Collisional-radiative ionization and recombination coefficients for quasi-stationary homogeneous hydrogen and hydrogenic ion plasmas

Drawin, H. W.

doi:10.1007/bf01392774

Cited by 90 publications

(41 citation statements)

References 2 publications

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“…For inelastic hydrogen collisions, we use quantum-based rates inspired by fitting quantum rates of Na, Mg, Al, and Si and applying them to Fe (R. Ezzeddine et al 2016, in preparation). This leads to more accurate abundances and avoids the Drawin (Drawin 1969) approximation that overestimates collisional rates by orders of magnitude. Using line-by-line NLTE corrections, we rebalance the Fe abundance with respect to excitation, ionization, and reduced equivalent width.…”

Section: Observations and Abundance Analysismentioning

confidence: 99%

Chemical Diversity in the Ultra-Faint Dwarf Galaxy Tucana Ii*

Frebel

Ezzeddine

et al. 2016

ApJL

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We present the first detailed chemical abundance study of the ultra-faint dwarf galaxy TucanaII, based on highresolution Magellan/MIKE spectra of four red giant stars. =−3 is carbon-normal, and exhibits distinct light element abundance ratios from the carbon-enhanced stars. This carbonnormal star implies that at least two distinct nucleosynthesis sources, both possibly associated with PopulationIII stars, contributed to the early chemical enrichment of this galaxy. Despite its very low luminosity, TucanaII shows a diversity of chemical signatures that preclude it from being a simple "one-shot" first galaxy yet still provide a window into star and galaxy formation in the early universe.

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Section: Observations and Abundance Analysismentioning

confidence: 99%

Chemical Diversity in the Ultra-Faint Dwarf Galaxy Tucana Ii*

Frebel

Ezzeddine

et al. 2016

ApJL

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“…Cross-sections for transitions caused by inelastic collisions with H i atoms are basically unknown for any atom heavier than Mg. The only available formula developed originally for collisions between equal H-like atoms (Drawin 1968(Drawin , 1969) that was subsequently slightly modified by Steenbock & Holweger (1984) and Lambert (1993) was shown to overestimate the rates of bound-bound transitions by two to seven orders of magnitude (Belyaev & Barklem 2003;Barklem et al 2012). Furthermore, the charge transfer processes cannot be described by this simple classical formula at all.…”

Section: Collision Ratesmentioning

confidence: 99%

NLTE analysis of Sr lines in spectra of late-type stars with new R-matrix atomic data

Bergemann

Hansen²,

Bautista

et al. 2012

A&A

100

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We investigate the statistical equilibrium of neutral and singly-ionized strontium in late-type stellar atmospheres. Particular attention is given to the completeness of the model atom, which includes new energy levels, transition probabilities, photoionization and electron-impact excitation cross-sections computed with the R-matrix method. The NLTE model is applied to the analysis of Sr I and Sr II lines in the spectra of the Sun, Procyon, Arcturus, and HD 122563, showing a significant improvement in the ionization balance compared to LTE line formation calculations, which predict abundance discrepancies of up to 0.5 dex. The solar Sr abundance is log = 2.93 ± 0.04 dex, in agreement with the meteorites. We present the grid of NLTE abundance corrections for Sr I and Sr II lines that covers a wide range of stellar parameters.

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“…Thus, we relied on the commonly-used approximations. Cross-sections for continuum and allowed discrete transitions due to inelastic collisions with H i were computed with the formulae of Drawin (1969). This original recipe was developed for collisions between equal hydrogen-like particles, hence we applied a scaling factor 0 ≤ S H ≤ 5 to the Drawin cross-sections.…”

Section: Statistical Equilibrium Calculationsmentioning

confidence: 99%

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

Bergemann

Cescutti

2010

A&A

159

131

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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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Collisional-radiative ionization and recombination coefficients for quasi-stationary homogeneous hydrogen and hydrogenic ion plasmas

Cited by 90 publications

References 2 publications

Chemical Diversity in the Ultra-Faint Dwarf Galaxy Tucana Ii*

Chemical Diversity in the Ultra-Faint Dwarf Galaxy Tucana Ii*

NLTE analysis of Sr lines in spectra of late-type stars with new R-matrix atomic data

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

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