The eclipsing binary star RZ Cas^*,**

I. First spectroscopic detection of rapid pulsations in an Algol system

¹ Thüringer Landessternwarte, 07778 Tautenburg, Germany
² Astrophysical Research Center of the Structure and Evolution of the Cosmos, Sejong University, Seoul 143-747, Korea e-mail: david@arcsec.sejong.ac.kr
³ Astronomical Observatory, Odessa National University, Shevchenko Park, 65014, Odessa, Ukraine

Corresponding author: H. Lehmann, lehm@tls-tautenburg.de

Received: 17 April 2003
Accepted: 19 September 2003

Abstract

In a first report on the results of a multi-site campaign in 2001 of photometric and spectroscopic observations of the active semi-detached Algol-type system RZ Cas, we concentrate on the radial velocity (RV) variations. Using weak absorption lines we obtain an improved orbital solution for both components. In the orbital RV curve we observe a strong, asymmetric rotation effect. For the first time we detect rapid spectroscopic multi-mode pulsations in an Algol system. Whereas the photometrically observed oscillations were dominated by monoperiodic pulsations at frequency 64.19 c d^-1 until the year 2000, we find in 2001 a multiperiodic behaviour with two dominant frequencies of  c d^-1 and  c d^-1.
Both modes show amplitude variations over the orbital phase with a a minimum at orbital phases 0.6–0.8 and a maximum just after the primary minimum (f₂) and at ϕ ≈ 0.25 (f₁). The different shape of amplitude modulation of the f₁ and f₂ modes points to different () mode structures. The modulation itself can be explained by assuming a variable extinction due to gas streams and an inhomogeneous accretion annulus that weakens the light from different regions of the primary depending on its orbital position. This assumption is well supported by the gas density distribution obtained in preliminary hydrodynamic simulations.
We found strong variations and cycle-to-cycle variable shapes of the orbital RV-curves of Balmer lines that have maximum magnitude in the line indicating a strong variability of mass-transfer rates and a non-stationary circumbinary envelope.