Chemically Self-consistent Modeling of the Globular Cluster NGC 2808 and its Effects on the Inferred Helium Abundance of Multiple Stellar Populations
Year: 2025
Type: article
Abstract: The helium abundances in the multiple populations that are now known to comprise all closely studied Milky Way globular clusters (GCs) are often inferred by fitting isochrones generated from stellar evolutionary models to GC 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 GC 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 data set 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% ± 3%. This is in agreement with previous studies of NGC 2808. This work, along with previous work by A. Dotter et al. focused on NGC 6752, demonstrates that chemically self-consistent models of GCs do not significantly alter inferred helium abundances and are therefore unlikely to be worth the significant additional time investment.
Source: The Astrophysical Journal
Authors: Emily M. Boudreaux, Brian C. Chaboyer, Amanda Ash, Renata Edaes Hoh, Gregory Feiden
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $4,499
Awards: G3725477825, IN THE LAST DECADE PHOTOMETRIC OBSERVATIONS BY THE HUBBLE SPACE TELESCOPE HAVE REVEALED THAT NEARLY ALL GLOBULAR CLUSTERS CONTAIN MULTIPLE POPULATIONS OF STARS. STELLAR MODELS AND ISCOHRONES HAVE BEEN USED TO INFER THE PROPERTIES OF THE DIFFERENT POPULATIONS WITH THE GENERAL CONSENSUS BEING THAT THE POPULATIONS DIFFER IN THEIR HELIUM AND LIGHT ELEMENT CONTENT. HOWEVER THE STELLAR MODELS WHICH ARE USED TO INTERPRET THE DATA DO NOT SELF CONSISTENTLY TAKE INTO ACCOUNT THE EFFECTS OF ENHANCED HELIUM ABUNDANCES ON THE STELLAR ATMOSPHERE BOUNDARY CONDITIONS AND WERE CREATED USING A PRESCRIPTION FOR CONVECTION WHICH IS SUITABLE FOR THE SUN BUT NOT FOR THE METAL-POOR STARS FOUND IN GLOBULAR CLUSTERS. WE PROPOSE TO COMBINE NEW MARCS MODELS ATMOSPHERES WITH THE DARTMOUTH STELLAR EVOLUTION CODE TO SELF-CONSISTENTLY MODEL THE MULTIPLE POPULATIONS IN GLOBULAR CLUSTERS. THESE MODELS WILL BE CREATED WITH A PRESCRIPTION FOR CONVECTION WHICH IS SUITABLE FOR METALPOOR STARS AND OUR NEW MODELS WILL PROVIDE INDEPENDENT ESTIMATES OF THE DEGREE OF HELIUM ENHANCEMENT IN THE MULTIPLE POPULATIONS OF A NUMBER OF GLOBULAR CLUSTERS A KEY CONSTRAINT ON FORMATION SCENARIOS FOR GLOBULAR CLUSTERS. FINALLY WE WILL ADOPT A MONTE CARLO APPROACH WHICH TAKES INTO ACCOUNT THE VARIOUS UNCERTAINTIES IN STELLAR MODELS (FOR EXAMPLE THE OPACITIES NUCLEAR REACTION RATES AND TREATMENT OF CONVECTION) TO DETERMINE THE UNCERTAINTY IN THE PREDICTED PROPERTIES OF THIS NEW GENERATION OF SELF-CONSISTENT STELLAR MODELS. THESE NEW MODELS WILL PROVIDE KEY CONSTRAINTS FOR MODELS OF THE FORMATION OF GLOBULAR CLUSTERS WHICH WERE AMONG THE FIRST MASSIVE OBJECTS FORMED IN THE UNIVERSE., G7531162841
OpenAlex ID: W4407405337