  Faint X-ray emission from hot plasma (T > 1 MK) 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 jet is
heated to X-ray temperatures is unknown. We present high spatial resolution 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(CIV) ~ 10$^{5}$ K within the inner jet (<1 arcsec) 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 arcsec and
in the redshifted jet out to 0.5 arcsec. C IV line centroid shifts give a
radial velocity in the blueshifted jet of -136 $\pm$ 10 km/s at an offset of
0.29 arcsec (39 au) and deceleration outward is detected. The deprojected jet
speed is subject to uncertainties in the jet inclination but values >200 km/s
are likely. The mass-loss rate in the blueshifted jet is at least 2.3 $\times$
10$^{-9}$ M_sun 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.
