[Paper Review] High-precision abundances of Sc, Mn, Cu, and Ba in solar twins. Trends of element ratios with stellar age

P. E. Nissen|Jun 27, 2016

Stellar, planetary, and galactic studies93 references3 citations

TL;DR

This study presents high-precision abundances of Sc, Mn, Cu, and Ba in 21 solar twins using HARPS spectra, revealing tight linear correlations between [X/Fe] ratios and stellar age for stars younger than 6 Gyr. The results indicate that Sc, Mn, and Cu are primarily produced in Type II supernovae and the weak s-process in massive stars, while Ba is enriched by low-mass AGB stars, with [Y/Mg] and [Y/Al] emerging as robust chemical clocks for age determination at ~1 Gyr precision.

ABSTRACT

A previous study of correlations between element abundance ratios, [X/Fe], and ages of solar twin stars is extended to include Sc, Mn, Cu, and Ba. HARPS spectra with S/N > 600 are used to derive very precise (+/- 0.01 dex) differential abundances, and stellar ages with internal errors less than 1 Gyr are obtained by interpolation in the logg - Teff diagram between isochrones calculated with the Aarhus Stellar Evolution Code. For stars younger than 6 Gyr, [X/Fe] is tightly correlated with stellar age for all elements. For ages between 6 and 9 Gyr, the [X/Fe] - age correlations break down and the stars split up into two groups having respectively high and low [X/Fe] for the odd-Z elements. It is concluded that while stars in the solar neighborhood younger than about 6 Gyr were formed from interstellar gas with a smooth chemical evolution, older stars have originated from regions enriched by supernovae with different neutron excesses. Furthermore, the correlations between abundance ratios and stellar age suggest that: i) Sc is made in Type II supernovae along with the alpha-capture elements, ii) the Type II to Ia SNe yield ratio is about the same for Mn and Fe, iii) Cu is mainly made by the weak s-process in massive stars, iv) the Ba/Y yield ratio for AGB stars increases with decreasing stellar mass, v) [Y/Mg] and [Y/Al] can be used as "chemical clocks" when determining ages of solar metallicity stars.

Motivation & Objective

• To extend the study of element abundance trends with stellar age to include Sc, Mn, Cu, and Ba in solar twins.
• To investigate the nucleosynthetic origins of these elements by analyzing their abundance ratios relative to Fe and other reference elements.
• To determine whether the observed abundance-age correlations reflect smooth Galactic chemical evolution or complex enrichment histories in older stars.
• To evaluate the potential of [Y/Mg] and [Y/Al] as chemical clocks for age determination in solar-metallicity stars.
• To explore the role of neutron excess in Type II supernovae and mass-dependent yields in AGB stars through observed abundance patterns.

Proposed method

• High-resolution HARPS spectra with signal-to-noise ratio >600 were used to derive differential abundances with a precision of ~0.01 dex.
• Stellar atmospheric parameters were determined using MARCS model atmospheres and iron line excitation/ionization balance.
• Stellar ages were derived via interpolation in the log g – T_eff diagram using isochrones from the Aarhus Stellar Evolution Code (ASTEC).
• Abundance ratios [X/Fe] were analyzed as functions of age, with attention to dependence on elemental condensation temperature (T_C).
• Linear fits to [X/Fe] vs. age were evaluated using reduced chi-squared (χ²_red ≈ 1) to assess goodness of fit.
• Comparisons were made with Yonsei-Yale isochrones to assess age scale consistency, accounting for metal diffusion effects in ASTEC models.

Experimental results

Research questions

• RQ1 How do the abundance ratios [Sc/Fe], [Mn/Fe], [Cu/Fe], and [Ba/Fe] vary with stellar age in solar twins?
• RQ2 Do the observed abundance trends with age support a smooth chemical evolution scenario for stars younger than 6 Gyr?
• RQ3 What do the abundance patterns of odd-Z elements (Na, Al, Sc, Cu) in older stars (6–9 Gyr) reveal about the chemical enrichment history of the Galactic disk?
• RQ4 Can [Y/Mg] and [Y/Al] serve as reliable chemical clocks for estimating stellar ages with ~1 Gyr precision?
• RQ5 How do the yields of Ba and Y from AGB stars depend on stellar mass, as inferred from abundance trends?

Key findings

• For stars younger than 6 Gyr, [X/Fe] ratios of Sc, Mn, Cu, and Ba show tight linear correlations with age, with reduced chi-squared values near 1, indicating high-fidelity age-abundance relations.
• The [Sc/Mg] ratio varies smoothly with age, suggesting Sc is produced in Type II supernovae alongside α-capture elements.
• The near-constant [Mn/Fe] with age implies that the Type Ia to Type II supernova yield ratio for Mn is similar to that for Fe.
• The small scatter in [Cu/Na] across all ages supports the hypothesis that Cu is primarily synthesized via the weak s-process in massive stars.
• The steeper increase of [Ba/Fe] with decreasing age compared to [Y/Fe] indicates that the Ba/Y yield ratio from AGB stars increases with decreasing stellar mass, consistent with recent theoretical predictions.
• The ratios [Y/Mg] and [Y/Al] show linear, age-dependent trends with slopes near -0.04 dex Gyr⁻¹, establishing them as robust chemical clocks for age estimation in solar-metallicity stars.