Resolving the Inner Arcsecond of the RY Tau Jet with HST

Faint X-ray emission from hot plasma (T-x > 10(6) K) has been detected extending outward a few arcseconds along the optically delineated jets of some classical T Tauri stars including RY Tau. The mechanism and location where the jets are heated to X-ray temperatures are unknown. We present high spatial resolution Hubble Space Telescope (HST) far-ultraviolet long-slit observations of RY Tau with the slit aligned along the jet. The primary objective was to search for C iv emission from warm plasma at T-C (IV) similar to 10(5) K within the inner jet (<1 '') that cannot be fully resolved by X-ray telescopes. Spatially resolved C IV emission is detected in the blueshifted jet extending outward from the star to 1 '' and in the redshifted jet out to 0.'' 15. C IV line centroid shifts give a radial velocity in the blueshifted jet of -136 +/- 10 km s(-1) at an offset of 0.'' 29 (39 au) and deceleration outward is detected. The deprojected jet speed is subject to uncertainties in the jet inclination, but values greater than or similar to 200 km s(-1) are likely. The mass loss rate in the blueshifted jet is at least M-jet,M-blue = 2.3 x 10(-9) M-circle dot yr(-1), consistent with optical determinations. We use the HST data along with optically determined jet morphology to place meaningful constraints on candidate jet heating models including a hot-launch model in which the jet is heated near the base to X-ray temperatures by an unspecified (but probably magnetic) process, and downstream heating from shocks or a putative jet magnetic field.