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.
knots in stellar jets : crossing shocks or internal working surfaces?
We present analytic and numerical models of a jet from a source with a time-dependent ejection velocity. This time-dependence results in the formation of internal working surfaces, which travel down the jet flow. Knots in stellar jets might correspond to such internal working surfaces. We also discuss the more general problem of a jet from a source with a general (velocity+direction) time- variability.
Circumstellar disks and rings: observational results
Presentation de resultats d'observations dans les regions de formations d'etoiles concernant les disques et les anneaux circumstellaires: discussion detaillee de la nebuleuse bipolaire 5106 entouree d'un nuage moleculaire et du nuage noir L1551 dans le complexe Taurus