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 ecl ipsing binary, is a very important test for the pre-main-sequence (PMS) mod els. In fact, its secondary (0.95 +/- 0.05 M.) is still in the Hayashi phas e, where the effective temperatures (T-eff) are highly dependent on the mod eling of the surface convective layers. We present new models for the PMS, including zero-order thermal modifications that are due to the magnetic fie ld, generated by the dynamo that is due to the coupling between rotation an d convection. This generally neglected physical input affects the T-eff loc ation of the PMS: tracks that do not allow for the effect of the magnetic f ield on the convective gradients provide an upper limit to the T-eff. The p resent models can match the H-R diagram location of the secondary of RX J05 29.4+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 o ut that more complete models are needed, including a self-consistency model between the dynamo magnetic field and the evolution of stellar rotation.