We present an overview of the spectral variability of the peculiar F-type hypergiant Cas, obtained from our long-term monitoring campaigns over the past 8.5 yr with four spectrographs in the northern hemisphere. Between 2000 June and September an exceptional variability phase occurred when the V brightness dimmed by about a full magnitude. The star recovered from this deep minimum by 2001 April. It is the third outburst of Cas on record in the last century. We observe TiO absorption bands in high-resolution 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 M ' 5:4 Â 10 À2 M yr À1 . Strong episodic mass loss and TiO have also been observed during the outbursts of 1945-1947 and 1985-1986. A detailed analysis of the exceptional outburst spectra is provided, by comparing with high-resolution optical spectra of the early M-type supergiants l 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 H 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 Â 10 À2 M during 200 days, caused by dynamic instability of the upper atmosphere of this pulsating massive supergiant near the Eddington luminosity limit. 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 Carinae. We present calculations that correctly predict the outburst timescale, whereby the shell ejection is driven by the release of hydrogen ionization recombination energy.
Context. We study the time history of the yellow hypergiant HR 8752 based on high-resolution spectra , the observed MK spectral classification data, B − V-and V-observations ) and yet earlier V-observations . Aims. Our local thermal equilibrium analysis of the spectra yields accurate values of the effective temperature (T eff ), the acceleration of gravity (g), and the turbulent velocity (v t ) for 26 spectra. The standard deviations average are 82 K for T eff , 0.23 for log g, and 1.1 km s −1 for v t . Methods. A comparison of B− V observations, MK spectral types, and T eff -data yields E(B− V), "intrinsic" B− V, T eff , absorption A V , and the bolometric correction BC. With the additional information from simultaneous values of B − V, V, and an estimated value of R, the ratio of specific absorption to the interstellar absorption parameter E(B − V), the "unreddened" bolometric magnitude m bol,0 can be determined. With Hipparcos distance measurements of HR 8752, the absolute bolometric magnitude M bol,0 can be determined. Results. Over the period of our study, the value of T eff gradually increased during a number of downward excursions that were observable over the period of sufficient time coverage. These observations, together with those of the effective acceleration g and the turbulent velocity v t , suggest that the star underwent a number of successive gas ejections. During each ejection, a pseudo photosphere was produced of increasingly smaller g and higher v t values. After the dispersion into space of the ejected shells and after the restructuring of the star's atmosphere, a hotter and more compact photosphere became visible. From the B − V and V observations, the basic stellar parameters, T eff , log M/M , log L/L , and log R/R are determined for each of the observational points. The results show the variation in these basic stellar parameters over the past near-century. Conclusions. We show that the atmospheric instability region in the HR-diagram that we baptize the yellow evolutionary void actually consists of two parts. We claim that the present observations show that HR 8752 is presently climbing out of the "first" instability region and that it is on its way to stability, but in the course of its future evolution it still has to go through the second potential unstable region.
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