Chemically Self-Consistent Modeling of the Globular Cluster NGC 2808 and its Effects on the Inferred Helium Abundance of Multiple Stellar Populations

TL;DR

This study develops chemically self-consistent stellar models for globular cluster NGC 2808, fitting photometry with a new software tool to accurately infer helium abundances of multiple stellar populations, confirming previous findings.

Contribution

First chemically self-consistent models of NGC 2808 built with advanced atmospheres and opacities, and a novel fitting algorithm to determine stellar populations and their helium content.

Findings

Helium abundance of second-generation stars is 15±3% higher than first-generation.

Chemically self-consistent models do not significantly change inferred helium abundances.

Fidanka effectively determines the number of populations and fits isochrones to photometry.

Abstract

The helium abundances in the multiple populations that are now known to comprise all closely studied Milky Way globular clusters are often inferred by fitting isochrones generated from stellar evolutionary models to globular cluster photometry. It is therefore important to build stellar models that are chemically self-consistent in terms of their structure, atmosphere, and opacity. In this work we present the first chemically self-consistent stellar models of the Milky Way globular cluster NGC 2808 using MARCS model atmospheres, OPLIB high-temperature radiative opacities, and AESOPUS low-temperature radiative opacities. These stellar models were fit to the NGC 2808 photometry using Fidanka , a new software tool that was developed to optimally fit cluster photometry to isochrones and for population synthesis. Fidanka can determine, in a relatively unbiased way, the ideal number of distinct populations that exist within a dataset and then fit isochrones to each population. We achieve this outcome through a combination of Bayesian Gaussian Mixture Modeling and a novel number density estimation algorithm. Using Fidanka and F275W-F814W photometry from the Hubble UV Globular Cluster Survey we find that the helium abundance of the second generation of stars in NGC 2808 is higher than the first generation by $15\pm3\%$. This is in agreement with previous studies of NGC 2808. This work, along with previous work by Dotter et al. (2015) focused on NGC 6752, demonstrates that chemically self-consistent models of globular clusters do not significantly alter inferred helium abundances, and are therefore unlikely to be worth the significant additional time investment.