An Analysis of the Absolute Energy Distribution in the Spectrum of SU Draconis.

Photoelectric spectrum scans of the cluster-type variable SU Dra have been obtained and used to determine the absolute energy distribution in the spectrum at different phases. By comparison with fluxes computed from model atmospheres, the effective temperature and gravity have been derived at each phase. From the observations and model atmospheres the relative displacement curve R/R is determined Slit spectra with a dispersion of 18 A/mm have been obtained and used to determine the radial- velocity curves of the metal lines, Hw, H , and H and K of Ca ii It is shown that differential motion exists in the atmospheres at all phases. Since the layer which one is observing changes during the cycle, because of changes in opacity, the velocity-curves cannot be interpreted literally, and integration of the velocity-curve does not yield the absolute displacement-curve, R = - R. Therefore, the Wesselink-Baade method gives incorrect radii for RR Lyrae stars. It is shown, however, that between phases 0 40 and 0 85 it is possible to determine R = - R from the radial-velocity curve. The -velocity which is needed to determine R = - R cannot be obtained unambiguously, but a good estimate is possible. The mean radius of SU Dra is found to be 3.60 X 100 km, or 5.2 solar radii. The uncertainty is approximately 20 per cent. The mean absolute visual magnitude is +08, with an uncertainty of roughly 0 4 if the absolute calibration of a Lyr is assumed correct. An alternative calibration of a Lyr yields an absolute visual magnitude which is 0 25 mag. brighter. The difference in absolute magnitude of cluster-type variables can be determined with an accuracy of 0 2. The relationship between the effective temperature (actually O ) and B - V for SU Dra is given along with the same relationship for a hypothetical star which has no metallic lines.