The [Feiv] Discrepancy: Constraining the Iron Abundances in Nebulae

Abstract

We study the current discrepancy between the model-predicted and measured concentrations of Fe ++ and Fe +3 in ionized nebulae. We calculate a set of photoionization models, updated with the atomic data relevant to the problem, and compare their results with those derived for the available nebulae where both [Fe iii] and [Fe iv] lines have been measured. Our new model results are closer to the measured values than the results of previous calculations, but a discrepancy remains. This discrepancy translates into an uncertainty in the derived Fe abundances of a factor of up to $4. We explore the possible causes of this discrepancy and find that errors in the Fe atomic data may be the most likely explanation. The discrepancy can be fully accounted for by any of the following changes: (1) an increase by a factor of $10 in the recombination rate (radiative plus dielectronic, or charge transfer) for Fe +3 , (2) an increase by a factor of 2-3 in the effective collision strengths for Fe ++ , or (3) a decrease by a factor of 2-3 in the effective collision strengths for Fe +3 . We derive the Fe abundances implied by these three explanations and use the results to constrain the degree of depletion of Fe in our sample nebulae. The Galactic H ii regions and planetary nebulae are found to have high depletion factors, with less than 5% of their Fe atoms in the gas phase. The extragalactic H ii regions (LMC 30 Doradus, SMC N88A, and SBS 0335À052) have somewhat lower depletions. The metal-deficient blue compact galaxy SBS 0335À052 could have from 13% to 40% of Fe in the gas phase. The depletions derived for the different objects define a trend of increasing depletion at higher metallicities.

s-process enrichment in the planetary nebula NGC 3918. Results from deep echelle spectrophotometry

“…As can be seen, the sum value is in very good agreement with the value derived from Eq. (2) of Rodríguez & Rubin (2005). Delgado-Inglada & Rodríguez (2014) also find several objects with high-ionization Fe ions.…”

“…The [Fe iii] lines are usually the brightest iron lines in photoionized nebulae. However, as is shown in Fig 9, Rodríguez & Rubin (2005), which is based on the detection of [Fe iii] lines and two ICFs, one from photoionization models (their Eq. 2) and one from an observational fit that takes into account all the uncertainties in the atomic data involved in the calculations and that allows us to constrain real iron abundances (Eq.…”

C/O Abundance Ratios, Iron Depletions, and Infrared Dust Features in Galactic Planetary Nebulae

“…The sample includes 28 Galactic PNe studied in Delgado-Inglada et al (2009), but we exclude nebulae with electron densities above 25,000 cm −3 since it is difficult to derive reliable physical conditions and chemical abundances in these objects (Delgado-Inglada et al 2009). Besides, we set an upper limit on the I (He ii λ4686)/I (Hβ) intensity ratio of 0.6, in order to avoid high excitation PNe for which the iron correction scheme adopted here may not apply (Rodríguez & Rubin 2005). The final sample of 56 PNe is presented in Table 1.…”

“…However the iron abundances computed in this way differ from the ones obtained by adding up the ionic abundances of Fe + , Fe ++ , and Fe +3 in some H ii regions and low-excitation PNe. Rodríguez & Rubin (2005) studied the uncertainties involved in the calculations and proposed that the iron abundances can be constrained using two ICFs, one derived from photoionization models:…”

The Mg IIλ2797,λ2803 emission in low-metallicity star-forming galaxies from the SDSS

“…These data indicate a moderate level of magnesium depletion. There are only a few determinations of the magnesium abundance in H ii regions, such as the Orion nebula in the Galaxy and the Tarantula nebula in the Large Magellanic Cloud and in some bright planetary nebulae (Rodríguez & Rubin 2005;Peimbert & Peimbert 2010;Dinerstein et al 2012). Dinerstein et al (2012) has investigated the gas-phase magnesium abundances in 25 planetary nebulae using the Mg ii λ4481 Å recombination line.…”