Light-Element Abundance Variations at Low Metallicity: the Globular Cluster NGC 5466

TL;DR

This study analyzes light-element abundance variations in low-metallicity globular cluster NGC 5466 by measuring CN, CH, and carbon abundances in 67 stars, revealing internal mixing effects with minimal primordial variation.

Contribution

First low-resolution spectral analysis of light-element variations in NGC 5466, demonstrating internal mixing effects and minimal primordial abundance differences.

Findings

Carbon abundance declines with luminosity on the giant branch.

CN band strength shows limited star-to-star variation.

Evidence of ongoing internal mixing in cluster stars.

Abstract

We present low-resolution (R~850) spectra for 67 asymptotic giant branch (AGB), horizontal branch and red giant branch (RGB) stars in the low-metallicity globular cluster NGC 5466, taken with the VIRUS-P integral-field spectrograph at the 2.7-m Harlan J. Smith telescope at McDonald Observatory. Sixty-six stars are confirmed, and one rejected, as cluster members based on radial velocity, which we measure to an accuracy of 16 km s-1 via template-matching techniques. CN and CH band strengths have been measured for 29 RGB and AGB stars in NGC 5466, and the band strength indices measured from VIRUS-P data show close agreement with those measured from Keck/LRIS spectra previously taken of five of our target stars. We also determine carbon abundances from comparisons with synthetic spectra. The RGB stars in our data set cover a range in absolute V magnitude from +2 to -3, which permits us to study the rate of carbon depletion on the giant branch as well as the point of its onset. The data show a clear decline in carbon abundance with rising luminosity above the luminosity function "bump" on the giant branch, and also a subdued range in CN band strength, suggesting ongoing internal mixing in individual stars but minor or no primordial star-to-star variation in light-element abundances.