Other
Scientific paper
Nov 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996dln.........21d&link_type=abstract
Daresbury Laboratory: Newsletter on Analysis of Astronomical Spectra, p. 21
Other
Molecular Gases, Star Formation, Turbulent Boundary Layer, Turbulent Mixing, Shock Waves, Herbig-Haro Objects, Boundary Layers, Entrainment, Steady State, Shrouds, Mach Number, Light Emission, Bow Waves
Scientific paper
There is growing support for the idea that massive molecular outflows from young stars are driven by collimated, high-velocity winds or 'jets'. The former are usually observed in maps of 'moderate-velocity' molecular line emission, most often in CO J=1-0 or 2-1. The jets, on the other hand, are only seen because of the transient optical and/or near-infrared line emission features which result from internal or boundary layer shocks - these shock features are well known as Herbig-Haro (HH) objects. However, we do not fully understand how the molecular outflows are driven by the collimated jets. Two general models prevail: (1) In the 'prompt entrainment' scenario, ambient, molecular gas is swept up through bow shocks which form at the end of or along the length of, a collimated stellar jet. Alternatively, (2) in the 'steady-state' model, ambient gas is entrained along the length of the jet via a turbulent boundary layer. Both models have their advantages: the steady state model is able to produce broad molecular outflows, as are often observed, although detailed calculations of turbulent mixing layers predict very low columns of entrained molecular gas. Conversely, the prompt entrainment model has difficulty in producing wide molecular outflows. It can, however, explain the occurrence of high-velocity molecular peaks or 'bullets' observed in many outflows, because of the build-up of mass behind each entraining bow shock. Notably, the prompt entrainment mechanism also inhibits steady-state entrainment, since the bow shock formed at the end of a high Mach number jet will push the ambient gas aside into a high-density 'shroud', which will be separated from the beam of the jet by a warm, low density 'cocoon'. Contact between the jet and the ambient gas is therefore restricted.
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