An Improved Bandstrength Index for the CH G Band of Globular Cluster Giants

TL;DR

This paper introduces a new spectral index, S2(CH), optimized for measuring carbon abundance in globular cluster giants by minimizing nitrogen dependence and maximizing carbon sensitivity, based on synthetic spectra.

Contribution

The paper presents a novel, optimized G-band spectral index, S2(CH), designed for more accurate [C/Fe] measurements in globular cluster giants, improving upon previous indices.

Findings

S2(CH) effectively measures [C/Fe] in bright globular cluster giants.

The index minimizes dependence on [N/Fe], reducing confounding effects.

Valid across a wide range of metallicities and stellar parameters.

Abstract

Spectral indices are useful tools for quantifying the strengths of features in moderate-resolution spectra and relating them to intrinsic stellar parameters. This paper focuses on the 4300AA CH G-band, a classic example of a feature interpreted through use of spectral indices. G-band index definitions, as applied to globular clusters of different metallicity, abound in the literature, and transformations between the various systems, or comparisons between different authors' work, are difficult and not always useful. We present a method for formulating an optimized G-band index, using a large grid of synthetic spectra. To make our new index a reliable measure of carbon abundance, we minimize its dependence on [N/Fe] and simultaneously maximize its sensitivity to [C/Fe]. We present a definition for the new index S2(CH), along with estimates of the errors inherent in using it for [C/Fe] determination, and conclude that it is valid for use with spectra of bright globular cluster red giants over a large range in [Fe/H], [C/Fe], and [N/Fe].