  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 [OI], [OII], [SII], [NI], and [NII], 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.

Solar and Stellar Astrophysics