Barium abundance in red giants of NGC 6752

Abstract

Aims. We study the effects related to departures from non-local thermodynamic equilibrium (NLTE) and homogeneity in the atmospheres of red giant stars, to assess their influence on the formation of Ba II lines. We estimate the impact of these effects on the barium abundance determinations for 20 red giants in Galactic globular cluster NGC 6752. Methods. One-dimensional (1D) local thermodynamic equilibrium (LTE) and 1D NLTE barium abundances were derived using classical 1D ATLAS9 stellar model atmospheres. The three-dimensional (3D) LTE abundances were obtained for 8 red giants on the lower RGB, by adjusting their 1D LTE abundances using 3D-1D abundance corrections, i.e., the differences between the abundances obtained from the same spectral line using the 3D hydrodynamical and classical 1D stellar model atmospheres. The 3D-1D abundance corrections were obtained in a strictly differential way using the 3D hydrodynamical and classical 1D codes CO 5 BOLD and LHD. Both codes utilized identical stellar atmospheric parameters, opacities, and equation of state. Results. The mean 1D barium-to-iron abundance ratios derived for 20 giants are [Ba/Fe] 1D LTE = 0.24 ± 0.05(stat.) ± 0.08(sys.) and [Ba/Fe] 1D NLTE = 0.05 ± 0.06(stat.) ± 0.08(sys.). The 3D-1D abundance correction obtained for 8 giants is small (∼+0.05 dex), thus leads to only minor adjustment when applied to the mean 1D NLTE barium-to-iron abundance ratio for the 20 giants, [Ba/Fe] 3D+NLTE = 0.10 ± 0.06(stat.) ± 0.10(sys.). The intrinsic abundance spread between the individual cluster stars is small and can be explained in terms of uncertainties in the abundance determinations. Conclusions. Deviations from LTE play an important role in the formation of barium lines in the atmospheres of red giants studied here. The role of 3D hydrodynamical effects should not be dismissed either, even if the obtained 3D-1D abundance corrections are small. This result is a consequence of subtle fine-tuning of individual contributions from horizontal temperature fluctuations and differences between the average temperature profiles in the 3D and 1D model atmospheres: owing to the comparable size and opposite sign, their contributions nearly cancel each other. This fine-tuning is characteristic of the particular set of atmospheric parameters and the element investigated, hence should not necessarily be a general property of spectral line formation in the atmospheres of red giant stars.

Atmospheric parameters and chemical properties of red giants in the CoRoT asteroseismology fields

“…Once again, this can be interpreted as arising from a time evolution along the history of the Galaxy, which is of relative proportion to the yields of the various stellar populations. The combined effect of departures from LTE and time-dependent/spatial variations in the atmospheric structure due to convection is relatively small in very low-metallicity environments such as globular clusters (Δ[Ba/Fe] ∼ 0.15 dex compared to a 1D LTE analysis; Dobrovolskas et al 2012), and the magnitude is likely much lower in our sample.…”

Neutron-capture element deficiency of the Hercules dwarf spheroidal galaxy

“…Following Short & Hauschildt (2006), we estimated the downward NLTE corrections for the unsaturated Ba II line at 6141 Å, at [Ba/H] ∼ −4 as observed in our stars (Table 1), to be on the order of 0.06 dex, which even lowers our upper limits. Likewise, Dobrovolskas et al (2012) found the same order of magnitude effect, albeit at higher metallicities ([Fe/H] = −1.6 dex).…”

“…The difference in the EWs measured from these two synthetic spectra yields an indication of the level on contamination by the Fe i line (e.g., Dobrovolskas et al 2012). On average, we found a difference of 7% in measured EWs, with a maximum of 26% for the most metal-rich star, #42441 (Her-2).…”

Abundances of lithium, oxygen, and sodium in the turn-off stars of Galactic globular cluster 47 Tucanae

“…Parameters of the 3D hydrodynamical CO 5 BOLD atmosphere models used in this work. with respect to those determined using 3D hydrodynamical model atmospheres (a non-exhaustive list includes Asplund et al 1999;Collet et al 2007;Caffau & Ludwig 2007;González Hernández et al 2008;Bonifacio et al 2010;Dobrovolskas et al 2012). The role of convection becomes especially important in the atmospheres of metal-poor stars ([Fe/H] < −2), where horizontal fluctuations of thermodynamic quantities and changes in the vertical temperature and velocity profiles may lead to significant differences in the predicted spectral line strengths.…”