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 sigma 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 (L-star similar to 10(-2)-1 L-circle dot; L-acc similar to 10(-5)-10(-1) L-circle dot;(M) over dot(acc) similar to 10(-11)-10(-7) M-circle dot/yr). The lines show the presence of a slow wind (V-peak < 20 km s(-1)) that is dense (n(H) > 10(8) cm(-3)), warm (T similar to 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 similar to 10(-12) M-circle dot and similar to 0.01 AU(3) for M-star similar to 0.1 M-circle dot, to similar to 3 x 10(-10) M-circle dot and similar to 1 AU(3) for M-star similar to 1 M-circle dot, respectively. These results provide quite stringent constraints to wind models in low-mass young stars, that need to be explored further.