  On UT 2009 January 16, we observed a white light megaflare on the dM4.5e star
YZ CMi as part of a long-term spectroscopic flare-monitoring campaign to
constrain the spectral shape of optical flare continuum emission. Simultaneous
U-band photometric and 3350A-9260A spectroscopic observations were obtained
during 1.3 hours of the flare decay. The event persisted for more than 7 hours
and at flare peak, the U-band flux was almost 6 magnitudes brighter than in the
quiescent state. The properties of this flare mark it as one of the most
energetic and longest-lasting white light flares ever to be observed on an
isolated low-mass star. We present the U-band flare energetics and a flare
continuum analysis. For the first time, we show convincingly with spectra that
the shape of the blue continuum from 3350A to 4800A can be represented as a sum
of two components: a Balmer continuum as predicted by the Allred et al
radiative hydrodynamic flare models and a T ~ 10,000K blackbody emission
component as suggested by many previous studies of the broadband colors and
spectral distributions of flares. The areal coverage of the Balmer continuum
and blackbody emission regions vary during the flare decay, with the Balmer
continuum emitting region always being significantly (~3-16 times) larger.
These data will provide critical constraints for understanding the physics
underlying the mysterious blue continuum radiation in stellar flares.
