Disks around T Tauri stars are known to lose mass , as best shown by the profiles of forbidden emission lines of low ionization species .
At least two separate kinematic components have been identified , one characterised by velocity shifts of tens to hundreds km/s ( HVC ) and one with much lower velocity of 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 \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 , blue-shifted HVC and red-shifted 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 } \sim 10 ^ { -2 } -1 L _ { \odot } ; L _ { acc } \sim 10 ^ { -5 } -10 ^ { -1 } L _ { \odot } ; \dot { M } _ { acc } \sim 10 ^ { -11 } -10 ^ { -7 } M _ { \odot } /yr ) .
The lines show the presence of a slow wind ( V _ { peak } < 20 km/s ) , dense ( n _ { H } > 10 ^ { 8 } cm ^ { -3 } ) , warm ( T \sim 5000 - 10000 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 \sim 10 ^ { -12 } M _ { \odot } and \sim 0.01 AU ^ { 3 } for M _ { star } \sim 0.1 M _ { \odot } , to \sim 3 \times 10 ^ { -10 } M _ { \odot } and \sim 1 AU ^ { 3 } for M _ { star } \sim 1 M _ { \odot } , respectively .
These results provide quite stringent constraints to wind models in low mass young stars , that need to be explored further .