Magnetic fields are at the heart of the observed stellar activity in late-type stars, and they are presumably generated by a dynamo mechanism at the interface layer between the radiative and the convective stellar regions. Since dynamo models are bas
ed on the interaction between differential rotation and convective motions, the introduction of rotation in the ATON 2.3 stellar code allows for explorations regarding a physically consistent treatment of magnetic effects in stellar structure and evolution, even though there are formidable mathematical and numerical challenges involved. As examples, we present theoretical estimates for both the local (tau_c) and global (tau_g) convective turnover times for rotating pre-main sequence solar-type stars, based on up-to-date input physics for stellar models. Our theoretical predictions are compared with the previous ones available in the literature. In addition, we investigate the dependence of the convective turnover time on convection regimes, the presence of rotation and atmospheric treatment. Those estimates, this quantities can be used to calculate the Rossby number, Ro, which is related to the magnetic activity strength in dynamo theories and, at least for main-sequence stars, shows an observational correlation with stellar activity. More important, they can also contribute for testing stellar models against observations. Our theoretical values of tau_c, tau_g and Ro qualitatively agree with those published by Kim & Demarque (1996). By increasing the convection efficiency, tau_g decreases for a given mass. FST models show still lower values. The presence of rotation shifts tau_g towards slightly higher values when compared with non-rotating models. The use of non-gray boundary conditions in the models yields values of tau_g smaller than in the gray approximation.
HIP96515A is a double-lined spectroscopic binary with a visual companion (HIP96515B) at 8.6 arcsec. It is included in the SACY catalog as a potential young star and classified as an eclipsing binary in the ASAS Catalog. We have analyzed spectroscopic
and photometric observations of the triple system. The high-resolution optical spectrum of HIP96515A has been used to derive a mass ratio, M_2/M_1, close to 0.9, with the SB2 components showing spectral types of M1 and M2. The ASAS and Hipparcos light-curves of HIP96515A show periodic variations with P=2.3456 days, confirming that HIP96515A is an eclipsing binary with preliminary parameters of i=89, M_Aa=0.59+-0.03 Msun and M_Ab=0.54+-0.03 Msun, for the primary and secondary, respectively, at an estimated distance of 42+-3 pc. This is a new eclipsing binary with component masses below 0.6 Msun. Multi-epoch observations of HIP 96515 A&B show that the system is a common proper motion pair. The optical spectrum of HIP 96515B is consistent with a pure helium atmosphere (DB) white dwarf. We estimate a total age (main-sequence lifetime plus cooling age) of 400 Myr for the white dwarf. If HIP 96515 A&B are coeval, and assuming a common age of 400 Myr, the comparison of the masses of the eclipsing binary members with evolutionary tracks shows that they are underestimated by ~15% and ~10%, for the primary and secondary, respectively.
CONTEXT: Accurate mass, radius, and abundance determinations from binaries provide important information on stellar evolution, fundamental to central fields in modern astrophysics and cosmology. AIMS: Within the long-term Copenhagen Binary Project,
we aim to obtain high-quality light curves and standard photometry for double-lined detached eclipsing binaries with late A, F, and G type main-sequence components, needed for the determination of accurate absolute dimensions and abundances, and for detailed comparisons with results from recent stellar evolutionary models. METHODS: Between March 1985 and July 2007, we carried out photometric observations of AD Boo, HW CMA, SW CMa, V636 Cen, VZ Hya, and WZ Oph at the Stromgren Automatic Telescope at ESO, La Silla. RESULTS: We obtained complete uvby light curves, ephemerides, and standard uvbybeta indices for all six systems.For V636 Cen and HW CMa, we present the first modern light curves, whereas for AD Boo, SW CMa, VZ Hya, and WZ Oph, they are both more accurate and more complete than earlier data. Due to a high orbital eccentricity (e = 0.50), combined with a low orbital inclination (i = 84.7), only one eclipse, close to periastron, occurs for HW CMa. For the two other eccentric systems, V636 Cen (e = 0.134) and SW CMa (e = 0.316), apsidal motion has been detected with periods of 5270 +/- 335 and 14900 +/- 3600 years, respectively.
