T Tauri Jet Physics Resolved Near the Launching Region with theHubble Space Telescope

Abstract

We present an analysis of the gas physics at the base of jets from five T Tauri stars based on high angular resolution optical spectra, using the Hubble Space Telescope Imaging Spectrograph (HST STIS). The spectra refer to a region within 100 AU of the star, i.e., where the collimation of the jet has just taken place. We form position-velocity (PV) images of the line ratios to get a global picture of the flow excitation. We then apply a specialized diagnostic technique to find the electron density, ionization fraction, electron temperature, and total density. Our results are in the form of PV maps of the obtained quantities, in which the gas behavior is resolved as a function of both radial velocity and distance from the jet axis. They highlight a number of interesting physical features of the jet collimation region, including regions of extremely high density, asymmetries with respect to the axis, and possible shock signatures. Finally, we estimate the jet mass and angular momentum outflow rates, both of which are fundamental parameters in constraining models of accretion-ejection structures, particularly if the parameters can be determined close to the jet footpoint. Comparing mass flow rates for cases where the mass accretion rate is available in the literature (i.e., for DG Tau, RW Aur, and CW Tau) reveals a mass ejection-to-accretion ratio of 0.01-0.07. Finally, where possible (i.e., for DG Tau and CW Tau), both mass and angular momentum outflow rates are resolved into higher and lower velocity jet material. For the clearer case of DG Tau, this reveals that the more collimated higher velocity component plays a dominant role in mass and angular momentum transport.