Properties of the dense, warm gas as a function of the stellar mass for the GTO sample. Red dots are objects in Lupus, blue filled squares in σ Ori; arrows are objects with upper limits of the [O i] 630.03 luminosity, from which we derive the properties of the emitting region. The top panel plots the mass of the gas in the region, the bottom panel the upper limit to the volume, computed for an electron density n_e = 10⁷ cm^-3, ionization fraction 0.1 (n_H = 10⁸ cm^-3). In both panels we have assumed a line emissivity that is the average between the values for T = 10 000 K (1.1 × 10^-15 erg s^-1 per O i) and T = 5000 K (1.2 × 10^-16 erg s^-1 per O i) once n_e ≫ n_cr.