The treatment of mixing in core helium burning models -- III. Suppressing core breathing pulses with a new constraint on overshoot

Abstract

Theoretical predictions for the core helium burning phase of stellar evolution are highly sensitive to the uncertain treatment of mixing at convective boundaries. In the last few years, interest in constraining the uncertain structure of their deep interiors has been renewed by insights from asteroseismology. Recently, Spruit (2015) proposed a limit for the rate of growth of helium-burning convective cores based on the higher buoyancy of material ingested from outside the convective core. In this paper we test the implications of such a limit for stellar models with a range of initial mass and metallicity. We find that the constraint on mixing beyond the Schwarzschild boundary has a significant effect on the evolution late in core helium burning, when core breathing pulses occur and the ingestion rate of helium is fastest. Ordinarily, core breathing pulses prolong the core helium burning lifetime to such an extent that models are at odds with observations of globular cluster populations. Across a wide range of initial stellar masses ($0.83\le M/{\text{M}}_{\odot }\le 5$), applying the Spruit constraint reduces the core helium burning lifetime because core breathing pulses are either avoided or their number and severity reduced. The constraint suggested by Spruit therefore helps to resolve significant discrepancies between observations and theoretical predictions. Specifically, we find improved agreement for $R_2$, the observed ratio of asymptotic giant branch to horizontal branch stars in globular clusters; the luminosity difference between these two groups; and in asteroseismology, the mixed-mode period spacing detected in red clump stars in the \textit{Kepler} field.

Accepted for publication in MNRAS; 11 pages, 6 figures

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• Impact of Convective Boundary Mixing on the TP-AGB
• Evidence of structural discontinuities in the inner core of red-giant stars
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• Bridging the Gap between Intermediate and Massive Stars I: Validation of MESA against the State-of-the-Art Monash Stellar Evolution Program for a 2$M_{\odot }$ AGB Star
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• Period Change Rates of Large Magellanic Cloud Cepheids using MESA
• Evolutionary and hydrodynamic models of short--period Cepheids
• Advancing Globular Cluster Constraints on the Axion-Photon Coupling
• One size does not fit all: Evidence for a range of mixing efficiencies in stellar evolution calculations
• Ensemble seismic study of the properties of the core of Red Clump stars
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