Biases in abundance derivations for metal-rich nebulae

Stasińska, G.

doi:10.1051/0004-6361:20042216

Cited by 171 publications

(221 citation statements)

References 81 publications

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“…Most of the models lead to derived abundances, (O/H) out , that are lower than the input ones. This is a well-known effect (Stasińska 2005 and references therein), due to the exponential dependence with temperature of the CELs used to derive T e and the O + and O ++ abundances. In the presence of a temperature gradient, this dependence leads to an overestimation of T e and hence of the emissivity of the CELs.…”

Section: Resultsmentioning

confidence: 97%

Temperature Structure and Metallicity in H Ii Regions

Rodríguez

García‐Rojas

2009

ApJ

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The metallicities implied by collisionally excited lines (CELs) of heavy elements in H ii regions are systematically lower than those implied by recombination lines (RLs) by factors of ∼2, introducing uncertainties of the same order in the metallicities inferred for the interstellar medium of any star-forming galaxy. Most explanations of this discrepancy are based on the different sensitivities of CELs and RLs to electron temperature, and invoke either some extra heating mechanism producing temperature fluctuations in the ionized region or the addition of cold gas in metal-rich inclusions or ionized by cosmic rays or X-rays. These explanations will change the temperature structure of the ionized gas from the one predicted by simple photoionization models, and depending on which one is correct, will imply different metallicities for the emitting gas. We select nine H ii regions with observed spectra of high quality and show that simple models with metallicities close to the ones implied by oxygen CELs reproduce easily their temperature structure, measured with T e ([N ii])/T e ([O iii]), and their oxygen CELs emission. We discuss the strong constraints that this agreement places on the possible explanations of the discrepancy and suggest that the simplest explanation, namely errors in the line recombination coefficients by factors ∼2, might be the correct one. In such case, CELs will provide the best estimates of metallicity.

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Section: Resultsmentioning

confidence: 97%

Temperature Structure and Metallicity in H Ii Regions

Rodríguez

García‐Rojas

2009

ApJ

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“…Concerning H ii regions, Stasińska (2005) made the point that abundances derived using temperature-based methods in metal-rich nebulae are prone to strong errors, due to important temperature gradients with the nebulae. From the behavior of the radial variation of O/H in the giant H ii regions observed by …”

Section: Uncertaintiesmentioning

confidence: 99%

“…While H ii regions have since long been used to probe the present-day chemical composition of the interstellar medium (Peimbert 1975), the use of PNe, in principle, offers the possibility to study its fossil record, since PNe are descendants of stars that were formed up to a few Gyrs ago (Peimbert 1990;Maciel & Köppen 1994). Another point is that, in the case of a high-metallicity medium, abundances derived in H ii regions can be biased due to significant temperature gradients in their interiors, while those derived in PNe are not (Stasińska 2005). On the other hand, the chemical composition of PNe is not necessarily identical to that of the material out of which the progenitors were Based on observations collected at the ESO-VLT Observatory, Paranal, Chile, program 077.B-0430.…”

Section: Introductionmentioning

confidence: 99%

Planetary nebulae and H ii regions in the spiral galaxy NGC 300

Stasińska

Peña

Bresolin

et al. 2013

A&A

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We have obtained deep spectra of 26 planetary nebulae (PNe) and 9 compact H ii regions in the nearby spiral galaxy NGC 300, and analyzed them together with those of the giant H ii regions previously observed. We have determined the physical properties of all these objects and their He, N, O, Ne, S and Ar abundances in a consistent way.We find that, globally, compact H ii regions have abundance ratios similar to those of giant H ii regions, while PNe have systematically larger N/O ratios and similar Ne/O and Ar/O ratios. We demonstrate that the nitrogen enhancement in PNe cannot be only due to second dredge-up in the progenitor stars, since their initial masses are around 2-2.5 M . An extra mixing process is required, perhaps driven by stellar rotation.Concerning the radial abundance distribution, PNe behave differently from H ii regions: in the central part of the galaxy their average O/H abundance ratio is 0.15 dex smaller. Their abundance dispersion at any galactocentric radius is significantly larger than that shown by H ii regions and many of them have O/H values higher than H ii regions at the same galactocentric distance. This suggests that not only nitrogen, but also oxygen is affected by nucleosynthesis in the PN progenitors, by an amount which depends at least on the stellar rotation velocity and possibly other parameters. The formal O/H, Ne/H and Ar/He abundance gradients from PNe are significantly shallower than those from H ii regions. We argue that this indicates a steepening of the metallicity gradient in NGC 300 during the last Gyr, rather than an effect of radial stellar motions, although the large observed dispersion makes this conclusion only tentative.

