(Left) SiO emission observed in the J = 3 → 2 (bottom) and J = 8 → 7 transitions towards the protostellar source. Fluxes are expressed in units of main-beam temperature. (Right) Observed variations of the ratio of the CO J = 8 → 7 to J = 3 → 2 brigthness temperatures R(8–7/3–2) as a function of velocity (top) and predicted variations as a function of the gas density n(H₂) and the kinetic temperature T_k (bottom) for a typical column density N(SiO) = 2 × 10¹⁴cm^-2. The contour level R = 0.7,1.5 are drawn with a solid black line; the contours R = 0.5,1.8 are drawn with a dashed red line and account for an uncertainty of 30%.