U-BAND PHOTOMETRY AS A SIGNATURE OF GAS ACCRETION IN YOUNG LOW-MASS STARS

Low-mass pre-main-sequence stars can be classified as accreting/non-accreting by using a spectral-index dependent criterion based on the equivalent width of the Ha emission line. This limit defines the boundary between Classical T Tauri Stars (CTTS) and Weak-lined T Tauri Stars (WTTS) (Barrado et. al 2003). Some of these stars in nearby associations also show an excess in the U magnitude with respect to the Main Sequence, which cannot be solely explained by chromospheric activity. We propose that this excess can be correlated to the He accretion criterion due to the fact that hot shocked gas, that accretes onto the stellar surface, generates high energy photons that can be traced by observations in the U band.