Distribution of the observed velocity-integrated CO line flux density versus the rotational quantum number J_up for each transition, i.e. CO SLEDs. Black dots with error bars are the velocity integrated flux densities from this work. Red dots are the data from other works: all CO(1–0) data are from Harris et al. (2012); CO(4–3) in G09v1.97 is from Riechers et al. (in prep.); CO(6–5) , CO(7–6) and CO(8–7) in SDP 17b are from Lupu et al. (2012); CO(8–7) and CO(10−9) in SDP 81 are from ALMA Partnership (2015); CO(3–2) in G12v2.30, CO(4–3)in NCv1.143 and CO(3–2) in NBv1.78 are from O13; CO(4–3) in NAv1.56 is from Oteo et al. (in prep.). For a comparison, we also plot the CO SLED of G15v2.779 (Cox et al. 2011). We mark an index number for each source in turquoise following Table 3 for the convenience of discussion.