A Propos Crossing the Hertzsprung Gap

TL;DR

This paper investigates the rapid evolution of intermediate-mass and massive stars after hydrogen core-burning, proposing that a nuclear timescale, rather than thermal instability, explains the Hertzsprung gap phenomenon observed in star clusters.

Contribution

It challenges the traditional view that thermal instability causes the Hertzsprung gap, suggesting instead that a nuclear timescale suffices to explain the rapid stellar evolution.

Findings

The Hertzsprung gap can be explained without invoking thermal instability.

A suitably defined nuclear timescale accounts for the rapid evolution.

Counterexamples show thermal instability is not necessary for the phenomenon.

Abstract

The evolution of intermediate-mass and massive stars speeds up considerably after they finish their hydrogen core-burning. Due to this accelerated evolution, the probability to observe stars during this episode is small. In suitable stellar aggregates, in particular star clusters of appropriate ages, the fast evolutionary phase causes a depopulated area~--~referred to as the Hertzsprung gap~--~in color-magnitude diagrams and derivatives therefrom. The explanation of the speed-up usually resorts to the star's Kelvin-Helmholtz timescale and the Sch\"onberg-Chandrasekhar instability is called upon. This exposition challenges this viewpoint with counterexamples and argues that a suitably defined nuclear timescale is enough to explain the fast evolution. A thermal instability, even though it develops in stars evolving through the Hertzsprung gap, is not a necessary condition to trigger the phenomenon.