Chemical Abundances for 855 Giants in the Globular Cluster Omega Centauri (NGC 5139)

Abstract: We present elemental abundances for 855 red giant branch (RGB) stars in the globular cluster Omega Centauri (w Cen) from spectra obtained with the Blanco 4m telescope and Hydra multifiber spectrograph. The sample includes nearly all RGB stars brighter than V=13.5, and span's w Cen's full metallicity range. The heavy alpha elements (Si, Ca, and Ti) are generally enhanced by ~+0.3 dex, and exhibit a metallicity dependent morphology that may be attributed to mass and metallicity dependent Type II supernova (SN) yields. The heavy alpha and Fe-peak abundances suggest minimal contributions from Type Ia SNe. The light elements (O, Na, and Al) exhibit >0.5 dex abundance dispersions at all metallicities, and a majority of stars with [Fe/H]>-1.6 have [O/Fe], [Na/Fe], and [Al/Fe] abundances similar to those in monometallic globular clusters, as well as O-Na, O-Al anticorrelations and the Na-Al correlation in all but the most metal-rich stars. A combination of pollution from intermediate mass asymptotic giant branch (AGB) stars and in situ mixing may explain the light element abundance patterns. A large fraction (27%) of w Cen stars are O-poor ([O/Fe]<0) and are preferentially located within 5-10' of the cluster center. The O-poor giants are spatially similar, located in the same metallicity range, and are present in nearly equal proportions to blue main sequence stars. This suggests the O-poor giants and blue main sequence stars may share a common origin. [La/Fe] increases sharply at [Fe/H]>-1.6, and the [La/Eu] ratios indicate the increase is due to almost pure s-process production.

• The paper identifies distinct stellar populations with five metallicity peaks ranging from [Fe/H] -1.75 to -0.75 in Omega Centauri.
• The researchers use high-resolution Hydra spectra to uncover significant light element anticorrelations indicative of proton-capture nucleosynthesis.
• The study highlights the combined influences of Type II supernovae and AGB stars in driving the cluster’s complex chemical evolution.

Overview of Chemical Abundances in Omega Centauri

The study titled "Chemical Abundances for 855 Giants in the Globular Cluster Omega Centauri (NGC 5139)" conducted by Johnson et al. provides a comprehensive analysis of the elemental abundances in red giant branch (RGB) stars within the Omega Centauri globular cluster. Utilizing high-resolution spectra obtained from the Blanco 4m telescope equipped with the Hydra multifiber spectrograph, the research encompasses data from 855 RGB stars that cover Omega Centauri’s full metallicity range.

The paper extensively investigates various elemental abundances, particularly examining light elements (O, Na, Al) and heavy elements (Si, Ca, Ti), alongside neutron-capture elements (La, Eu). The study reveals intricate abundance patterns suggesting a complex chemical evolution within the cluster, influenced by both supernovae and asymptotic giant branch (AGB) stars.

Key Numerical Results and Findings

• Omega Centauri's Metallicity Distribution: The study identifies five peaks within the metallicity distribution function at [Fe/H] values of -1.75, -1.50, -1.15, -1.05, and -0.75. These peaks correspond to distinct stellar populations. Approximately 61% of the stars are in the RGB-Metal-Poor (RGB-MP) population with [Fe/H] ≤ -1.6.
• Light Element Variations: Significant [O/Fe], [Na/Fe], and [Al/Fe] variations are present, indicative of proton-capture nucleosynthesis activity. Notably, an O--Na and O--Al anticorrelation, along with a Na--Al correlation, is observed in stars with [Fe/H] ≤ -1. This suggests substantial pollution from intermediate mass AGB stars.
• Heavy α and Fe-peak Elements: Heavy elements such as Si, Ca, and Ti generally exhibit +0.3 dex enhancement across varying metallicities. The uniformity of elements like Sc and Ni suggests a dominant Type II supernovae influence, with minimal Type Ia supernova contribution.
• Neutron-Capture Elements and S-process Enhancement: At [Fe/H] ≳ -1.5, there is a notable increase in [La/Fe] due to s-process production. The [La/Eu] ratio indicates a significant contribution from low to intermediate mass stars, unlike the typical trend in globular clusters.

Implications and Speculative Future Directions

The abundance patterns in Omega Centauri provide insights into the cluster's complex star formation and chemical enrichment history. The findings support a scenario involving multiple generations of star formation, each contributing differently due to their unique nucleosynthetic outputs.

A critical inquiry is the origin of the observed abundance distributions. The data imply a combination of contributions from both Type II supernovae and AGB stars, highlighting distinct self-enrichment episodes. The study's identification of a significant s-process contribution at intermediate metallicities underlines the role of AGB stars as key contributors to the s-process elements.

Future research could explore isotopic ratios, particularly for elements like magnesium, to further explore the mechanism behind light element enhancements. Investigations into radial distributions of enriched material and comparisons of chemical signatures between Omega Centauri and other massive globular clusters or satellite galaxies could yield valuable evolutionary insights. Additionally, high-resolution observations of fainter stars could provide more data on the extent of chemical self-pollution and internal mixing processes.

In conclusion, Johnson et al. have @@@@1@@@@ our comprehension of Omega Centauri's complex chemical landscape, advancing our understanding of the evolutionary processes in massive globular clusters.