Enrichment of the interstellar medium by metal-rich droplets and the abundance bias in H ii regions

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We critically examine a scenario for the enrichment of the interstellar medium (ISM) in which supernova ejecta follow a long (108 yr) journey before falling back onto the galactic disk in the form of metal-rich “droplets”, These droplets do not become fully mixed with the interstellar medium until they become photoionized in H ii regions. We investigate the hypothesis that the photoionization of these highly metallic droplets can explain the observed “abundance discrepancy factors” (ADFs), which are found when comparing abundances derived from recombination lines and from collisionally excited lines, both in Galactic and extragalactic H ii regions. We derive bounds of 1013–1015 cm on the droplet sizes inside H ii regions in order that (1) they should not have already been detected by direct imaging of nearby nebulae, and (2) they should not be too swiftly destroyed by diffusion in the ionized gas. From photoionization modelling we find that, if this inhomogeneous enrichment scenario holds, then the recombination lines strongly overestimate the metallicities of the fully mixed H ii regions. The abundances derived from collisionally excited lines also suffer some bias, although to a much lesser extent. In the absence of any recipe for correcting these biases, we recommend the discarding of all objects showing large ADFs from studies of galactic chemical evolution. These biases must also be kept in mind when comparing the galactic abundance gradients for elements derived from recombination lines with those derived from collisionally excited lines. Finally, we propose a set of observations that could be undertaken to test our scenario and improve our understanding of element mixing in the ISM.