2014A&A...569A...5N

Disks around T Tauri stars are known to lose mass, as best shown by the profiles of the forbidden emission lines of low-ionization species. At least two separate kinematic components have been identified, one characterized by velocity shifts of tens to hundreds of km/s (HVC) and one with a much lower velocity of a few km/s (LVC). The HVC are convincingly associated to the emission of jets, but the origin of the LVC is still unknown. In this paper we analyze the forbidden line spectrum of a sample of 44 mostly low-mass young stars in Lupus and σ Ori observed with the X-shooter ESO spectrometer. We detect forbidden line emission of OI, OII, SII, NI, and NII, and characterize the line profiles as LVC, blueshifted HVC, and redshifted HVC. We focus our study on the LVC. We show that there is a good correlation between line luminosity and both L_star and the accretion luminosity (or the mass accretion rate) over a large interval of values (L_star∼10^–2-1L_☉; L_acc∼10^–5-10^–1L_☉; {dot}(M)_acc∼10^–11-10^–7M_☉/yr). The lines show the presence of a slow wind (V_peak<20km/s) that is dense (n_H>10⁸cm^–3), warm (T∼5000-10000K), mostly neutral. We estimate the mass of the emitting gas and provide a value for the maximum volume it occupies. Both quantities increase steeply with the stellar mass, from ∼10^–12M_☉ and ∼0.01AU³ for M_star∼0.1M_☉, to ∼3x10^–10M_☉ and ∼1AU³ for M_star∼1M_☉, respectively. These results provide quite stringent constraints to wind models in low-mass young stars, that need to be explored further.