Gas-phase Fe/O and Fe/N abundances in star-forming regions. Relations between nucleosynthesis, metallicity, and dust

Context . In stars, metallicity is usually traced using Fe, while in nebulae, O serves as the preferred proxy. Both elements have different nucleosynthetic origins and are not directly comparable. Additionally, in ionized nebulae, Fe is heavily depleted onto dust grains. Aims . We investigate the distribution of Fe gas abundances in a sample of 452 star-forming nebulae with [Fe III] λ4658 detections and their relationship with O and N abundances. Additionally, we analyze the depletion of Fe onto dust grains in photoionized environments. Methods . We homogeneously determined the chemical abundances with direct determinations of electron temperature ( T e ), considering the effect of possible internal variations of this parameter. We adopted a sample of 300 Galactic stars to interpret the nebular findings. Results . We find a moderate linear correlation ( r = −0.59) between Fe/O and O/H. In turn, we report a stronger correlation ( r = −0.80) between Fe/N and N/H. We interpret the tighter correlation as evidence that Fe and N are produced on similar timescales while Fe- dust depletion scales with the Fe availability. The apparently flat distribution between Fe/N and N/H in Milky Way stars supports this interpretation. We find that when 12+log(O/H)<7.6, the nebulae seem to reach a plateau value around log(Fe/O) ≈ −1.7. If this trend were confirmed, it would be consistent with a very small amount of Fe dust in these systems, similar to what is observed in high-z galaxies discovered by the James Webb Space Telescope (JWST). We derive a relationship that allows us to approximate the fraction of Fe trapped into dust in ionized nebulae. If the O-dust scales in the same way, its possible contribution in low-metallicity nebulae would be negligible. After analyzing the Fe/O abundances in J0811+4730 and J1631+4426, we do not see evidence of the presence of very massive stars with M init > 300 M ⊙ in these systems. Conclusions . The close relation observed between the N and Fe abundances has the potential to serve as a link between stellar and nebular chemical studies. This requires an expansion of the number of abundance determinations for these elements in both stars and star-forming nebulae, especially at low metallicities.