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 RXJ 0529.4 +0041, a double lined eclipsing binary, is a very important test for the pre-main sequence (PMS) models. In fact, its secondary (0:95 0:05M ) is still in the Hayashi phase, where the Teff’s are highly dependent on the modelling of the surface convective layers. We present new models for the PMS, including zero–order thermal modifications due to the magnetic field, generated by the dynamo due to the coupling between rotation and convection. This generally neglected physical input affects the Teff location of the PMS: tracks not allowing for the effect of the magnetic field on the convective gradients provide an upper limit to the Teff. The present models can fit the HR diagram location of the secondary of RXJ 0529.4 +0041, and the low lithium depletion expected for stars of 1M . 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 self–consistency between the dynamo magnetic field and the evolution of stellar rotation. Subject headings: stars: pre main sequence evolution —stars: T Tauri — magnetic fields — lithium depletion