[Paper Review] Gravonuclear Instabilities in Post-Horizontal-Branch Stars

A. V. Sweigart, John C. Lattanzio+2 more|arXiv (Cornell University)|Sep 23, 1999

Stellar, planetary, and galactic studies3 citations

TL;DR

This paper investigates gravonuclear instabilities in post-horizontal-branch stars, finding they arise not from intrinsic stellar physics but from artificial suppression methods used in models to quench breathing pulses. When central helium abundance is fixed, a sharp helium discontinuity forms, enabling thin-shell thermal instability; however, once nuclear burning broadens the shell into an S-shape, the instability vanishes, indicating the phenomenon is a numerical artifact of model assumptions rather than a physical reality.

ABSTRACT

We investigate the gravonuclear instabilities reported by Bono et al. (1997a,b) during the onset of helium-shell burning at the end of the horizontal-branch (HB) phase. These instabilities are characterized by relaxation oscillations within the helium shell which lead to loops in the evolutionary tracks. We find the occurrence of these instabilities depends critically on how the breathing pulses are suppressed near the end of the HB phase. If they are suppressed by omitting the gravitational energy term in the stellar structure equations, then the helium profile within the core at the end of the HB phase will contain a broad region of varying helium abundance. The helium-burning shell which forms in this region is too thick to be unstable, and gravonuclear instabilities do not occur. If, on the other hand, the breathing pulses are suppressed by prohibiting any increase in the central helium abundance, then the final helium profile can exhibit a large discontinuity at the edge of the helium-exhausted core. The helium shell which forms just exterior to this discontinuity is then much thinner and can be thermally unstable. Even in this case, however, the gravonuclear instabilities disappear as soon as the nuclear burning broadens the helium shell into its characteristic S-shape. We conclude that the gravonuclear instabilities found by Bono et al. are a consequence of the ad hoc procedure used to suppress the breathing pulses.

Motivation & Objective

• To examine the origin of gravonuclear instabilities reported by Bono et al. (1997) in post-horizontal-branch stars.
• To determine whether these instabilities are physically real or artifacts of modeling choices in stellar structure equations.
• To assess how different methods of suppressing breathing pulses affect helium profile formation and shell stability.
• To evaluate the role of helium abundance gradients and shell thickness in triggering thermal instabilities.

Proposed method

• Modeling post-horizontal-branch stellar evolution using stellar structure equations with and without the gravitational energy term.
• Comparing two suppression techniques: omitting the gravitational energy term and fixing the central helium abundance.
• Tracking the evolution of helium abundance profiles at the end of the horizontal branch phase.
• Analyzing the thickness and stability of the helium-burning shell formed in each scenario.
• Assessing whether relaxation oscillations and looped evolutionary tracks emerge under different conditions.
• Evaluating the transition from thin-shell instability to broad S-shaped shells as a function of nuclear burning.

Experimental results

Research questions

• RQ1 Do gravonuclear instabilities in post-HB stars arise from physical processes or modeling artifacts?
• RQ2 How does suppressing breathing pulses via omission of the gravitational energy term affect helium profile formation?
• RQ3 What happens to shell stability when the central helium abundance is artificially held constant?
• RQ4 Under what conditions does a thin helium shell become thermally unstable?
• RQ5 Why do gravonuclear instabilities disappear once the helium shell broadens into an S-shape?

Key findings

• Gravonuclear instabilities are not physically real but are numerical artifacts resulting from the ad hoc suppression of breathing pulses in stellar models.
• When the gravitational energy term is omitted, the helium profile develops a broad, varying region, preventing the formation of a thin, unstable shell.
• Fixing the central helium abundance leads to a sharp discontinuity at the core edge, creating a thin helium shell prone to thermal instability.
• However, as nuclear burning broadens the shell into a characteristic S-shape, the instability vanishes, even if the shell remains thin.
• The occurrence of instabilities is thus contingent on model-specific assumptions rather than intrinsic stellar behavior.
• The study concludes that the reported instabilities are not robust physical phenomena but are instead consequences of inconsistent or artificial modeling procedures.