Fe I/Fe II ionization equilibrium in cool stars: NLTE versus LTE

Mashonkina, L.; Gehren, T.; Shi, Jianrong; Korn, A. J.; Grupp, F.

doi:10.1017/s1743921310000530

Cited by 18 publications

(22 citation statements)

References 11 publications

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“…The validity of the LTE approach to 1D line formation calculations for Fe ii was demonstrated in very accurate analyses of the statistical equilibrium of Fe in the Sun (Gehren et al 2001a,b) and metal-poor stars (Mashonkina et al 2010). These studies employ the same type of atmospheric models, as we use in the current work.…”

Section: Observations and Stellar Parametersmentioning

confidence: 84%

Chromium: NLTE abundances in metal-poor stars and nucleosynthesis in the Galaxy

Bergemann

Cescutti

2010

A&A

153

124

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Aims. We investigate statistical equilibrium of Cr in the atmospheres of late-type stars to ascertain whether the systematic abundance discrepancy between Cr I and Cr II lines, as often found in previous work, is due to deviations from local thermodynamic equilibrium (LTE). Furthermore, we attempt to interpret the Non-LTE (NLTE) trend of [Cr/Fe] with [Fe/H] using chemical evolution models for the solar neighborhood. Methods. NLTE calculations are performed for the model of the Cr atom, comprising 340 levels and 6806 transitions in total. We use the quantum-mechanical photoionization cross-sections of Nahar (2009) and investigate the sensitivity of the model to uncertain crosssections for H I collisions. NLTE line formation is performed for the MAFAGS-ODF model atmospheres of the Sun and 10 metal-poor stars with −3.2 < [Fe/H] < −0.5, and Cr abundances are derived by comparing the synthetic and observed flux spectra. Results. We achieve good ionization equilibrium of Cr for models with different stellar parameters, if inelastic collisions with H I atoms are neglected. The solar NLTE abundance based on Cr I lines is 5.74 dex with σ = 0.05 dex, which is ∼0.1 dex higher than the LTE abundance. For the metal-poor stars, the NLTE abundance corrections to Cr I lines range from +0.3 to +0.5 dex. The resulting [Cr/Fe] ratio is roughly solar for the range of metallicities analyzed here, which is consistent with current views on the production of these iron peak elements in supernovae. Conclusions. The tendency of Cr to become deficient with respect to Fe in metal-poor stars is an artifact caused by the neglect of NLTE effects in the line formation of Cr i, and has no relation to any peculiar physical conditions in the Galactic ISM or deficiencies of nucleosynthesis theory.

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Section: Observations and Stellar Parametersmentioning

confidence: 84%

Chromium: NLTE abundances in metal-poor stars and nucleosynthesis in the Galaxy

Bergemann

Cescutti

2010

A&A

153

124

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“…Collet et al (2005) treats it as a free parameter and tests values S H = 1 and 0.001. Korn et al (2003) found a higher value, S H = 3, however in more recent work (Mashonkina et al 2010) the same group has constrained it to 0.1 based on an improved Fe atom. Here we adopt the values 1 and 0.001; this gives us two sets of synthetic spectra, one with the Drawin (S H = 1) description of H collisions, and a second close to maximal NLTE effects (S H = 0.001).…”

