Bipolar Molecular Outflows: T Tauri Stars and

Herbig-Haro Objects

by Seung-Urn Choe

Under the supervision of Professor Thomas W. Jones

ABSTRACT

The relations of Herbig-Haro objects to the observed bipolar molecular outflows with T Tauri stars are studied. An evaporation disk model is proposed to obtain the shape of the disk where gas evaporates and to explain the collimation of the central T Tauri wind. In this case the collimation angel is about 10°. The collimated T Tauri wind making a form of the de Laval nozzle viscously interacts with the surrounding medium. This interaction enhances the second collimation (about 40°) of the resulting flow, mixing stellar and disk winds with external molecular gas. These viscous outflows are observed in the bipolar molecular outflow of the T Tauri stars.
 
It is also proposed in the model that a Kelvin-Helmholtz instability in the throat of the de Laval nozzle produces clumps, which can be accelerated by the ram pressure of the collimated wind up to the wind speed. The clumps eventually ass through a shock in the outflow, which results from its encounter with the ambient cloud. the clumps are then moving faster than the surrounding flow. These clumps are identified with Herbig-Haro objects. As they move supersonically outward relative to the surrounding flow, they produce bow shocks in the direction of their motions. The bow shocks produce characteristic emission line profiles, which can be predicted by a simple calculations. During their outward motions, the pressure nonuniformity around them change their own shapes.