Luiz T. S. Mendes

Theoretical values of convective turnover times and Rossby numbers for solar-like, pre-main sequence stars

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, T 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.

Combined effects of tidal and rotational distortions on the equilibrium configuration of low-mass, pre-main sequence stars

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) 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 massconcentrated stars at the zero-age main-sequence than standard models.

Non-gray rotating stellar models and the evolutionary history of the Orion Nebular Cluster

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 effi cient 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.

Rossby numbers of fully convective and partially convective stars

In this work, we investigate the stellar magnetic activity in the theoretical point of view, through the use of stellar structure and evolution models. We present theoretical values of convective turnover times and Rossby numbers for low-mass stars, calculated with the ATON stellar structure and evolution code. We concentrate our analysis on fully convective and partially convective stars motivated by recent observations of X-ray emission of slowly rotating fully convective stars, which suggest that the presence of a tachocline is not a central key for magnetic fields generation. We investigate the behavior of the convective turnover time evolution, as well as its radial profile inside the star. A discussion about the location where the convective turnover time is calculated in the stellar interior is also addressed. Our theoretical results are compared to observational data from low-mass stars.

Stellar models of rotating, pre-main sequence low-mass stars with magnetic fields

We report our present efforts for introducing magnetic fields in the ATON stellar evolution code code, which now evolved to truly modifying the stellar structure equations so that they can incorporate the effects of an imposed, large-scale magnetic field. Preliminary results of such an approach, as applied to low-mass stellar models, are presented and discussed.

Rotational properties of the Orion nebular cluster revised

The observational data of the Orion Nebula Cluster (ONC) is reanalyzed by means of new sets of pre-main sequence evolutionary tracks including rotation, non-gray boundary conditions and either low (LCE) or high convection efficiency (HCE). Our goal is to get a better understanding of the appropriate physical constraints for the rotational evolution of the stars within the ONC, with particular emphasis on the choice of their initial angular momentum and its variation with time. The role played by convection is a key aspect of our analysis, since there are conflicting results from theory and observations. For example, 2-D radiation hydrodynamic simulations predict HCE in the pre-main sequence, while semi-empirical calibrations either seem to require that convection is less efficient in pre-MS than in the following main-sequence phase. We derived stellar masses and ages for the ONC by using both LCE and HCE. Another relevant aspect that we considered was the role of non-gray atmospheres. Our results show that the resulting mass distribution for the bulk of the ONC population is in the range 0.2−0.4M for our non-gray models, and in the range 0.1-0.3M for gray models. In agreement with previous works, we found that a large percentage (∼70%) of low-mass stars (M≤Mtr, where Mtr is a transition mass) in the ONC appears to be fast rotators (P<4days). Mtr depends on the model choosen, being Mtr=0.5 for LCE, Mtr=0.35 for HCE and, as found in previous works, Mtr=0.25 for gray models. Finally, our analysis indicates that a “second parameter” is needed for a proper description of convection in the pre-main sequence phase.

Theoretical Values of the Rossby Number for Low‐Mass, Rotating Pre‐Main Sequence Stars

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. 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 gives us the opportunity to begin explorations regarding a physically consistent treatment of magnetic effects in stellar structure and evolution, in spite of the formidable mathematical and numerical challenges involved.As an example of such explorations, we present theoretical estimates of the convective turnover time τc for rotating pre‐main sequence solar‐type stars, based on up‐to‐date input physics for stellar models. Those estimates, as opposed to the use of empirically derived values of τc for such matters, can be used to calculate the Rossby number Ro, which is related to the magnet...