X-shooter spectroscopy of young stellar objects. V. Slow winds in T Tauri stars

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^-1 (HVC) and one with a much lower velocity of a few km s^-1 (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 O i, O ii, S ii, N i, and N ii, 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 (Lstar ~ 10^-2 - 1 L⊙; Lacc ~ 10^-5-10^-1L⊙; Macc ~ 10^-11 - 10^-7 M⊙/yr). The lines show the presence of a slow wind (Vpeak < 20 km s^-1) that is dense (nH > 10^8 cm^-3), warm (T ~ 5000-10 000 K), 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^-12 M⊙ and ~0.01 AU^3 for Mstar ~ 0.1 M⊙, to ~ 3 10^-10 M⊙ and ~1 AU^3 for Mstar ~ 1 M⊙, respectively. These results provide quite stringent constraints to wind models in low-mass young stars, that need to be explored further.