Local associations and the barium puzzle

Abstract

We have observed high-dispersion echelle spectra of main-sequence stars in five nearby young associations -Argus, Carina-Near, Hercules-Lyra, Orion and Subgroup B4 -and derived abundances for elements ranging from Na to Eu. These are the first chemical abundance measurements for two of the five associations, while the remaining three associations are analysed more extensively in our study. Our results support the presence of chemical homogeneity among association members with a typical starto-star abundance scatter of about 0.06 dex or less over many elements. The five associations show log ǫ(Li) consistent with their age and share a solar chemical composition for all elements with the exception of Ba. We find that all the heavy elements (Y, Zr, La, Ce, Nd, Sm and Eu) exhibit solar ratios, i.e., [X/Fe] ≃ 0, while Ba is overabundant by about 0.2−0.3 dex. The origin of the overabundance of Ba is a puzzle. Within the formulation of the s-process, it is difficult to create a higher Ba abundance without a similar increase in the s-process contributions to other heavy elements (LaSm). Given that Ba is represented by strong lines of Ba ii and La-Sm are represented by rather weak ionized lines, the suggestion, as previously made by other studies, is that the Ba abundance may be systematically overestimated by standard methods of abundance analysis perhaps because the upper reaches of the stellar atmospheres are poorly represented by standard model atmospheres. A novel attempt to analyse the Ba i line at 5535Å gives a solar Ba abundance for stars with effective temperatures hotter than about 5800 K but increasingly subsolar Ba abundances for cooler stars with apparent Ba deficiencies of 0.5 dex at 5100 K. This trend with temperature may signal a serious non-LTE effect on the Ba i line.

First determination ofs-process element abundances in pre-main sequence clusters

“…Thus, in agreement with what was found for slightly older associations (age ∼ 100 Myr) by D'Orazi et al (2012) and Reddy & Lambert (2015), we still face a barium conundrum. Crucially, the behaviour is much more extreme in this case, compared to the young local associations, because for those objects Ba abundances were at 0.2 -0.3 dex level, whereas IC 2602, IC 2391 and Argus show values up to ∼ 0.65 dex.…”

High-precision stellar abundances of the elements: methods and applications

“…As seen from Fig. 16 Reddy and Lambert 2015). According to Reddy and Lambert (2017), this large spread in the derived [Ba/Fe] values for young stars may be spurious due to problems in determining Ba abundances from the rather strong Ba ii lines available.…”

Origin of α-rich young stars: clues from C, N, and O

“…As the sample of red giants analysed in this paper spans a limited range in T eff , log g and ξ t , the abundance sensitivity to stellar parameters is carried out for a representative star KIC 6664950 with T eff = 4800 K, log g = 2.4 dex and ξ t = 1.7 km s −1 . The incremental changes about the average abundance of an element caused by varying T eff , log g, ξ t and the model metallicity by ± 50 K, 0.2 dex, 0.2 km s −1 and 0.1 dex (see for details of these choices Reddy & Lambert 2015), respectively, with respect to the chosen model parameters are summarised in Table A4. The square root of the quadratic sum of the mean values of all four contributors are presented in the column headed σ 2 .…”