First results on pre-main-sequence evolution, including a magnetic field

The recent preliminary determination of masses and physical parameters for the ROSAT-discovered, weak T Tauri star RX J0529.4+0041, a double-lined eclipsing binary, is a very important test for the pre-main-sequence (PMS) models. In fact, its secondary (0.95 +/- 0.05 M.) is still in the Hayashi phase, where the effective temperatures (T-eff) are highly dependent on the modeling of the surface convective layers. We present new models for the PMS, including zero-order thermal modifications that are due to the magnetic field, generated by the dynamo that is due to the coupling between rotation and convection. This generally neglected physical input affects the T-eff location of the PMS: tracks that do not allow for the effect of the magnetic field on the convective gradients provide an upper limit to the T-eff. The present models can match the H-R diagram location of the secondary of RX J0529.4+0041 and the low lithium depletion expected for stars of similar to1 M.. We show that this new feature in the computation is not equivalent to an artificial parametric change of the convection efficiency. We also point out that more complete models are needed, including a self-consistency model between the dynamo magnetic field and the evolution of stellar rotation.