Velocity study of axisymmetric protostellar jets with molecular cooling

Abstract

Jets of gas released from young stars excavate cavities and drive bipolar outflows. The outflow properties may be related to the speed of the jets. To test this, we study the propagation of supersonic overdense jets through axisymmetric hydrodynamic simulations with radiative cooling and chemistry, building on previous studies by injecting molecular and atomic jets with a wide range of speeds, between 50 and 300 km s -1 , into both molecular and atomic media. We show that the high collimation of outflows driven by molecular jets holds for all jet speeds. At the higher speeds, we find that the jet Mach number is the critical parameter which determines the shape of the cavity and the cavity is filled with atomic gas. However, at low speeds the jet material is the key factor with atomic jets producing much wider cavities, whereas molecular jets produce narrow cool molecular sheaths. A Mach disc is associated with the leading edge of the atomic simulations, whereas oblique shocks which refocus the jet are found in molecular flows. We also examine the mass spectra (distribution of mass with radial velocity), generally finding quite shallow relationships for all jet speeds (i.e the y index is typically 1-2). Steep molecular mass spectra are, however, associated with the atomic-jet-molecular-medium combination. We conclude that the properties of bipolar outflows possess signatures related to the jet speed but are probably more sensitive to other factors.