Evidence for a sharp structure variation inside a red-giant star

TL;DR

This study detects a seismic signature of a sharp internal structure variation in a red giant star, interpreted as a depression in sound speed in the helium second-ionization zone, and estimates the star's fundamental parameters.

Contribution

It provides the first seismic evidence of a sharp internal structure variation in a red giant and links it to a specific physical region within the star.

Findings

Detected a periodic seismic signature indicating a sharp structure variation.

Interpreted the feature as a sound speed depression in the helium second-ionization zone.

Estimated the star's mass and radius consistent with stellar models.

Abstract

The availability of precisely determined frequencies of radial and non-radial oscillation modes in red giants is finally paving the way for detailed studies of the internal structure of these stars. We look for the seismic signature of regions of sharp structure variation in the internal structure of the CoRoT target HR7349. We analyse the frequency dependence of the large frequency separation and second frequency differences, as well as the behaviour of the large frequency separation obtained with the envelope auto-correlation function. We find evidence for a periodic component in the oscillation frequencies, i.e. the seismic signature of a sharp structure variation in HR7349. In a comparison with stellar models we interpret this feature as caused by a local depression of the sound speed that occurs in the helium second-ionization region. Using solely seismic constraints this allows us to estimate the mass (M=1.2^{+0.6}_{-0.4} Msun) and radius (R=12.2^{+2.1}_{-1.8} Rsun) of HR7349, which agrees with the location of the star in an HR diagram.