A model for collimated outflows in molecular clouds and the case of HH 7–11
Collimated outflows of high-velocity gas, with typical scale lengths of 0.01–0.2 pc, are often observed to be associated with linear chains of optical emission knots1, sometimes connected by faint nebulosities. The flows have been ascribed to the interaction between an energetic wind ejected by a young stellar object and circumstellar material. However, none of the acceleration and collimation mechanisms proposed so far can easily account for the most peculiar properties of the outflows, namely the almost constant spacing between the knots, their radial motion and the abrupt velocity changes along the flow, well illustrated by the prototype we use for our modelling, HH 7–11. We suggest here a new interpretation of the phenomenon, based on flows in a channel of variable cross-sectional area due to Kelvin–Helmholtz instabilities between the flow and the ambient medium; and present solutions of the Mach number equation for such a channel, which possess multiple critical points and shocks identified with the observed optical knots.
Evolution of Perturbations and Shock Formation in Stellar Winds and Jets.
We study the temporal evolution of disturbances in spherically symmetric and polytropic winds from a central source. Such disturbances may be due to localized momentum addition in the outflow, or localized deviation from spherically symmetric expansion. We follow the evolution of an initial steady state which is perturbed to a continuous or discontinuous final equilibrium state, as predicted by previous calculations of stationary flows. We show that some of the predicted discontinuous equilibrium solutions are not physically accessible, while the attainment of the other equilibrium solutions depends on both temporal and spatial parameters characterizing the perturbation.
The subsonic structure of radiatively driven winds of early-type stars.
Methods are developed for the calculation of extended nongray model atmospheres for early-type stars with radiatively driven mass loss. Models for a central star of a planetary nebula and a Wolf-Rayet star are calculated, and compared with hydrostatic model calculations. Scaling laws are given for explaining the differences and similarities of stars of different stellar mass. It is found that transonic flow cannot occur if only continuum sources of opacity are present, because a moderate outward increase of the flux mean opacity is required for a physically acceptable structure. Observations of the Of star zeta Pup and the Wolf-Rayet star HD 50896 are summarized that indicate the continuum formation regions are spatially extended. (AIP)