The Millennium Outburst of the Cool Hypergiant ρ Cassiopeiae : Spectroscopy and Modeling

Between 2000 June and September an exceptional variability phase occurred in the peculiar F-type hypergiant ρ 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 ρ 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 ˙ M ≃5.4×10 −2 M ⊙ yr −1. 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-1 Guest investigator of the UK Astronomy Data Centre. 1 2 A. Lobel et al. type supergiants µ Cep (Ia) and Betelgeuse (Iab). During the outburst, central emission appears above the local continuum level in the split Na D lines. A prominent optical emission line spectrum appears in variability phases of fast wind expansion. The radial velocity curves of Hα, and of photospheric metal absorption lines signal a very extended, and velocity stratified dynamic atmosphere. The outburst spectra indicate the formation of a low-temperature, optically thick circumstellar gas shell of 3×10 −2 M ⊙ 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 timescale , whereby the shell ejection is driven by the release of hydrogen recombination energy. The observations reveal that during the outburst the atmospheric hydrogen recombination zone dissipates ∼5 % of the stellar luminosity into the circumstellar environment, by driving a su-perwind. 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 η Carinae. These phases of punctuated mass-loss represent the major mass-loss mechanism of cool massive hypergiants. 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.