  Between 2000 June and September an exceptional variability phase occurred in
the peculiar F-type hypergiant Rho Cas, when the V-brightness dimmed by at
least a full magnitude. The star recovered from this deep minimum by 2001
April. It is the third outburst of Rho Cas on record in the last century. We
observe TiO absorption bands in high-resolution optical and near-IR spectra
obtained with the Utrecht Echelle Spectrograph during the summer of 2000. TiO
formation in the outer atmosphere occurred before the deep brightness
minimum.Atmospheric models reveal that the effective temperature decreases by
at least 3000 K, and the TiO shell is driven supersonically with Mdot =~5.4
10^-2 Msun/yr. Strong episodic mass loss and TiO have also been observed during
the outbursts of 1945-47 and 1985-86. An analysis of the exceptional outburst
spectra of 2000-01 is provided, by comparing with high-resolution optical
spectra of the early M-type supergiants Mu Cep (Ia) and Betelgeuse (Iab).
During the outburst, central emission appears above the local continuum level
in the split Na D lines. The outburst spectra indicate the formation of a low-
temperature, optically thick circumstellar gas shell of 3 10^-2 Msun during
\~200 d, caused by dynamic instability of the upper atmosphere of this
pulsating massive supergiant near the Eddington luminosity limit. We present an
equation that correctly predicts the outburst time-scale, whereby the shell
ejection is driven by the release of hydrogen recombination energy. We observe
that the mass-loss rate during the outburst is of the same order of magnitude
as has been proposed for the outbursts of Eta Carinae. The research results in
this paper are described in further detail in Lobel et al. (2003). A spectral
movie sequence of the outburst is available at
http://cfa-www.harvard.edu/~alobel/
