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

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 (ρ 10 8 g/cm 3 ), the acceptable conditions range from ≈ 100 MK at ≈ 10 8 g/cm 3 to ≈ 200 MK at ≈ 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 firstgeneration 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.

Sensitivity to Thermonuclear Reaction Rates in Modeling the Abundance Anomalies of NGC 2419

“…We also consider how this affects which potential polluter sites (e.g., classical novae, AGB stars, super AGB stars) are thought to reproduce the observed abundances. A complete discussion of these candidates is given in Iliadis et al (2016). Their findings are summarized in Section 1.…”

“…In this work, we adopt the methodology of Iliadis et al (2016) to explore possible self-enrichment scenarios in NGC 2419. For a thorough exposition, we refer the reader to that work.…”

“…The elemental abundances of X mix (i.e., Mg, K, Ca, Ti, Si, Sc, and V) are then compared with those of the observed, extreme population. For these stars, we adopt the same abundance conditions as Iliadis et al (2016), which is based on the scatter in the data and abundance uncertainties of individual stars. These are given by 1.3…”

“…They found that the chemical abundances of the enriched material could be made to agree with the observations provided both model parameters and thermonuclear reaction rates were fine-tuned. Later, Iliadis et al (2016) broadened the scope of this investigation. They used Monte Carlo nuclear reaction network calculations to constrain the physical conditions that could give rise to the observed abundances.…”

“…Therefore, reaction rate uncertainties must be considered for the most robust predictions. Iliadis et al (2016) already demonstrated that these uncertainties affect the range of plausible polluter sites. They found that the allowed region of the temperature-density parameter space widens considerably when the impact of reaction rate uncertainties are considered.…”