The structure of the instability strip and mode identification for β Cep stars in three young open clusters

Abstract

We use a grid of stellar models to calculate isochrones. These are used to determine the ages of NGC 3293, NGC 4755 and NGC 6231. Using a non-adiabatic pulsation code, we compute the instability strip for β Cep variables. We find that there is a correlation between the frequencies of the excited modes and the mass for stars of a given age. A calibration is presented which allows age determination of β Cep stars in clusters with greater accuracy than isochrone fitting. The strong dependence of the frequency-mass relationship on age explains the temperature difference between the instability strips in NGC 3293 and NGC 4755. The second part of the paper examines the question of mode identification for β Cep stars in NGC 3293 and NGC 4755. We present new uvby observations for these stars and use the dependence of the amplitude on wavelength to estimate the modes. We are able to isolate a few radial pulsators in the two clusters and to deduce their masses, effective temperatures and luminosities. © 1997 RAS.