E. Oblak

Disentangling discrepancies between stellar evolution theory and sub-solar mass stars The influence of the mixing length parameter for the UV Psc binary

Serious discrepancies have recently been observed between predictions of stellar evolution models in the 0.7-1.1 M� mass range and accurately measured properties of binary stars with components in this mass range. We study one of these objects, the eclipsing binary UV Piscium, which is particularly interesting because Popper (1997) derived age estimates for each component that differed by more than a factor of two. In an attempt to solve this significant discrepancy (a difference in age of 11 Gyr), we compute a large grid of stellar evolution models with the CESAM code for each component. By fixing the masses to their accurately determined values (relative error smaller than 1% for both stars), we consider a wide range of possible metallicities Z (0.01 to 0.05), and helium content Y (0.25 to 0.34) uncorrelated to Z. In addition, the mixing length parameter αMLT is left as another free parameter. We obtain a best fit in the Teff-radius diagram for a common chemical composition (Z, Y) = (0.012, 0.31), but a different MLT parameter αMLT,A = 0.95 ± 0.12(statistical)+0.30(systematic) and αMLT,B = 0.65 ± 0.07(stat)+0.10(syst). The apparent age discrepancy found by Popper (1997) disappears with this solution, the components being coeval to within 1%. This suggests that fixing αMLT to its solar value (∼1.6), a common hypothesis assumed in most stellar evolutionary models, may not be correct. Secondly, since αMLT is smaller for the less massive component, this suggests that the αMLT parameter may decrease with stellar mass, showing yet another shortcoming of the mixing length theory to explain stellar convection. This trend needs further confirmation with other binary stars with accurate data.

Speckle observations of binary systems measured by Hipparcos

From speckle observations made with the PISCO speckle camera at the Pic du Midi Observatory, we present high angular resolution astrometric data for 43 binary stars already observed by the Hipparcos satellite. This sample consists of mainly new Hipparcos eclipsing binaries with a visual companion closer than one arcsecond, chosen with the aim to study the dynamical implications of a third component on the observational parameters of the eclipsing system. In addition, we also included a selection of close visual binaries with few speckle data in order to analyse possible systematic departures between the speckle and the non-speckle orbits. The reduction method and the results are presented in detail. For the close visual binaries we confront our observations with the ephemerides based on the best known orbits. For the wide visual binaries the confrontation is made directly with the Hipparcos data. Our observations are consistent both with previous speckle data and with most of the Hipparcos measurements.