Collimation of stellar winds by nonadiabatic de Laval nozzles

Abstract

The interaction between an isotropic stellar wind and a stratified environment can lead to the formation of de Laval nozzles (in the adiabatic case) or to the formation of an elongated cavity surrounded by a dense, cold shell of shocked gas (in the limit of short cooling distances, i.e., in the highly nonadiabatic case). A preliminary exploration of the intermediate regime between the adiabatic and the highly nonadiabatic regimes yields very interesting results. We find that while for large cooling distances (i.e., cooling distances larger than ~5 times the environmental scale height) the flow resembles the adiabatic de Laval nozzle, for short cooling distances (i.e., cooling distances smaller than ~ 5 times the environmental scale height) the flow is considerably different from the adiabatic case, leading to the formation of very narrow, well collimated, cold jets. A preliminary comparison between observations of the HH 1/2 source and radio free-free spectra computed from our models gives very encouraging results.