Stellar Expansion or Inflation?

TL;DR

This paper distinguishes between stellar expansion and inflation, clarifies their physical mechanisms, and reinterprets some phenomena previously attributed to inflation as expansion, using detailed stellar models.

Contribution

It provides a detailed comparison of stellar expansion and inflation mechanisms, clarifies terminology, and reinterprets recent observations with advanced stellar models.

Findings

Some phenomena labeled as stellar inflation are actually due to stellar expansion.

The removal of inflated zones should be called deflation, not contraction.

Models show differences in radius changes depending on mass loss and binary interactions.

Abstract

While stellar expansion after core-hydrogen exhaustion related to thermal imbalance has been documented for decades, the physical phenomenon of stellar inflation that occurs close to the Eddington limit has only come to the fore in recent years. We aim to elucidate the differences between these physical mechanisms for stellar radius enlargement, especially as additional terms such as `bloated' and `puffed-up' stars have been introduced in the recent massive star literature. We employ single and binary star MESA structure and evolution models for both constant mass, as well as models allowing for the mass to change, due to winds or binary interaction. We find cases that were previously attributed to stellar inflation in fact to be due to stellar expansion. We also highlight that while the opposite effect of expansion is contraction, the removal of an inflated zone should not be referred to as contraction but {\it deflation} as the star is still in thermal balance.