Spectral evidences of density variations in HII regions. Effects on metallicity determinations

We present recent results based on a homogeneous analysis of the DEep Spectra of Ionized REgions Database (DESIRED), a compilation of about two hundred high signal-to-noise ratio optical spectra of Galactic and extragalactic HII regions and other photoionized nebulae. We show important insights and caveats about the effects of electron density inhomogeneities and their impact on the temperature structure and metallicity determinations in ionized nebulae. In particular, we demonstrate how internal density variations can be critical for the analysis of infrared fine structure lines, which can lead to significant underestimations of metallicity if not taken into account.