Evidence of structural discontinuities in the inner core of red-giant stars

TL;DR

This study uses Kepler asteroseismic data to identify large core structural discontinuities in a subset of red-giant clump stars, revealing insights into stellar core mixing processes.

Contribution

It provides the first observational evidence of core discontinuities in red-giant stars and suggests these features are intermittently formed during stellar evolution.

Findings

6.7% of studied stars show core discontinuities

Discontinuities imply radiative stratification beyond convective boundary

Discontinuities may form intermittently during evolution

Abstract

Red giants are stars in the late stages of stellar evolution. Because they have exhausted the supply of hydrogen in their core, they burn the hydrogen in the surrounding shell. Once the helium in the core starts fusing, the star enters the clump phase, which is identified as a striking feature in the color-magnitude diagram. Since clump stars share similar observational properties, they are heavily used in astrophysical studies, as probes of distance, extinction through the galaxy, galaxy density, and stellar chemical evolution. In this work, we perform the detailed observational characterization of the deepest layers of clump stars using asteroseismic data from Kepler. We find evidence for large core structural discontinuities in about 6.7% of the stars in our sample, implying that the region of mixing beyond the convective core boundary has a radiative thermal stratification. These stars are otherwise similar to the remaining stars in our sample, which may indicate that the building of the discontinuities is an intermittent phenomenon.