Thermal self-similar disc winds: transonic nature

Abstract

ABSTRACT Thermally driven two-dimensional disc winds are examined under the fully self-similar treatment, focusing the attention on the transonic nature. In the spherical potential, such as point-mass and logarithmic ones, the usual transonic solutions cannot be constructed, because the critical conditions themselves do not exist. That is, the flow can reach the sonic point in the polar direction, while there is no gravitational force in the polar direction, and the regularity condition does not hold. In the non-spherical potential, such as the Mestel self-gravitating one, the gravity works in the polar direction, and therefore, there are critical points in the flow. However, the usual transonic solutions do not exist, since the topology of the critical points is always centre (O-type). Hence, in the fully self-similar disc winds, the usual transonic solutions passing through the critical points do not exist, at least for typical cases. Instead, the new-type wind solutions are found in the point-mass case. In contrast to the usual transonic flows, the new-type solutions are continuously accelerated along the streamline by the thermal pressure, and eventually become supersonic in the streamline direction without passing through the critical points. These new-type solutions are possible, when the density distribution is centrally concentrated, and when the initial subsonic velocity is not so large to avoid singularities.