Understanding the angular momentum evolution of T Tauri and Herbig Ae/Be stars.

We investigate a sample of 6 Herbig Ae/Be stars belonging to the Orion OB1 association, as well as 73 low mass objects, members of the $\sigma$ Orionis cluster, in order to explore the angular momentum evolution at early stages of evolution, and its possible connection with main-sequence Ap/Bp magnetic stars. Using FIES and HECTOCHELLE spectra, we obtain projected rotational velocities through two independent methods. Individual masses, radii, and ages were computed using evolutionary models, distance, and cluster extinction. Under the assumption that similar physical processes operate in both, T Tauri and Herbig Ae/Be stars, we construct snapshots of the protostar's rotation against mass during the first 10 Myr with the aid of a rotational model that includes a variable disc lifetime, changes in the stellar moment of inertia, a dipolar magnetic field with variable strength, and angular momentum loss through stellar winds powered by accretion. We use these snapshots, as well as the rotational data, to infer a plausible scenario for the angular momentum evolution. We find that magnetic field strengths of a few k$G$ at 3 Myr are required to match the rotational velocities of both groups of stars. Models with masses between 2-3 $M_{\odot}$ display larger angular momentum by a factor of $\sim 3$, in comparison to stars of similar spectral types on the main-sequence. Even though some quantitative estimates on this dramatic decrease with age, for Ap/Bp magnetic main-sequence stars are presented, the results obtained for the angular momentum evolution do not explain their low rotation.