated around the star. The extent of the photoionized region is small (tenths of au) in dense outflows and close to the disc plane; however, it may cover the whole wind extent in diffuse winds, e.g. disc winds generated by small accretion rates (≤10-9 M⊙ yr-1). Photoionization also modifies the electron density in the plasma. As a consequence, the ambipolar diffusion heating decreases in the inner part of the wind by roughly one order of magnitude with respect to that derived by other authors. In fact, radiative heating controls the thermal properties of the inner 0.3 and 1 au of the disc wind for accretion rates of 10-7 and 10-8 M⊙ yr-1, respectively. The temperature of the densest region (base) of the wind is, at most, ∼ 10 000 K. Therefore, although densities as high as ∼109 cm-3 can be achieved by disc winds, the temperature is significantly smaller than the ∼5 × 105-8 × 105 K derived from the ultraviolet (UV) observations of the base of the optical jets. Also, it is shown that densities as high as ∼109 cm-3 cannot be achieved at the jet recollimation point for the accretion rates observed in the T Tauri stars. In summary, we conclude that the flow traced by the UV semiforbidden lines is not associated with cold disc winds but, most likely, it is tracing the hot inner jet, postulated in cold disc wind theory, which prevents the radial collapse of the wind.