The helium-weak silicon star HR 7224

Abstract

Aims. HR 7224 is a rapidly rotating He-weak silicon star known as photometrically variable with a period of 1.123 d. Drastic changes of photometric amplitude and period were reported in 2003. We investigate the star spectroscopically to determine the time scales of variability, to derive basic atmospheric parameters, and to give first estimates on possible surface structures. Methods. We investigate time series of more then 570 high-resolution spectra of HR 7224 for radial velocity and line profile variations. The rotation period is determined from period analysis of data. Comparing radial velocity and line profile variations we draw first conclusions on possible surface structures in terms of line strengths. From Balmer and metal lines we determine Teff ,l ogg, v sini ,a nd metal abundances depending on rotation phase. Results. The star shows a remarkable amplitude of radial velocity variation of ± 6k m s −1 . From this variation we deduce a rotation period of 1.12324 d. The period and the double-wave shaped radial velocity curve are in agreement with earlier photometric findings. The observed radial velocity variations can be explained by the rotation period and its harmonics and no further periodic contributions were found in the residuals. HR 7224 has enhanced Si and Fe abundances whereas He is extremely depleted. The star shows line strength variations of all observed elements with the rotation period. Variations of radial velocity, line strength and line shape of silicon lines and their relationships can be explained by assuming two large spots on the stellar surface showing enhanced line strength. From Balmer lines we deduce different temperatures of the star for different rotation phases. The observed difference in amplitudes of the variation of Mg ii and Si ii lines supports the assumption of surface regions of different temperature on this presumably magnetic Cp star. Synthetic uvby colors computed from adapted synthetic spectra confirm the high photometric amplitude reported for the 2003 light curves.