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“…Under these conditions, the oxygen abundance is under-estimated, possibly by large factors (Stasińska 2005).…”

Section: Caveat 2: Temperature Gradientsmentioning

confidence: 99%

Measuring chemical abundances in extragalactic metal-rich H ii regions

Bresolin

2008

The Metal-Rich Universe

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The analysis of metal-rich H ii regions has a profound impact on the calibration of abundance diagnostics widely used to measure the chemical content of star-forming galaxies, both locally and at high redshift. I review the main difficulties that affect direct abundance determinations from temperature-sensitive collisionally excited lines, and briefly discuss strong-line methods, in particular their empirical calibration. In the near future it will be possible to calibrate strong-line methods using metal recombination lines, providing abundances that are virtually insensitive to uncertainties on the nebular temperature structure. Chemical abundances of metal-rich H II regions: why?Ionized nebulae (H ii regions) trace the sites of massive star formation in spiral and irregular galaxies. The rapid evolution of these stars, ending in supernovae explosions, and the subsequent recycling of nucleosynthesis products into the interstellar medium, make H ii regions essential probes of the present-day chemical composition of star-forming galaxies across the Universe. The study of nebular abundances is therefore crucial for understanding the chemical evolution of galaxies. In the following pages I will provide an optical astronomer's perspective on some of the issues concerning the measurement of abundances in metal-rich H ii regions, by focusing on the observational difficulties that are peculiar to the high metallicity regime, discussing some of the most recent abundance determinations from H ii regions in the metal-rich zones of spiral galaxies, and indicating some possibilities for further progress. Throughout this paper I will use the oxygen abundance as a proxy for the metallicity (oxygen makes up roughly half of the metal content of the interstellar medium), and assume the solar value from Asplund et al. (2004), 12 + log(O/H) ⊙ = 8.66. Elements besides oxygen will not be discussed in great detail. MotivationsWhy measure abundances of metal-rich H ii regions? After all, as we will see in Section 2, metal-rich H ii regions pose difficulties to the observer that are not present at lower metallicities, i.e. roughly below half the solar O/H value. However, high abundances are encountered in a variety of astrophysical contexts, and the study of ionized nebulae often provides the only way to measure these abundances. Here are a few examples that motivate detailed studies of metal-rich H ii regions, aimed at improving our abundance diagnostics:(a) galactic abundance gradients have been known in spiral galaxies since the pioneering work carried out in the 1970's (see the compilations by Vila-Costas & Edmunds 1992, Zaritsky et al. 1994. Even after the recent downward revision of the metallicity in metalrich H ii regions from empirical methods , the central parts of spiral galaxies reach or exceed the solar metallicity (Pilyugin et al. 2004(Pilyugin et al. , 2006a.(b) chemical evolution models of galaxies aim at explaining the observed abundance gradients and abundance ratios on the basis of assumptions about the stellar yield...

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Biases in abundance derivations for metal-rich nebulae

Cited by 171 publications

References 81 publications

Temperature Structure and Metallicity in H Ii Regions

Temperature Structure and Metallicity in H Ii Regions

Planetary nebulae and H ii regions in the spiral galaxy NGC 300

Measuring chemical abundances in extragalactic metal-rich H ii regions

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