Distribution of extinction-corrected infrared spectral indices (α’, solid bars) and measured spectral indices (α, grey-dashed steps) for all the 4930 YSOs in SFRs, as discussed in Section 4. Blue bars denote α’ values estimated for sources exclusively from Marton et al. (2016). Orange bars denote α’ values for sources from Dunham et al. (2015). Red vertical lines mark the range of values for a YSO to be classified as a Class II source (−1.6 ≤ α′<  − 0.3).