The spectral variability of the cool hypergiant rho Cassiopeiae

We checked the change of the effective temperature with the atmospherical pulsation of rho Cas from combined analyses of optical spectra of 1993-95 and IUE spectra of 1979-81. We find an upper range for bigtriangleup T_eff =~ 750 K over a period of 16-17 months. We present a thorough study of the related absorption line profile changes from which a velocity stratification with excitation energy could not be detected for the metallic lines. The distinct evolution of Hα displays variable distortions of filling-in by emission with the pulsation. Our theoretical line profile fits yield a value for vsini of 25 kms(-1) . We report the finding of two different causes for the splitting of absorption line cores in the spectra of rho Cas. For the phase near highest effective temperature we have analyzed the development of very far shortward extended line wings assuming velocities up to 150 kms(-1) . These violet absorptions originate about 2.5 R_{*} above the photosphere in a cool and variable supersonic wind from which we determine an upper value of the mass-loss rate of 9.2 10(-5) M_⊙y(-1) . We also report the finding of an increase of the supersonic wind velocity by ~ 15 kms(-1) per electronvolt decrease, measured from the maximum velocities of these violet absorptions for Fei lines. A separate analysis of the IUE spectra reveals tremendous changes of the Balmer continuum flux by a factor 4 to 5 in only 26 hours, clearly distinct from the long-term variations of Hα . Partly based on observations taken at the La Palma Observatory, the Special Astrophysical Observatory and with the International Ultraviolet Explorer