Dust optical depth, τ_v, at 225 GHz as function of the dust temperature, T_d, derived from the observations of Alves et al. (2019) and following the method described in Bouvier et al. (2021). The lines show τ_v as derived from Eq. (7). The errors are calculated using the error propagation method described in Appendix C.2. of Bouvier et al. (2021) and include the calibration uncertainty of 15% (Alves et al. 2019) and the source size uncertainty. The left and right panels refer to 11A and 11B, respectively.