Indirect imaging of an accretion disk rim in the long-period interacting binary W Crucis

¹ Department of Physics, University of Zagreb, Bijenička 32, 10$\,$000 Zagreb, Croatia e-mail: pavlovski@sirius.phy.hr
² Observatoire de Genève, 51 Ch. des Maillettes, 1290 Sauverny, Switzerland
³ Max-Planck-Institut für Astrophysik, Postfach 1312, 85741 Garching, Germany

Received: 20 December 2005
Accepted: 7 March 2006

Abstract

Context.Light curves of the long-period Algols are known for their complex shape (asymmetry in the eclipse, light variations outside the eclipse, changes from cycle-to-cycle), but their interpretation is not possible in the standard model of binary stars.

Aims. We determined that complex structures present in these active Algol systems could be studied with the eclipse-mapping method that was successfully applied to the new 7-color photometric observations in the Geneva system of W Cru, belonging to the isolated group of these active Algols.

Methods. Several cycles of this long-period (198.5 days) eclipsing binary were covered by observations. We used a modified Rutten's approach to the eclipse-mapping. The optimization of the system's parameters and the recovery of the disk intensity distribution are performed using a genetic algorithm (GA).

Results.The finding of a primary (hot) component is hidden in thick accretion disk confirms previous solutions. The mass of the primary component,  , indicates that it is a mid-B type star. The mass-losing component fills its critical lobe, which, for the system's parameters, means it is a G-type supergiant with a mass  . The disk is very extended geometrically, and its outer radius is about 80% of the primary's critical lobe. A reconstructed image reveals the rather clumpy and nonuniform brightness distribution of an accretion disk rim in this system that is seen almost edge-on. This clumpiness accounts for light curve distortions and asymmetries, as well as for secular changes.