¹²CO J = 10–9 (left) and ¹³CO J = 10–9 (right) spectra for a low-mass Class 0 protostar (top, Ser SMM1), low-mass Class I source (Elias 29), intermediate-mass object (NGC 2071) and high-mass YSO (bottom, W3-IRS5). The green line indicates the baseline level and the red dashed line the 0 km s^-1 value. All spectra have been rebinned to 0.27 km s^-1 and shifted with respect to their relative local standard-of-rest velocity.