[Paper Review] Two distinct halo populations in the solar neighborhood. II. Evidence from stellar abundances of Mn, Cu, Zn, Y, and Ba

P. E. Nissen, W. J. Schuster|Mar 24, 2011

Stellar, planetary, and galactic studies72 references3 citations

TL;DR

This study analyzes stellar abundances of Mn, Cu, Zn, Y, and Ba in 94 solar neighborhood halo stars to investigate chemical differences between two previously identified halo populations—high- and low-α stars. Using high-resolution spectroscopy and differential abundance analysis, it finds systematic differences in [Cu/Fe], [Zn/Fe], and [Ba/Y], indicating distinct chemical enrichment histories: high-α stars formed in high-star-formation-rate environments dominated by Type II SNe, while low-α stars originated in systems with slower evolution and contributions from Type Ia SNe and AGB stars.

ABSTRACT

A previous study (Nissen & Schuster 2010) of 94 dwarf stars with -1.6 < [Fe/H] < -0.4 has revealed the existence of two distinct halo populations with a systematic difference in [alpha/Fe] at a given metallicity. In continuation of that work, abundances of Mn, Cu, Zn, Y, and Ba are determined for the same sample of stars. Equivalent widths of atomic lines are measured from high resolution VLT/UVES and NOT/FIES spectra and used to derive precise abundance ratios from an LTE analysis based on MARCS model atmospheres. Systematic differences between the `high-alpha' and `low-alpha' halo populations are found for [Cu/Fe], [Zn/Fe], and [Ba/Y], whereas there is no significant difference in the case of [Mn/Fe]. At a given metallicity, [Cu/Fe] shows a large scatter that is closely correlated with a corresponding scatter in [Na/Fe] and [Ni/Fe]. The metallicity trends of [Cu/Fe], [Zn/Fe], and [Ba/Y] can be explained from existing nucleosynthesis calculations if the high-alpha stars formed in regions with such a high star formation rate that only massive stars and Type II supernovae contributed to the chemical enrichment. The low-alpha stars, on the other hand, most likely originate from systems with a slower chemical evolution, characterized by additional enrichment from Type Ia supernovae and low-mass AGB stars.

Motivation & Objective

• To investigate whether the two distinct halo populations—high- and low-α stars—identified by [α/Fe] differences also differ in other abundance ratios.
• To determine if abundance ratios of Mn, Cu, Zn, Y, and Ba can serve as chemical tracers of different star formation histories and enrichment mechanisms.
• To test whether the observed abundance trends can be explained by existing nucleosynthesis models for Type II and Type Ia supernovae and AGB stars.
• To explore the origin of the low-α population by comparing its abundance patterns with those of present-day dSph galaxies and the ω Cen globular cluster.
• To assess whether the lack of difference in [Mn/Fe] between the two populations challenges current models of chemical evolution.

Proposed method

• High-resolution VLT/UVES and NOT/FIES spectra were used to measure equivalent widths of atomic lines in 94 F, G, and K dwarf stars with -1.6 < [Fe/H] < -0.4.
• Abundance ratios were derived using LTE analysis based on MARCS model atmospheres.
• Differential abundance analysis was performed relative to two thick-disk stars (HD 22879 and HD 76932) to achieve high precision.
• The analysis focused on [Mn/Fe], [Cu/Fe], [Zn/Fe], [Y/Fe], and [Ba/Fe], with [Ba/Y] used as a proxy for neutron-capture element evolution.
• Chemical evolution models were compared to observed trends, particularly assessing contributions from Type II SNe, Type Ia SNe, and low-mass AGB stars.
• Abundance patterns were compared with those in present-day dSph galaxies and the ω Cen globular cluster to evaluate accretion scenarios.

Experimental results

Research questions

• RQ1 Do the high- and low-α halo populations exhibit systematic differences in abundance ratios beyond [α/Fe], particularly for Cu, Zn, Y, and Ba?
• RQ2 Can the observed abundance trends in [Cu/Fe], [Zn/Fe], and [Ba/Y] be explained by nucleosynthesis models involving different contributions from Type II and Type Ia supernovae?
• RQ3 Why is there no significant difference in [Mn/Fe] between the two populations despite their distinct chemical histories?
• RQ4 Do the abundance patterns of the low-α stars resemble those of present-day dSph galaxies or the ω Cen globular cluster, suggesting a common origin?
• RQ5 What role do AGB stars and delayed enrichment from SNe Ia play in shaping the chemical evolution of the low-α population?

Key findings

• The high-α and low-α halo populations show significant differences in [Cu/Fe], [Zn/Fe], and [Ba/Y], with the former having higher ratios at a given [Fe/H].
• The [Cu/Fe] ratio exhibits a large scatter that correlates strongly with scatter in [Na/Fe] and [Ni/Fe], suggesting a link to star formation rate and enrichment processes.
• The abundance trends in [Cu/Fe], [Zn/Fe], and [Ba/Y] are consistent with nucleosynthesis models if high-α stars formed in high-star-formation-rate environments dominated by massive stars and Type II SNe.
• The low-α stars likely originated in systems with slower chemical evolution, where delayed enrichment from Type Ia SNe and low-mass AGB stars contributed significantly.
• The lack of difference in [Mn/Fe] between the two populations challenges current models, as Mn is primarily produced in Type II SNe and should show a clear distinction if SFR differences were the main driver.
• The abundance patterns of the low-α stars do not exactly match those of present-day dSph galaxies or ω Cen, though [Ni/Fe] and [Cu/Fe] show partial overlap, suggesting a more complex origin involving differential mass loss or selective retention of AGB products.