Kinetic equilibrium of iron in the atmospheres of cool dwarf stars

Gehren, T.; Korn, A. J.; Shi, Jianrong

doi:10.1051/0004-6361:20011479

Cited by 40 publications

(34 citation statements)

References 30 publications

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“…Inspection of the literature shows that very different choices for S H are made for different species. For example Gehren et al (2001) advocate S H = 5 for Fe i in the Sun.…”

Section: Corrections For Departure From Local Thermodynamic Equilibriummentioning

confidence: 99%

The photospheric solar oxygen project

Caffau¹,

Ludwig

Steffen

et al. 2008

A&A

264

305

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Context. The solar oxygen abundance has undergone a major downward revision in the past decade, the most noticeable one being the update including 3D hydrodynamical simulations to model the solar photosphere. Up to now, such an analysis has only been carried out by one group using one radiation-hydrodynamics code. Aims. We investigate the photospheric oxygen abundance considering lines from atomic transitions. We also consider the relationship between the solar model used and the resulting solar oxygen abundance, to understand whether the downward abundance revision is specifically related to 3D hydrodynamical effects. Methods. We performed a new determination of the solar photospheric oxygen abundance by analysing different high-resolution high signal-to-noise ratio atlases of the solar flux and disc-centre intensity, making use of the latest generation of CO5BOLD 3D solar model atmospheres. Results. We find 8.73 ≤ log (N O /N H ) + 12 ≤ 8.79. The lower and upper values represent extreme assumptions on the role of collisional excitation and ionisation by neutral hydrogen for the NLTE level populations of neutral oxygen. The error of our analysis is ± (0.04± 0.03) dex, the last being related to NLTE corrections, the first error to any other effect. The 3D "granulation effects" do not play a decisive role in lowering the oxygen abundance. Conclusions. Our recommended value is log (N O /N H ) = 8.76 ± 0.07, considering our present ignorance of the role of collisions with hydrogen atoms on the NLTE level populations of oxygen. The reasons for lower O abundances in the past are identified as (1) the lower equivalent widths adopted and (2) the choice of neglecting collisions with hydrogen atoms in the statistical equilibrium calculations for oxygen.

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“…Inspection of the literature shows that very different choices for S H are made for different species. For example Gehren et al (2001) advocate S H = 5 for Fe i in the Sun.…”

Section: Corrections For Departure From Local Thermodynamic Equilibriummentioning

confidence: 99%

The photospheric solar oxygen project

Caffau¹,

Ludwig

Steffen

et al. 2008

A&A

264

305

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“…Thus, in principle, any rescaling of the Drawin's cross-sections, e.g. using variable S H for levels of different excitation energies as exemplified by Baumueller & Gehren (1996) (Gehren et al 2001b). For Δξ t = ±0.2 km s −1 with respect to the reference value 0.9 km s −1 , the abundances derived from the Cr i lines of multiplet 18 change by roughly ∓0.1 dex, and any combination of ξ t and log ε leads to an equally good profile fit (Fig.…”

Section: Solar Lines Of Cr I and Cr Iimentioning

confidence: 99%

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

Bergemann

Cescutti

2010

A&A

157

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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“…Although unpublished as a stellar atmosphere code this model is widely used and has proven its reliability for A-, F-and G-type stars of different evolutionary stage and for various scientific questions. Some of the most recent publications based on Gehrens ODF Version of MAFAGS are: Korn et al (2003), Mashonkina et al (2003), Fuhrmann (2002), Gehren et al (2001a), Gehren et al (2001b), Mashonkina & Gehren (2001), Fuhrmann (1999), Mashonkina et al (1999), Mashonkina et al (1999), Fuhrmann et al (1997), Fuhrmann et al (1994), Fuhrmann et al (1993) and Gehren et al (1991).…”

Section: Introductionmentioning

confidence: 99%

MAFAGS-OS: New opacity sampling model atmospheres for A, F and G stars

Grupp

2004

A&A

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Abstract. We present a new opacity sampling model atmosphere code, named MAFAGS-OS. This code, designed for stars reaching from A0 down to G9 on a solar and metal poor main sequence and up to an evolutionary stage represented by the turnoff is introduced in its basic input physics and modelling techniques. Fe I bound-free cross-sections of Bautista (1997) are used and convection is treated according to Canuto & Mazzitelli (1991). An α cm -parameter for the efficiency of convection of 0.82 is used as determined by Bernkopf (1998) from stellar evolution requirements. Within the process of opacity sampling, special attention is drawn to the matter of line selection. We show that a selection criterion, in which lines are chosen by their opacity weighted relative to the continuous background opacity, is useful and valid. The solar model calculated using this new code is shown to fit the measured solar flux distribution. It is also tested against the measured solar colours and leads to U − B = 0.21 and B − V = 0.64, in good agreement with observation. Comparison with measured centre-to-limb continuum data show only small improvement with respect to opacity-sampling type model atmospheres. This is the first of a series of 2 papers. Paper II will deal with the matter of temperature determination using Balmer lines and the infrared-flux method; furthermore it will present three "standard" stars analysed using this new model.

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Kinetic equilibrium of iron in the atmospheres of cool dwarf stars

Cited by 40 publications

References 30 publications

The photospheric solar oxygen project

The photospheric solar oxygen project

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

MAFAGS-OS: New opacity sampling model atmospheres for A, F and G stars

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