Helicoidal Astrophysical Outflows

Abstract

We present three classes of analytic solutions of the basic hydrodynamic equations that we assume to govern the flow of plasma in the atmosphere of a rotating astrophysical object. In the first class, we have accelerating helical wind-type outflows wherein a finite speed at the stellar surface accelerates monotonically outwards to an asymptotic speed at large radial distances, with a sinusoidal dependence in the latitude corresponding to a maximum flow at the equator and no flow at the poles. In the second, we have decelerating helical outflows wherein a finite initial speed decelerates monotonically to a lower speed at infinity, with a latitudinal dependence such that the flow is concentrated at the poles. As a special subcase of this class we obtain an accretion-type helical inflow. Finally, the third class of solutions with a spherically symmetric radial flow speed in a non-spherically symmetric plasma density may be regarded as a superposition of the previous two classes, i.e., it accelerates close to the base and decelerates further away to a constant asymptotic speed. The solutions are written in terms of dimensionless physical parameters in order to facilitate their direct application to specific astrophysical flows? such as in winds from massive pre-main sequence objects and T Tauri stars, or, bipolar flows in young stellar objects.