No abstract

Context. Rotational evolution in the pre-main sequence is described with new sets of pre-MS evolutionary tracks including rotation, non-gray boundary conditions (BCs) and either low (LCE) or high convection efficiency (HCE). Aims. Using observational data and our theoretical predictions, we aim at constraining (1) the differences obtained for the rotational evolution of stars within the ONC by means of these different sets of new models; (2) the initial angular momentum of low mass stars, by means of their templates in the ONC. Methods. We discuss the reliability of current stellar models for the pre-MS. While the 2D radiation hydrodynamic simulations predict HCE in pre-MS, semi-empirical calibrations either seem to require that convection is less efficient in pre-MS than in the following MS phase (lithium depletion) or are still contradictory (binary masses). We derive stellar masses and ages for the ONC by using both LCE and HCE. Results. The resulting mass distribution for the bulk of the ONC population is in the range 0.2−0.4 M for our new non-gray models and, as in previous analyse, in the range 0.1−0.3 M for models having gray BCs. In agreement with Herbst et al. (2002) we find that a large percentage (∼70%) of low-mass stars (M < ∼ 0.5 M for LCE; M < ∼ 0.35 M for HCE) in the ONC appears to be fast rotators (P < 4 days). Three possibilities are open: 1) ∼70% of the ONC low mass stars lose their disk at early evolutionary phases; 2) their "locking period" is shorter; 3) the period evolution is linked to a different morphology of the magnetic fields of the two groups of stars. We also estimate the range of initial angular momentum consistent with the observed periods. Conclusions. The comparisons made indicate that a second parameter is needed to describe convection in the pre-MS, possibly related to the structural effect of a dynamo magnetic field.

Context. 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 (tachocline) between the radiative core and the base of the convective envelope. Aims. Since dynamo models are based on the interaction between differential rotation and convective motions, the introduction of rotation in the ATON 2.3 stellar evolutionary 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. Methods. As examples of such explorations, we present theoretical estimates for both the local convective turnover time (τ c ), and global convective times (τ g ) 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. Results. Those estimates, as opposed to the use of empirically derived values of τ c for such matters, 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. Conclusions. Our theoretical values of τ c , τ g and Ro qualitatively agree with those published by Kim & Demarque (1996, ApJ, 457, 340). By increasing the convection efficiency, τ g decreases for a given mass. FST models show still lower values. The presence of rotation shifts τ g towards slightly higher values when compared with non-rotating models. The use of non-gray boundary conditions in the models yields values of τ g smaller than in the gray approximation.

Context. In close binary systems, the axial rotation and the mutual tidal forces of the component stars deform each other and destroy their spherical symmetry by means of the respective disturbing potentials. Aims. We present new models for low-mass, pre-main sequence stars that include the combined distortion effects of tidal and rotational forces on the equilibrium configuration of stars. Using our theoretical results, we aim at investigating the effects of interaction between tides and rotation on the stellar structure and evolution. Methods. The Kippenhahn & Thomas (1970, in Stellar Rotation, ed. A. Slettebak) approximation, along with the Clairaut-Legendre expansion for the gravitational potential of a self-gravitating body, is used to take the effects of tidal and rotational distortions on the stellar configuration into account. Results. We obtained values of internal structure constants for low-mass, pre-main sequence stars from stellar evolutionary models that consider the combined effects of rotation and tidal forces due to a companion star. We also derived a new expression for the rotational inertia of a tidally and rotationally distorted star. Our values corresponding to standard models (with no distortions) are compatible with those available in literature. Our distorted models were successfully used to analyze the eclipsing binary system EK Cep, reproducing the stellar radii, effective temperature ratio, lithium depletion, rotational velocities, and the apsidal motion rate in the age interval of 15.5-16.7 Myr. Conclusions. In the low-mass range, the assumption that harmonics greater than j = 2 can be neglected seems not to be fully justified, although it is widely used when analyzing the apsidal motion of binary systems. The non-standard evolutionary tracks are cooler than the standard ones, mainly for low-mass stars. Distorted models predict more mass-concentrated stars at the zero-age main-sequence than standard models.

We investigate stellar magnetic activity from the theoretical point of view, by using stellar evolution models to calculate theoretical convective turnover times (τc) and Rossby numbers (Ro) for pre-main-sequence and main-sequence stars. The problem is that the canonical place where τc is usually determined (half a mixing length above the base of the convective zone) fails for fully convective stars and there is no agreement on this in the literature. Our calculations were performed with the ATON stellar evolution code. We concentrated our analysis on fully and partially convective stars motivated by recent observations of slowly rotating fully convective stars, whose X-ray emissions correlate with their Rossby numbers in the same way as in solar-like stars, suggesting that the presence of a tachocline is not required for magnetic field generation. We investigate the behaviour of τc over the stellar radius for stars of different masses and ages. As Ro depends on τc, which varies strongly with the stellar radius, we use our theoretical results to determine a better radial position at which to calculate it for fully convective stars. Using our alternative locations, we fit a sample of 847 stars in the rotation-activity diagram (LX/Lbol versus Ro) with a two-part power-law function. Our fit parameters are consistent with previous work, showing that stars with Ro ≤ Rosat are distributed around a saturation level in LX/Lbol and, for stars with Ro &gt; Rosat, LX/Lbol clearly decays with Ro with an exponent of −2.4 ± 0.1.