The details of the physical arguments concerning the amount of oxygen available in the droplets, the mixing processes and the simulation of the ORL-CEL discrepancy with a multi-zone photoionization model are described in Stasinska et al. (2007). Here, we simply give the most important conclusions.
Photoionization of the oxygen-rich droplets predicted by the T-T96 scenario can reproduce the observed abundance discrepancy factors (ADFs, i.e. the ratios of abundances obtained from ORLs and from CELs) derived for Galactic and neutral oxygen-rich droplets neutral oxygen-rich droplets
fully mixed H ii gas
fully mixed H ii gas
Figure 42.5. When a next generation of massive stars forms, they photoionize the surrounding interstellar medium (ISM), including the metal-rich droplets. It is only after the droplets have been photoionized that their matter is intimately mixed with the matter from the ISM and that proper chemical enrichment has occurred. The whole process since the explosion of the supernovae that provided fresh oxygen has taken at least 100 Myr.
extragalactic H ii regions. The recombination lines arising from the highly metallic droplets thus show mixing at work.
We find that, if our scenario holds, the recombination lines strongly overestimate the metallicities of the fully mixed H ii regions. The collisionally excited lines may also overestimate them, although to a much smaller extent. In the absence of any recipe to correct for these biases, we recommend that one should discard objects with large ADFs when one is attempting to probe the chemical evolution of galaxies.
To proceed further with this question of inhomogeneities, one needs as many observational constraints as possible. On the theoretical side, one needs robuster estimates of the integrated stellar yields as well as better knowledge of the impact of massive stars on the ISM and of the role of turbulence. All these issues are relevant to our understanding of the metal enrichment of the Universe.
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