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Astron. Astrophys. 363, 585-592 (2000)

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3. Synthetic spectra and line profile variations

We modeled the stellar surface with a modified version of the program developed and described by Townsend (1997a, 1997b). This package was originally developed for modeling rapidly rotating non-radially pulsating stars. For our purpose, we switched off the non-radial pulsations and modified the program to handle spots on the star instead. The modifications allow for local differences of element abundances.

The stellar surface is divided into a grid consisting of about 51 000 points. For each point, a local line profile has to be specified. We derived these line profiles by interpolating in a grid of line profiles derived in the following way: The model atmosphere structures were computed with ATLAS9 (Kurucz 1979, 1993). Line profiles were calculated with these model atmospheres using the BHT code (Baschek et al. 1966). With this code it is possible to modify the abundances of He and metals. For the Hei lines the broadening theories of Griem et al. (1966) (Hei[FORMULA]4713) and Barnard et al. (1974) (Hei[FORMULA]4471, 4921) were used. Our grid of profiles covers the whole range of [FORMULA], log g and [FORMULA] which occur at the stellar surface.

Input parameters for the modeling of the complete surface are the polar radius [FORMULA], the polar effective temperature [FORMULA], the stellar mass M, the equatorial rotational velocity [FORMULA] and the inclination of the rotation axis i. For each surface point [FORMULA], log g, [FORMULA] (the rotational velocity projected towards the observer) and µ (the angle under which the surface element is seen) are calculated. Rotatational flattening and the dependence of temperature and gravity on stellar latitude (von Zeipel 1924) are taken into account. The element abundance [FORMULA] is specified for each surface element. In this way, abundance spots at the surface are modeled.

The effect of rotation is simulated by modeling 20 snapshots of the star during one rotational cycle. The visible points of the stellar surface and their contribution to the flux are determined and integrated for each frame using the line profiles interpolated from the grid of line profiles computed with BHT.

In contrast to the analysis applied by Groote & Hunger (1997) - where the equivalent widths were calculated directly - we first synthesized the spectra and afterwards measured their equivalent widths. This procedure allows us to investigate variations in the line profiles as well as to compare the integrated equivalent widths.

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© European Southern Observatory (ESO) 2000

Online publication: December 11, 2000