Mixed modes in red-giant stars observed with CoRoT

TL;DR

This study utilizes mixed-mode oscillation analysis of CoRoT red-giant star data to determine their evolutionary stages and identify seismic characteristics, revealing distinctions between red-giant branch and clump stars.

Contribution

The paper introduces a novel method based on envelope autocorrelation to analyze mixed modes and determine the evolutionary status of red giants observed with CoRoT.

Findings

Mixed modes help distinguish red-giant evolutionary stages.

High-mass secondary clump stars show unique seismic properties.

Stars in the clump experienced significant mass loss.

Abstract

The CoRoT mission has provided thousands of red-giant light curves. The analysis of their solar-like oscillations allows us to characterize their stellar properties. Up to now, the global seismic parameters of the pressure modes remain unable to distinguish red-clump giants from members of the red-giant branch. As recently done with Kepler red giants, we intend to analyze and use the so-called mixed modes to determine the evolutionary status of the red giants observed with CoRoT. We also aim at deriving different seismic characteristics depending on evolution. The complete identification of the pressure eigenmodes provided by the red-giant universal oscillation pattern allows us to aim at the mixed modes surrounding the l=1 expected eigenfrequencies. A dedicated method based on the envelope autocorrelation function is proposed to analyze their period separation. We have identified the mixed-mode signature separation thanks to their pattern compatible with the asymptotic law of gravity modes. We have shown that, independent of any modelling, the g-mode spacings help to distinguish the evolutionary status of a red-giant star. We then report different seismic and fundamental properties of the stars, depending on their evolutionary status. In particular, we show that high-mass stars of the secondary clump present very specific seismic properties. We emphasize that stars belonging to the clump were affected by significant mass loss. We also note significant population and/or evolution differences in the different fields observed by CoRoT.