Discovery of post-mass-transfer helium-burning red giants using asteroseismology

TL;DR

This paper uses asteroseismology to identify helium-burning red giants that have undergone significant mass loss due to binary interactions, revealing new insights into post-mass-transfer stellar evolution.

Contribution

It introduces a novel asteroseismic method to detect and characterize post-mass-transfer red giants, expanding understanding of binary star evolution.

Findings

Identified two classes of post-mass-transfer red giants with distinct properties.

Detected stars with significantly reduced masses indicating dramatic mass loss.

Results align with binary evolution models and suggest new research avenues.

Abstract

A star expands to become a red giant when it has fused all the hydrogen in its core into helium. If the star is in a binary system, its envelope can overflow onto its companion or be ejected into space, leaving a hot core and potentially forming a subdwarf-B star. However, most red giants that have partially transferred envelopes in this way remain cool on the surface and are almost indistinguishable from those that have not. Among $\sim$7000 helium-burning red giants observed by NASA's Kepler mission, we use asteroseismology to identify two classes of stars that must have undergone dramatic mass loss, presumably due to stripping in binary interactions. The first class comprises about 7 underluminous stars with smaller helium-burning cores than their single-star counterparts. Theoretical models show that these small cores imply the stars had much larger masses when ascending the red giant branch. The second class consists of 32 red giants with masses down to 0.5 M$_\odot$, whose implied ages would exceed the age of the universe had no mass loss occurred. The numbers are consistent with binary statistics, and our results open up new possibilities to study the evolution of post-mass-transfer binary systems.