Multiwavelength observations of RV Tauri stars III.R Scuti

We present ultraviolet, optical and infrared observations of the RV Tauri star R Sct. We argue that changes in the ultraviolet spectrum with pulsational phase cannot be due to changes in effective temperature and gravity alone, and that variable circumstellar extinction in the form of Rayleigh scattering is also necessary. The intrinsic ultraviolet spectra can then be used to determine the spectral type as a function of light curve phase. We find UV spectral types in the range G0-K2, consistent with optical spectra and with the fitting of model atmospheres to ultraviolet-optical-infrared data. Changes in the angular diameter of R Sct with pulsational phase suggest that it undergoes non-radial pulsations. Furthermore, we find no evidence for changes in the stellar parameters during long-term light variations, implying that the cause of these variations must lie outside the star. The Rayleigh scattering seems to be due to very small grains in the environment of R Sct; such a conclusion is consistent with the result that Mg II emission seems to be due to atmospheric shocks rather than chromospheric activity, the shocks inhibiting the formation of large grains in the circumstellar shell of R Sct.