  Near-ultraviolet spectroscopic data obtained with the HST STIS instrument on
the dMe flare star YZ Canis Minoris (YZ CMi) were analyzed. Flare and quiet
intervals were identified from the broadband near-UV light curve, and the
spectrum of each flare was separately extracted. Mg II and Fe II line profiles
show similar behavior during the flares. Two large flares allowed time-resolved
spectra to be analyzed, revealing a very broad component to the Mg II k line
profile in at least one flare spectrum (F9b). If interpreted as a velocity,
this component requires chromospheric material to be moving with FWHM ~ 250
km/sec, implying kinetic energy far in excess of the radiative energy. The Mg
II k flare line profiles were compared to recent radiative hydrodynamic models
of flare atmospheres undergoing electron beam heating. The models successfully
predict red enhancements in the line profile with typical velocity of a few
km/sec, but do not reproduce the flares showing blue enhancements, or the
strongly broadened line observed in flare F9b. A more complete calculation of
redistribution into the line wings, including the effect of collisions with the
electron beam, may resolve the origin of the excess line broadening.
