High‐Resolution Spectroscopy of the Yellow Hypergiant ρ Cassiopeiae from 1993 through the Outburst of 2000–2001

We present an overview of the spectral variability of the peculiar F-typehypergiant Rho Cas, obtained from our long-term monitoring campaigns over thepast 8.5 years with four spectrographs in the northern hemisphere. Between 2000June and September an exceptional variability phase occurred when theV-brightness dimmed by about a full magnitude. The star recovered from thisdeep minimum by 2001 April. It is the third outburst of Rho Cas on record inthe last century. We observe TiO absorption bands in high-resolution near-IRspectra obtained with the Utrecht Echelle Spectrograph during the summer of2000. TiO formation in the outer atmosphere occurred before the deep brightnessminimum. Atmospheric models reveal that the effective temperature decreases byat least 3000 K, and the TiO shell is driven supersonically with Mdot ~= 5.410^{-2} Msun/yr. Strong episodic mass loss and TiO have also been observedduring the outbursts of 1945-47 and 1985-86. A detailed analysis of the exceptional outburst spectra is provided, bycomparing with high-resolution optical spectra of the early M-type supergiantsMu Cep (Ia) and Betelgeuse (Iab). The outburst spectra indicate the formationof a low-temperature, optically thick circumstellar gas shell of 3 10^{-2} Msunduring ~200 d, caused by dynamic instability of the upper atmosphere of thispulsating massive supergiant near the Eddington luminosity limit. We observethat the mass-loss rate during the outburst is of the same order of magnitudeas has been proposed for the outbursts of Eta Carinae. We present calculationsthat correctly predict the outburst time-scale, whereby the shell ejection isdriven by the release of hydrogen ionization-recombination energy.