Asiago Eclipsing Binaries Program

Double-lined eclipsing binaries (SB2 EBs) provide the most accurate source of information on fundamental stellar parameters, first of all masses and radii. Measuring parameters with high accuracy, is a formidable benchmark for the current generation of stellar evolutionary models. In fact the models have to simultaneously fit both stars of the binary, by adopting exactly the same age and the same chemical compositions.

Figure 1:Lightcurves, B-V, and radial velocity curves of the eclipsing binary star V505 Per

 In order to properly use binary systems to constrain the accuracy of stellar models, masses and radii have to be known at the 1% level. Accurate determination of effective temperatures and metallicities are also necessary. It is possible to achieve an equally good fit with low metallicity and incomplete mixing or higher metallicity and more realistic physics. Determining stellar abundances by fitting evolutionary tracks is unreliable. Accurate spectroscopic abundances of stars used as test of stellar evolution are crucial. The aim of this program is to derive the orbit and fundamental physical parameters for a selection of SB2 EBs by means of Echelle high resolution, high S/N spectroscopy and B, V photometry. Spectroscopic observations are carried out with the Echelle+CCD spectrograph mounted at Copernico telescope, operated by INAF OAPd on Mt. Ekar-Asiago. In addition, effective temperatures, gravities, rotational velocities and chemical abundances are obtained from spectra analysis, while reddening comes from direct measurements of interstellar NaI (5890 & 5896 Å) and KI (7665 & 7699 Å) doublets.

Figure 2:Comparison of BaSTI evolutionary tracks with observed parameters for V505 Per

 Our well determined data are used to provide strong constraints on the different approaches and assumptions adopted in stellar model calculations. We compute tailored evolutionary models for the exact masses and chemical mixtures observed in each binary, to test the effects of overshooting, diffusion and solar metallicity. This objective is accomplished by selecting binary systems whose stellar components have suitable masses and are in a appropriate evolutionary stage, i.e. core and shell H-burning phases.

Figure 3:Distribution in mass of the eclipsing binaries selected for Asiago program

 The reliability of stellar models is still partially hampered by our poor knowledge of some physical processes in real stars, such as (a) the efficiency of core convective overshooting during the core H-burning phase in intermediate mass stars (M≥1.1-1.2 Msol, and its dependence on the chemical composition); (b) the amount of convective core overshooting; (c) how much the convective core overshooting reduces with decreasing of stellar mass; (d) the efficiency of convection in the super-adiabatic layers; (e) the real efficiency of diffusive processes. The most suitable type of binaries are those with the mass of at least one of the two components around 1.2 Msol. Up till now in the Asiago eclipsing binaries program are included for observation thirty-five SB2 EBs. The distribution in mass for the primary star of this selection is shown in the histogram, as estimated from spectroscopic classification. Photometric data acquisition is complete for half of the systems and in progress for the rest. Final radial velocity curves have been obtained for about 10 systems, completion for the others is underway.

People: L. Tomasella, U. Munari, A. Siviero

Collaboration: S. Cassisi (INAF OA Teramo), T. Zwitter (Ljubljana Univ.), D. Terrell (Southwest Research Institute, USA), E.F. Milone (Calgary Univ., Canada)

Publications: Tomasella et al. (2008) A&A 480,464; Tomasella et al. (2008) A&A 483,263