Influence of the composition of metallic iron inclusions (for carbonaceous grains with a 10 nm thick aromatic-rich carbon mantle and silicates with a 5 nm mantle). We show models, where 30% (yellow), 50% (pink), and 100% (turquoise) of the iron inclusions are made of FeS. The first two models almost perfectly overlap in the figures (yellow and pink lines). In panel a), the turquoise line is out of the plot with β ~ 1.25. The radiation field is also varied with . See Sect. 2.2 for description of the black and white lines and symbols.