Medium-resolution Spectroscopy of Red Giant Branch Stars in $\omega$ Centauri

TL;DR

This study analyzes the chemical composition of approximately 600 red giant stars in $ ext{ω}$ Centauri using medium-resolution spectroscopy, revealing insights into their metallicity, calcium abundance, and cluster chemical evolution.

Contribution

It provides the first large-scale medium-resolution spectroscopic survey of RGB stars in $ ext{ω}$ Centauri, highlighting limitations of fiber spectroscopy in crowded regions and challenging previous abundance trends.

Findings

Fewer intermediate-metallicity stars than previous high-res surveys.

Calcium abundance [Ca/Fe] remains consistent across most metallicities.

No clear evidence of a radial metallicity gradient.

Abstract

We present [Fe/H] and [Ca/Fe] of $\sim600$ red giant branch (RGB) members of the globular cluster $\omega$ Centauri. We collect medium-resolution ($R\sim2000$) spectra using the Blanco 4 m telescope at the Cerro Tololo Inter-American Observatory equipped with Hydra, the fiber-fed multi-object spectrograph. We demonstrate that blending of stellar light in optical fibers severely limits the accuracy of spectroscopic parameters in the crowded central region of the cluster. When photometric temperatures are taken in the spectroscopic analysis, our kinematically selected cluster members, excluding those that are strongly affected by flux from neighboring stars, include relatively fewer stars at intermediate metallicity ([Fe/H]$\sim-1.5$) than seen in the previous high-resolution survey for brighter giants in Johnson & Pilachowski. As opposed to the trend of increasing [Ca/Fe] with [Fe/H] found by those authors, our [Ca/Fe] estimates, based on Ca II H & K measurements, show essentially the same mean [Ca/Fe] for most of the metal-poor and metal-intermediate populations in this cluster, suggesting that mass- or metallicity-dependent SN II yields may not be necessary in their proposed chemical evolution scenario. Metal-rich cluster members in our sample show a large spread in [Ca/Fe], and do not exhibit a clear bimodal distribution in [Ca/Fe]. We also do not find convincing evidence for a radial metallicity gradient among RGB stars in $\omega$ Centauri.