Projection effects in the spectra of early-type pulsating stars

Armagh Observatory, College Hill, Armagh BT61 9DG, Northern Ireland, UK e-mail: pmr@star.arm.ac.uk, csj@star.arm.ac.uk

Corresponding author: P. Montañés Rodriguez, pmr@star.arm.ac.uk

Received: 13 July 2000
Accepted: 7 June 2001

Abstract

The relation between the surface velocity of a pulsating star in the star's inertial frame and the apparent expansion velocity measured by a remote observer has been classically formulated in terms of a projection factor p. This factor depends on the relative limb darkening in continuum and spectral lines and may therefore be a function of composition. We have computed synthetic spectra to investigate the behaviour of p and, indeed, of the apparent line profile for radially pulsating stars over a range of temperature and gravity with hydrogen-rich and hydrogen-poor atmospheres. We have subsequently derived values of p suitable for the interpretation of velocity measurements of pulsating early type stars. Effective temperature has the most important influence on p: a change of 10 000 K in T_eff introduces a change of 2-3 per cent with important consequences for measurements of stellar radii to an accuracy .