The influence of the C+N+O abundances on the determination of the relative ages of Globular Clusters: the case of NGC 1851 and NGC 6121 (M4)

Abstract

The color magnitude diagram (CMD) of NGC 1851 presents two subgiant branches (SGB), probably due the presence of two populations differing in total CNO content. We test the idea that a difference in total CNO may simulate an age difference when comparing the CMD of clusters to derive relative ages. We compare NGC 1851 with NGC 6121 (M4), a cluster of very similar [Fe/H]. We find that, with a suitable shift of the CMDs that brings the two red horizontal branches at the same magnitude level, the unevolved main sequence and red giant branch match, but the SGB of NGC 6121 and its red giant branch "bump" are fainter than in NGC 1851. In particular, the SGB of NGC 6121 is even slightly fainter than the the faint SGB in NGC 1851. Both these features can be explained if the total CNO in NGC 6121 is larger than that in NGC 1851, even if the two clusters are coeval. We conclude by warning that different initial C+N+O abundances between two clusters, otherwise similar in metallicity and age, may lead to differences in the turnoff morphology that can be easily attributed to an age difference.

accepted for publication in MNRAS

References in corpus (12)

• Formation and Dynamical Evolution of Multiple Stellar Generations in Globular Clusters
• The ACS Survey of Galactic Globular Clusters. I. Overview and Clusters Without Previous HST Photometry
• The ACS Survey of Galactic Globular Clusters. III. The Double Subgiant Branch of NGC 1851
• Rotational mixing in low-mass stars II. Self-consistent models of Pop II RGB stars
• The double Subgiant Branch of NGC 1851: the role of the CNO abundance
• An abundance analysis of bright giants in the globular cluster NGC 1851
• A large C+N+O abundance spread in giant stars of the globular cluster NGC 1851
• Origin of Abundance Inhomogeneity in Globular Clusters
• Heavy element abundances in giant stars of the globular clusters M4 and M5
• The Radial Distribution of the Two Stellar Populations in NGC 1851
• Rubidium and lead abundances in giant stars of the globular clusters M4 and M5
• Properties of second generation stars in Globular Clusters

Cited by in corpus (18)

• The second and third parameters of the Horizontal Branch in Globular Clusters
• Abundance patterns of multiple populations in Globular Clusters: a chemical evolution model based on yields from AGB ejecta
• Sodium-Oxygen anticorrelation among Horizontal Branch Stars in the Globular Cluster M4
• The C+N+O abundance of Omega Centauri giant stars: implications on the chemical enrichment scenario and the relative ages of different stellar populations
• CNO abundances in the globular clusters NGC 1851 and NGC 6752
• The double sub-giant branch of NGC 6656 (M22): a chemical characterization
• Detection of solar-like oscillations in relics of the Milky Way: asteroseismology of K giants in M4 using data from the NASA K2 mission
• Star Formation Histories of Globular Clusters with Multiple Populations. I. ω Cen, M22, and NGC 1851
• Constraining stellar population models - I. Age, metallicity, and abundance pattern compilation for Galactic globular clusters
• C and N abundances of MS and SGB stars in NGC 1851
• The oxygen vs. sodium (anti)correlation(s) in omega Cen
• Atmospheric Parameters and Metallicities for 2191 stars in the Globular Cluster M4
• Multichromatic colour-magnitude diagrams of the globular cluster NGC 6366
• The Horizontal Branch of NGC 1851: Constraints from its RR Lyrae Variables
• The impact of enhanced He and CNONa abundances on globular cluster relative age-dating methods
• The M4 Core Project with HST - IV. Internal Kinematics from Accurate Radial Velocities of 2771 Cluster Members
• Chemical evolution of star clusters
• Globular Cluster Abundances and What They Can Tell Us About Galaxy Formation