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Astronomy & Astrophysics manuscript no. (will be inserted by hand later)

Summary: Astronomy & Astrophysics manuscript no.
(will be inserted by hand later)
Projection e ects in the spectra of early-type pulsating stars
P. Monta~nes Rodriguez and C.S. Je ery
Armagh Observatory, College Hill, Armagh BT61 9DG, Northern Ireland.
Received . . . , 2001; accepted . . .
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 pro le 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. E ective temperature has the most important in uence on p: a change
of 10 000 K in Te introduces a change of 2 { 3 per cent with important consequences for measurements of stellar
radii to an accuracy < 1%.
Key words. stars: early-type { stars: pulsation { stars: atmospheres.
1. Introduction
The expansion of a pulsating stellar atmosphere intro-
duces a Doppler shift to the absorption spectrum. As a
consequence of stellar surfaces being approximately spher-


Source: Armagh Observatory


Collections: Physics