A plot showing the dependence of the near- to mid-infrared spectral slope (α) to the luminosity of the associated outflow derived from the H₂ 1−0 S(1) line flux. The vertical hatched area around α = 3 shows the upper tentative border for the Class I and 0 protostars according to Lada (1987); the vertical dashed line at α = 0.3 indicates the border between Class I and Flat Spectra type sources according to Greene et al. (1994). The error for the flux values was assumed to be of the order of 20%. The solid line represents the derived best-fit for the plotted data points of the form: y = (26.5 ± 2.9)x − (8.8 ± 4.6) with χ² = 77.6.