Column density ratio between SO₂ and SO as a function of T_bol (left) and L_bol (right). Red dots are values calculated from ³⁴SO fluxes and later on multiplied by 22 (Wilson 1999), while blue dots indicate those sources where³⁴ SO is not detected and the ratios are calculated from the SO 6₆−5₅ transition. Black triangles represent lower limits (optically thick SO₂ emission) and green triangles indicate upper limits, given by those sources where SO emission is detected but SO₂ is not. The gray dashed line in the left panel represents T_bol = 70, the separation between Class 0 and Class I sources. Error bars in the y-axis represent ranges for T_ех between 50 and 150 K.