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Abstract: The origin of the abundance discrepancy is one of the key problems in thephysics of photoionized nebula. In this work, we analize and discuss data for asample of Galactic and extragalactic HII regions where this abundancediscrepancy has been determined. We find that the abundance discrepancy factorADF is fairly constant and of the order of 2 in all the available sample ofHII regions. This is a rather different behaviour than that observed inplanetary nebulae, where the ADF shows a much wider range of values. We do notfind correlations between the ADF and the O-H, O++-H+ ratios, the ionizationdegree, TeHigh, TeLow- TeHigh, FWHM, and the effective temperature of themain ionizing stars within the observational uncertainties. These resultsindicate that whatever mechanism is producing the abundance discrepancy in HIIregions it does not substantially depend on those nebular parameters. On thecontrary, the ADF seems to be slightly dependent on the excitation energy, afact that is consistent with the predictions of the classical temperaturefluctuations paradigm. Finally, we obtain that Te values obtained from OIIrecombination lines in HII regions are in agreement with those obtained fromcollisionally excited line ratios, a behaviour that is again different fromthat observed in planetary nebulae. These similar temperature determinationsare in contradiction with the predictions of the model based on the presence ofchemically inhomogeneous clumps but are consistent with the temperaturefluctuations paradigm. We conclude that all the indications suggest that thephysical mechanism responsible of the abundance discrepancy in HII regions andplanetary nebulae are different.



Autor: Jorge Garcia-Rojas, Cesar Esteban Instituto de Astrofisica de Canarias, Spain

Fuente: https://arxiv.org/







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