A photometric and spectroscopic study of the brightest northern Cepheids -- III. A high-resolution view of Cepheid atmospheres

Abstract

We present new high-resolution (R=40000) spectroscopic observations of 18 bright northern Cepheids carried out at David Dunlap Observatory, in 1997. The measurements mainly extend those of presented in Paper I adding three more stars (AW Per, SV Vul, T Mon). The spectra were obtained in the interval of 5900 A and 6660 A. New radial velocities determined with the cross-correlation technique and the bisector technique are presented. We found systematic differences between the spectroscopic and CORAVEL-type measurements as large as 1-3 km/s in certain phases. We performed Baade-Wesselink analysis for CK Cam discovered by the Hipparcos satellite. The resulting radius is 31+/-1 Ro, which is in very good agreement with recent period-radius relation by Gieren et al. (1999). Observational pieces of evidence of possible velocity gradient affecting the individual line profiles are studied. The FWHM of the metallic lines, similarly to the velocity differences, shows a very characteristic phase dependence, illustrating the effect of global compression in the atmosphere. The smallest line widths always occur around the maximal radius, while the largest FWHM is associated with the velocity reversal before the minimal radius. Three first overtone pulsators do not follow the general trend: the largest FWHM in SU Cas and SZ Tau occurs after the smallest radius, during the expansion, while in V1334 Cyg there are only barely visible FWHM-variations. The possibility of a bright yellow companion of V1334 Cyg is briefly discussed. The observed line profile asymmetries can be partly associated with the velocity gradient, which is also supported by the differences between individual line velocities of different excitation potentials.