[Paper Review] A double stellar generation in the Globular Cluster NGC6656 (M 22). Two stellar groups with different iron and s-process element abundance

A. F. Marino, A. P. Milone+5 more|arXiv (Cornell University)|May 25, 2009

Stellar, planetary, and galactic studies2 references154 citations

TL;DR

This study identifies a double stellar population in the globular cluster M 22, with two groups of stars showing distinct iron and s-process element abundances. High-resolution UVES and lower-resolution GIRAFFE spectroscopy reveal a 0.14 dex iron spread correlated with s-process elements (Y, Zr, Ba), suggesting two generations of stars formed from chemically enriched material, with the second generation enriched by AGB winds and core-collapse supernovae ejecta.

ABSTRACT

AIMS. In this paper we present the chemical abundance analysis from high resolution UVES spectra of seventeen bright giant stars of the Globular Cluster M~22. RESULTS. We obtained an average iron abundance of [Fe/H]=-1.76\pm0.02 (internal errors only) and an αenhancement of 0.36\pm0.04 (internal errors only). Na and O, and Al and O follow the well known anti-correlation found in many other GCs. We identified two groups of stars with significantly different abundances of the s-process elements Y, Zr and Ba. The relative numbers of the two group members are very similar to the ratio of the stars in the two SGBs of M22 recently found by Piotto (2009). Y and Ba abundances do not correlate with Na, O and Al. The s-element rich stars are also richer in iron and have higher Ca abundances. The results from high resolution spectra have been further confirmed by lower resolution GIRAFFE spectra of fourteen additional M22 stars. GIRAFFE spectra show also that the Eu -- a pure r-process element -- abundance is not related to the iron content. We discuss the chemical abundance pattern of M22 stars in the context of the multiple stellar populations in GC scenario.

Motivation & Objective

• To investigate the chemical abundance patterns of bright giant stars in the globular cluster M 22 to understand its stellar population complexity.
• To determine whether iron and s-process element variations in M 22 are linked to multiple stellar generations.
• To test the hypothesis that the split subgiant branch (SGB) observed in M 22 is caused by a small but significant iron abundance spread correlated with s-process elements.
• To examine the role of AGB stars and supernovae in enriching the interstellar medium with s-process and iron-peak elements, respectively.

Proposed method

• High-resolution UVES spectroscopy was used to measure iron-peak, light, and s-process element abundances in 17 bright giant stars in M 22.
• Lower-resolution GIRAFFE spectra were obtained for 14 additional stars to confirm the UVES results and extend the sample.
• Abundance ratios were derived using synthetic spectral synthesis and model atmospheres, with careful attention to line list and atmospheric parameters.
• The correlation between s-process elements (Y, Zr, Ba) and iron abundance was quantified to assess the presence of a metallicity spread.
• The photometric SGB morphology from HST/ACS data was compared with the spectroscopic abundance patterns to link chemical differences to observed color-magnitude sequence splits.
• The role of r-process (Eu) and α-elements (Ca) was analyzed to distinguish between r- and s-process contributions and constrain nucleosynthetic origins.

Experimental results

Research questions

• RQ1 Is there a measurable spread in iron abundance among stars in M 22, and is it correlated with s-process element abundances?
• RQ2 Do the two distinct subgiant branch (SGB) sequences in M 22 correspond to two stellar populations with different chemical compositions?
• RQ3 Are the s-process element-rich stars also enriched in iron and calcium, and is this enrichment consistent with pollution from AGB stars and core-collapse supernovae?
• RQ4 Is the iron abundance in M 22 correlated with sodium or other light elements, or is it independent as expected for a primordial metallicity spread?
• RQ5 Can the observed SGB split in M 22 be explained by a small iron abundance difference of ~0.14 dex, as predicted by theoretical models?

Key findings

• The average iron abundance in M 22 is [Fe/H] = -1.76 ± 0.02 (UVES) and -1.75 ± 0.02 (weighted mean with GIRAFFE), confirming its metal-poor nature.
• A clear bimodal distribution in s-process elements (Y, Zr, Ba) was found, with no correlation to Na, O, or Al abundances.
• Stars enriched in s-process elements show a higher iron abundance by ∼0.14 dex, indicating a metallicity spread correlated with s-process content.
• The s-process-rich stars also exhibit enhanced calcium, magnesium, and silicon abundances, consistent with pollution by core-collapse supernovae.
• The r-process element europium (Eu) shows no correlation with iron, supporting the idea that iron enrichment is not due to r-process but likely from core-collapse supernovae.
• The observed SGB split in M 22 (0.17 mag separation in F606W) is consistent with a 0.14 dex iron spread, and the faint SGB is likely populated by s-process-rich, more metal-rich stars.