Why do low-mass stars become red giants?

TL;DR

This paper investigates the physical factors influencing why stars become red giants, focusing on the roles of mean molecular weight and energy generation, and finds that different mechanisms may operate depending on stellar mass.

Contribution

The study modifies stellar evolution models to isolate effects of mean molecular weight and energy generation, revealing multiple pathways to red giant formation.

Findings

Mean molecular weight change is necessary for 1 solar mass stars to become giants.

For 5 solar mass stars, mean molecular weight change alone is insufficient.

Multiple mechanisms may lead to red giant evolution depending on stellar mass.

Abstract

We revisit the problem of why stars become red giants. We modify the physics of a standard stellar evolution code in order to determine what does and what does not contribute to a star becoming a red giant. In particular, we have run tests to try to separate the effects of changes in the mean molecular weight and in the energy generation. The implications for why stars become red giants are discussed. We find that while a change in the mean molecular weight is necessary (but not sufficient) for a 1 solar mass star to become a red giant, this is not the case in a star of 5 solar masses. It therefore seems that there may be more than one way to make a giant.