On Potassium and Other Abundance Anomalies of Red Giants in NGC 2419

TL;DR

This study uses Monte Carlo nuclear reaction network calculations to identify the temperature-density conditions that explain the observed elemental abundance anomalies, including Mg-K anticorrelation, in the globular cluster NGC 2419, ruling out several stellar sources.

Contribution

It constrains the stellar conditions responsible for abundance anomalies in NGC 2419 and evaluates potential polluter stars, highlighting super-AGB stars and novae as plausible candidates.

Findings

Identifies temperature-density conditions matching observed abundances in NGC 2419.

Rules out low-mass, AGB, massive, and supermassive stars as primary polluters.

Suggests super-AGB stars and novae as potential polluters under certain conditions.

Abstract

Globular clusters are of paramount importance for testing theories of stellar evolution and early galaxy formation. Strong evidence for multiple populations of stars in globular clusters derives from observed abundance anomalies. A puzzling example is the recently detected Mg-K anticorrelation in NGC 2419. We perform Monte Carlo nuclear reaction network calculations to constrain the temperature-density conditions that gave rise to the elemental abundances observed in this elusive cluster. We find a correlation between stellar temperature and density values that provide a satisfactory match between simulated and observed abundances in NGC 2419 for all relevant elements (Mg, Si, K, Ca, Sc, Ti, and V). Except at the highest densities ($\rho \gtrsim 10^8$~g/cm$^3$), the acceptable conditions range from $\approx$ $100$~MK at $\approx$ $10^8$~g/cm$^3$ to $\approx$ $200$~MK at $\approx$ $10^{-4}$~g/cm$^3$. This result accounts for uncertainties in nuclear reaction rates and variations in the assumed initial composition. We review hydrogen burning sites and find that low-mass stars, AGB stars, massive stars, or supermassive stars cannot account for the observed abundance anomalies in NGC 2419. Super-AGB stars could be viable candidates for the polluter stars if stellar model parameters can be fine-tuned to produce higher temperatures. Novae, either involving CO or ONe white dwarfs, could be interesting polluter candidates, but a current lack of low-metallicity nova models precludes firmer conclusions. We also discuss if additional constraints for the first-generation polluters can be obtained by future measurements of oxygen, or by evolving models of second-generation low-mass stars with a non-canonical initial composition.