Abstract
We conducted a photometric study of a sample of pre-main sequence and TTauri stars in a spectral range between G-K and some early M of masses from 0.5 to 3 M_⊙, in associations near the Sun (20-200 pc). We measured the excess of the UV band and found that for stars of spectral type early M and late K, the UV band increases considerably. There are two possible scenarios for such an increase: (1) Chromospheric activity, because the stars have radiative and convective cores, giving rise to magnetic activity which generates this excess present in the continuum and (2) the processes of accretion present in stars that have a disk of gas and dust, which they accrete to them by means of the magnetic lines field. Because of the ambiguity in the two effects (since they affect the same emission lines) it is difficult to distinguish which dominates. We propose that an excess of 0.2 magnitudes in the U band for our sample allows us to determine if both processes are at work, or if chromospheric activity dominates, since accretion stops at a very early age, 10 million years.
