1996A&A...308..151C

Analysis of high precision uvby light curves (Clausen et al. 1995b) of the detached, double-lined eclipsing binary V539 Ara has uncovered, that its secondary component belongs to the group of slowly pulsating B (or SPB) stars (Waelkens 1991). The B4V star is situated in the instability domain of the HR diagram predicted for Z=0.02 (Dziembowski et al. 1993), whereas the B3V primary, which appears stable, lies just at its border. Low amplitude oscillations with periods of 1.36, 1.78 and possibly 1.08 days have been identified. The components of V539 Ara are located in the lower half of the main sequence band. New absolute dimensions have been established: (M,R,logg,logT_eff,logL) values are (6.24±0.07, 4.50±0.08, 3.93±0.02, 4.26±0.01, 3.29± 0.05) and (5.31±0.06, 3.42±0.08, 4.23±0.01, 2.95±0.06) for primary and secondary, respectively. V539 Ara is consequently among our best sources for accurate parameters of mid-B stars. Stellar evolutionary models (Claret & Gimenez 1992a) based on OPAL opacities and including moderate overshooting fit the components very well for an assumed composition of (Y,Z)=(0.28,0.02); a common age of 4.5x10⁷yr is predicted. Models of significantly lower/higher metal abundance represent the observations less satisfactorily. The orbit of V539 Ara is slightly eccentric (e=0.053±0.001), and apsidal motion with a period of U=150±15yr has been detected. Relativistic effects are negligible, and the corresponding mean density concentration coefficient logk₂=-2.32±0.07 confirms the theoretical prediction of the Claret & Gimenez (1992a, 1993) models. For a yet unknown reason it is, however, not possible to describe all available times of minima through "normal" apsidal motion (Gimenez & Bastero 1995); more observations are needed. The combination of well established SPB oscillations and accurate dimensions is unique. V539 Ara appears to offer a highly needed possibility to study the structure of convective cores and their boundaries for intermediate mass stars. Dedicated (rotationally deformed) models are needed, as well as further photometric observations for a more detailed frequency study of V539 Ara. Several other well studied binaries are located in or near the predicted SPB domain.
Result. from a critical review of their properties, a confrontation with OPAL based models, and a search for yet overlooked SPB oscillations will be published separately (Clausen 1995).