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

Abstract

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.

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

“…The ratio of Sr i/Sr ii falls below 10 −3 above continuum optical depth unity, and the departures from LTE in the distribution of atomic level populations are almost entirely caused by overionization. The behavior of the Sr i departure coefficients with stellar parameters therefore resembles that of similar trace atoms, such as Co i or Cr i (Bergemann & Cescutti 2010). The levels are underpopulated and their b i -factors monotonously decrease outward.…”

Metal-poor stars towards the Galactic bulge: A population potpourri

“…The abundance ratios we derived in our sample of bulge stars bear little surprise in comparison with halo stars of similarly low metallicity and, as for the α-elements, we note a remarkable homogeneity in the iron-peak across the covered metallicity range. However, a NLTE analysis of Cr in metal-poor, warm dwarfs by Bergemann & Cescutti (2010), led them to conclude that the deficiencies found in LTE analyses could be explained by NLTE effects, rather than genuine deficiencies in [Cr/Fe]. This conclusion is supported by the lack of ionisation equilibrium obtained in this study: from our single Cr II line we find Cobolt in all stars is solar to moderately elevated, although we do not trace the slightly decreasing trend, towards higher metallicity, commencing at ∼−2 dex, seen in the Roederer et al (2014) [Co/Fe] for the one metal-poor giant in their sample with parameters similar to our stars.…”

GaiaFGK benchmark stars: abundances ofαand iron-peak elements

“…NLTE corrections were computed for selected lines of magnesium (Osorio et al 2015), silicon (Bergemann et al 2013 calcium (Lind et al 2013), chromium (Bergemann & Cescutti 2010), and manganese (Bergemann & Gehren 2008). The main uncertainty in these calculations is typically related to the inelastic hydrogen collision rates.…”

“…However, several works in the literature reported that while for Cr ii this is the case, for Cr i the [Cr/Fe] ratio increases with metallicity (Gratton & Sneden 1991;Bai et al 2004;Cayrel et al 2004;Lai et al 2008;Adibekyan et al 2012). Bergemann & Cescutti (2010) explained this discrepancy by finding that neutral Cr lines are strongly sensitive to NLTE for metal-poor stars.…”