Section: D Nlte Fe Line Analysismentioning

confidence: 95%

The barium isotopic fractions in five metal-poor stars

Gallagher

Ryan

Hosford

et al. 2012

A&A

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Context. Theory and observations of heavy element nucleosynthesis are in conflict with one-another. Theory states that in the most metal-poor stars, the rapid (r-) neutron-capture nucleosynthetic process would be dominant over the slow (s-) process. The most recent determinations of r-and s-process yields do not support this. Aims. We provide measurements of the Ba isotopic fractions for five metal-poor stars derived with a local thermodynamic equilibrium (LTE) analysis with 1D model stellar atmospheres. This increases the comparisons with heavy element nucleosynthesis theory. Methods. We use high resolution (R ≡ λ/Δλ = 90 000−95 000), very high signal-to-noise (S /N > 500) spectra to determine the fraction of odd Ba isotopes ( f odd ) by measuring subtle asymmetries in the profile of the Ba ii line at 4554 Å. We also use two different macroturbulent broadening techniques, Gaussian and radial-tangential, to model the Fe lines of each star, and propagate each technique to model macroturbulent broadening in the Ba 4554 Å line. We conduct a 1D non-LTE (NLTE) treatment of the Fe lines in the red giant HD 122563 and the subgiant HD 140283 in an attempt to improve the fitting. We determine [Ba/Eu] ratios for the two giants in our study, HD 122563 and HD 88609, which can also be used to determine the relative contribution of the s-and r-processes to heavy-element nucleosynthesis, for comparison with f odd . Results. We find mathematical solutions of f odd for HD 122563, HD 88609 and HD 84937 of −0.12 ± 0.07, −0.02 ± 0.09, and −0.05 ± 0.11 respectively. BD+26 • 3578 yielded a value for f odd = 0.08 ± 0.08. Only BD−04 • 3208 was found to have a physical f odd ratio of 0.18 ± 0.08. This means that all stars examined here show isotopic fractions more compatible with an s-process dominated composition. The [Ba/Eu] ratios in HD 122563 and HD 88609 are found to be −0.20 ± 0.15 and −0.47 ± 0.15 respectively, which indicate instead an r-process signature. We report a better statistical fit to the majority of Fe profiles in each star when employing a radial-tangential broadening technique during our 1D LTE investigation. Conclusions. With the increase of the number of stars for which the Ba isotope fraction f odd has been measured, and the nature of their results, there is now a stronger argument to suggest that other synthesis codes that employ alternative approaches to radiative transfer (e.g. 3D hydrodynamics) have to be considered to tackle the high level of precision required for the determination of isotopic ratios. We have shown that, from a statistical point of view, one must consider using a radial-tangential broadening technique rather than a Gaussian one to model Fe line macroturbulences when working in 1D. No improvement to Fe line fitting is seen when employing a NLTE treatment of the Fe lines.

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“…For five species, Sr ii, Zr ii, Ba ii, Pr ii, and Eu ii, we performed NLTE calculations using the methods described in our earlier studies (Belyakova & Mashonkina 1997;Velichko et al 2010;Mashonkina et al 1999Mashonkina et al , 2009Mashonkina & Gehren 2000) and determined the NLTE element abundances. They are presented in Table 8 In contrast to Sr ii and Ba ii, the term structure of the other NLTE species is produced by multiple electronic configurations and consists of hundreds and thousands of energy levels.…”

Section: Nlte Effectsmentioning

confidence: 99%

The Hamburg/ESOR-process enhanced star survey (HERES)

Mashonkina

Christlieb

Barklem

et al. 2010

A&A

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AstrophysicsThe Hamburg/ESO R-process enhanced star survey (HERES) V ABSTRACTAims. We present a detailed abundance analysis of a strongly r-process enhanced giant star discovered in the HERES project, HE 2327−5642, for which [Fe/H] = −2.78, [r/Fe] = +0.99. Methods. We determined the stellar parameters and element abundances by analyzing the high-quality VLT/UVES spectra. The surface gravity was calculated from the non-local thermodynamic equilibrium (NLTE) ionization balance between Fe i and Fe ii, and Ca i and Ca ii.Results. Accurate abundances for a total of 40 elements and for 23 neutron-capture elements beyond Sr and up to Th were determined in HE 2327−5642. For every chemical species, the dispersion in the single line measurements around the mean does not exceed 0.11 dex. The heavy element abundance pattern of HE 2327−5642 is in excellent agreement with those previously derived for other strongly r-process enhanced stars, such as CS 22892-052, CS 31082-001, and HE 1219-0312. Elements in the range from Ba to Hf match the scaled Solar r-process pattern very well. No firm conclusion can be drawn about the relationship between the fisrt neutroncapture peak elements, Sr to Pd, in HE 2327−5642 and the Solar r-process, due to the uncertainty in the Solar r-process. A clear distinction in Sr/Eu abundance ratios was found between the halo stars of different europium enhancement. The strongly r-process enhanced stars contain a low Sr/Eu abundance ratio at [Sr/Eu] = −0.92 ± 0.13, while the stars with 0 < [Eu/Fe] < 1 and [Eu/Fe] < 0 have 0.36 dex and 0.93 dex higher Sr/Eu values, respectively. Radioactive dating for HE 2327−5642 with the observed thorium and rare-earth element abundance pairs results in an average age of 13.3 Gyr, when based on the high-entropy wind calculations, and 5.9 Gyr, when using the Solar r-residuals. We propose that HE 2327−5642 is a radial-velocity variable based on our high-resolution spectra covering ∼4.3 years.

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Fe I/Fe II ionization equilibrium in cool stars: NLTE versus LTE

Cited by 18 publications

References 11 publications

Chromium: NLTE abundances in metal-poor stars and nucleosynthesis in the Galaxy

Chromium: NLTE abundances in metal-poor stars and nucleosynthesis in the Galaxy

The barium isotopic fractions in five metal-poor stars

The Hamburg/ESOR-process enhanced star survey (HERES)

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