Solar Twins and the Barium Puzzle

Abstract

Several abundance analyses of Galactic open clusters (OCs) have shown a tendency for Ba but not for other heavy elements (La−Sm) to increase sharply with decreasing age such that Ba was claimed to reach [Ba/Fe] ≃ +0.6 in the youngest clusters (ages < 100 Myr) rising from [Ba/Fe]= 0.00 dex in solar-age clusters. Within the formulation of the s-process, the difficulty to replicate higher Ba abundance and normal La−Sm abundances in young clusters is known as the barium puzzle. Here, we investigate the barium puzzle using extremely highresolution and high signal-to-noise spectra of 24 solar twins and measured the heavy elements Ba, La, Ce, Nd and Sm with a precision of 0.03 dex. We demonstrate that the enhanced Ba II relative to La−Sm seen among solar twins, stellar associations and OCs at young ages (<100 Myr) is unrelated to aspects of stellar nucleosynthesis but has resulted from overestimation of Ba by standard methods of LTE abundance analysis in which the microturbulence derived from the Fe lines formed deep in the photosphere is insufficient to represent the true line broadening imposed on Ba II lines by the upper photospheric layers from where the Ba II lines emerge. As the young stars have relatively active photospheres, Ba overabundances most likely result from the adoption of too low a value of microturbulence in the spectum synthesis of the strong Ba II lines but the change of microturbulence in the upper photosphere has only a minor affect on La−Sm abundances measured from the weak lines.

The Gaia-ESO Survey: a new approach to chemically characterising young open clusters

“…The standard analysis of the Ba ii 5853 Å and 6496 Å lines gave values up to +0.60 dex in clusters younger than 50 Myr, but solar values in older clusters (age 1-2 Gyr). The increasing trend as a function of the age has been confirmed by other authors (Maiorca et al 2011;Jacobson & Friel 2013;Mishenina et al 2015;Reddy & Lambert 2017;Magrini et al 2018;Delgado Mena et al 2019), and it has been interpreted by Maiorca et al (2012) as due to the recent production by nucleosynthesis in asymptotic giant branch (AGB) stars of low mass. This can explain mild enhancements (up to 0.1-0.2 dex), but not the much higher values measured in very young clusters, however.…”

From Scattered-light to Millimeter Emission: A Comprehensive View of the Gigayear-old System of HD 202628 and its Eccentric Debris Ring

“…As Sun-like stars deplete their Li as they age, trends among age-dated solar twins show that HD 202628's high Li abundace (log (Li)LTE = 2.23 ± 0.02) would similarly appear unusual if the star were >2 Gyr (see Fig. 4 of Reddy & Lambert 2017), but appears to be clearly older than the ∼0.7 Gyr-old Hyades (see Fig. 6 of Sestito et al 2003).…”

High-precision abundances of elements inKeplerLEGACY stars

“…Hence, 3D non-LTE studies for other elements would be highly interesting to further improve the accuracy of the abundance determinations. Effects of stellar activity (Flores et al 2016;Reddy & Lambert 2017) on abundance determinations also deserve further studies.…”

“…Flores et al (2016) have discovered that one of the solar twins included in Paper I, HD 45184, has a 5.14 yr activity cycle over which the equivalent widths of strong Fe ii lines change with an amplitude of about ±1 mÅ, and Reddy & Lambert (2017) Table 1. For each star, the age is calculated as the mean of the results from the seven different analyses of the available seismic data.…”