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Red Giant Seismology: Seismic Signatures of Convective Overshoot

Presentation #327.02 in the session Stellar Populations and Evolution I.

Published onJun 29, 2022
Red Giant Seismology: Seismic Signatures of Convective Overshoot

Improving our understanding of stellar interior mixing physics is important in many areas of astrophysics since astronomers rely on stellar modeling to produce reliable estimates for stellar ages. The complex process of convective fluid parcels moving past convection zone boundaries into radiative zones in stars is currently modeled in 1D stellar evolution codes by changing the mixing efficiency or temperature gradient profiles near the convective boundary. How exactly these stellar modeling codes should treat overshoot in different stars is currently unknown. Most red giant models do not reproduce the position of the observed luminosity bump, a diagnostic of the maximum extension of the convective envelope during the first-dredge up. Global asteroseismic parameters, the large frequency separation and frequency of maximum oscillation, show that overshoot below the convective envelope into the radiative core helps match red giant model luminosity bump positions to observed bump positions. The global seismic properties, however, cannot be used to probe envelope overshoot in a star-by-star manner. Red giant mixed modes (modes that are p-like (i.e., acoustic modes) at the surface and g-like (i.e., gravity modes) in the core) contain important information about the interior structure of the star and therefore can be used to study interior processes like overshoot. We present the results of a theoretical study to investigate the seismic signature of different modeling procedures of convective overshoot in red giants of varying masses and metallicities. We find that the details of how convective overshoot in the envelopes of red giants is modeled in 1D stellar evolution codes has clear effects on the oscillation properties of the star. In particular we show that the gravity-mode phase offset parameter, a measure of the phase shift given to the g-modes at the location of the convective boundary, evolves differently up the red giant branch for varying prescriptions of overshoot. Our intention is to use these modeling studies to determine the general overshoot prescription which best matches the observed mixed mode frequencies of actual red giants in cluster observed by Kepler.

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