Astron. Astrophys. 356, 200-208 (2000)

1. Introduction

Rapidly oscillating Ap (roAp) stars are a cool subgroup of CP2 stars and oscillate usually with multiple periods ranging from a few minutes up to about 20 minutes. Considering their position on the HR-diagram which implies a mass between one and two solar masses, such short periods are indicative for non-radial, high overtone, low order acoustic p-modes. According to the hitherto most successful oblique pulsator model (Kurtz 1982) pulsation and magnetic field axes are aligned, but oblique to the axis of rotation. This scenario and other characteristics of roAp stars are well described in various reviews (e.g., Kurtz 1990, Matthews 1997, and references therein).

The purpose of our project on abundance analyses of roAp stars is to provide accurate fundamental parameters of this astrophysically very interesting group of stars which constitutes an unique laboratory for testing important domains of stellar pulsation theory and of magneto-hydrodynamics communicated through oscillation (Gautschy et al. 1999, Matthews et al. 1999). In particular we derive , , abundances and magnetic field parameters which provide important boundary conditions needed for stellar pulsation models and we investigate the effects of a surface magnetic field on abundance determination.

The roAp star HD 166473 (CD 12303, V694 CrA) with mag is classified by Houk (1982) as an Ap(SRCREU ) star. Hauck & Mermilliod (1980) list Strömgren indices of = 0.213, = 0.311 and = 0.538. Based on (Martinez 1993), and using Crawford's (1979) A-type star calibration we determine and . Both indices are the most negative of all known roAp stars, except for Przybylski's star. The dominating pulsation frequency is at 1.892 mHz with a peak-to-peak amplitude of 0.49 mmag (Kurtz & Martinez 1987).

A variable surface magnetic field ranging from 8.6 to 6.4 kG was discovered by Mathys et al. (1997) using high-resolution spectra in the region centered on 6150 Å. The average of these spectra was used for our abundance analysis. The variation of the magnetic field modulus indicates a very long period beyond 3 yr which implies a of clearly less than 1 km s^-1. Both effects, slow rotation and large magnetic field, are responsible for intensification effects in specific spectral regions which strongly depend on the atomic line Zeeman pattern, and which cannot be described any more in terms of magnetic "pseudo-microturbulence", a successful concept, e.g., for Equ (Ryabchikova et al. 1997a) and HD 24712 (Ryabchikova et al. 1997b). In the following we present the results of our abundance analysis for HD 166473 based on synthetic spectra which include magnetic field effects in the radiation transfer.

© European Southern Observatory (ESO) 2000

Online publication: March 28, 2000
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