[Paper Review] One size does not fit all: Evidence for a range of mixing efficiencies in stellar evolution calculations

C. Johnston|arXiv (Cornell University)|Jan 1, 2021

Stellar, planetary, and galactic studiesPhysics and Astronomy114 references51 citations

TL;DR

This paper demonstrates that stellar evolution models consistently underpredict convective core masses in intermediate- and high-mass stars when compared to high-precision observations from eclipsing binaries and asteroseismology. Using core mass and hydrogen content inferences from multiple independent studies, the authors show that models require significantly enhanced mixing efficiency—beyond standard prescriptions—to match observations, implying a range of effective mixing mechanisms rather than a single universal value.

ABSTRACT

Contains fulltext : 239748.pdf (Publisher’s version ) (Open Access)

Motivation & Objective

• To address the long-standing uncertainty in stellar evolution models regarding internal chemical mixing in intermediate- and high-mass stars.
• To investigate why standard 1D stellar models fail to reproduce observed convective core masses in stars with well-developed cores.
• To evaluate whether current observational constraints on mixing efficiency are consistent across different stellar systems.
• To argue that a single 'one-size-fits-all' mixing efficiency cannot explain the full range of observed core masses.
• To advocate for a range of mixing efficiencies in models to match high-precision observational data.

Proposed method

• Gathered convective core mass and fractional core hydrogen content from 41 independent studies of eclipsing binaries and asteroseismic systems.
• Compiled observational inferences from diverse sources including spectroscopy, light curves, radial velocity curves, and asteroseismic period spacing.
• Compared these observational inferences directly to stellar evolution models computed with the MESA code using standard and modified mixing prescriptions.
• Used a consistent framework to assess model predictions against observed core masses and core hydrogen fractions across a wide mass range (1.1–24 M⊙).
• Evaluated the impact of different mixing mechanisms (e.g., overshooting, penetration, rotational mixing) by comparing model outcomes to observations.
• Assessed the statistical consistency of inferred core masses across multiple independent systems to identify systematic model underprediction.

Experimental results

Research questions

• RQ1 Why do standard stellar evolution models systematically underpredict the masses of convective cores in intermediate- and high-mass stars?
• RQ2 To what extent can current observational data constrain the efficiency of chemical mixing at convective boundaries?
• RQ3 Can a single, universal mixing efficiency parameter adequately explain the observed range of convective core masses across diverse stellar systems?
• RQ4 How do observational constraints from eclipsing binaries and asteroseismology compare in their sensitivity to core mass and mixing efficiency?
• RQ5 What are the implications of a range of inferred mixing efficiencies for post-main-sequence evolution and stellar population modeling?

Key findings

• Convective core masses inferred from observations are consistently more massive than those predicted by standard 1D stellar evolution models without or with minimal convective boundary mixing.
• The discrepancy between model predictions and observations is robust across multiple independent systems, including eclipsing binaries and asteroseismic stars with masses ranging from 1.2 to 24 M⊙.
• For stars like KIC 12009504 (1.2 M⊙), observed core mass (0.08 M⊙) exceeds model predictions by ~30–50% when no extra mixing is included.
• In high-mass systems such as V578 Mon (14.55 M⊙), the observed core mass (5.12 M⊙) is significantly larger than the 3.5–4.0 M⊙ predicted by models without efficient mixing.
• The fractional core hydrogen content (Xc/Xini) inferred from observations is systematically lower than model predictions, indicating enhanced mixing and core mass growth.
• The data collectively imply that a single, universal mixing efficiency parameter cannot explain the full range of observed core masses, necessitating a distribution of mixing efficiencies across stellar populations.