Top: best fitting star formation efficiency f_⋆ for the given mean radius r_h and surface density profile slopes β. Primordial boundary conditions as discussed in the text would imply f_⋆ ≲ 0.4 and r_h ≲ 2 pc. Bottom: best fitting pre-enrichment level for the same range in mean radius r_h and surface density profile slopes β as above. A physically motivated lower limit for the pre-enrichment level is about ~ − 1.8 dex, see text for details.