The red-giant branch bump: towards a better understanding of the physical nature and efficiency of convective boundary mixing

Khan, Saniya ORCID: 0000-0001-5998-5885 (2021). The red-giant branch bump: towards a better understanding of the physical nature and efficiency of convective boundary mixing. University of Birmingham. Ph.D.

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Several uncertainties are still affecting our ability to model low-mass stars today --- with significant implications on studies of exoplanetary systems, and of stellar populations and Galactic evolution. One of these is related to convection, and more specifically to convective overshooting processes which transport chemical elements beyond the formal convective boundary defined using the Schwarzschild criterion. In this respect, the luminosity of the red-giant branch bump can be used as a diagnostic for extra-mixing processes occurring below the envelopes of red-giant stars, and this has been well-studied in former investigations of Galactic globular clusters.

In this thesis, we use a novel set of constraints from asteroseismology to revisit the red-giant branch bump in field stars observed by Kepler and APOGEE. We also extend the analysis to clusters in our Galaxy and in the Magellanic Clouds, in order to work towards an envelope overshooting calibration applicable in a wide range of masses and metallicities. Both studies seem to indicate that the extra-mixing efficiency increases as we move towards lower metallicities. This offers interesting leads that are potentially relevant to test convection as a physical process.

Because such tests of stellar evolution theory rely strongly on the accuracy of asteroseismic constraints, we also dedicated a substantial part of the work to comparing asteroseismically-derived parallaxes with estimates from Gaia DR2 and EDR3. These analyses provide new estimates of the Gaia parallax zero-point in the Kepler, K2, and TESS fields. They also demonstrate the strong potential of asteroseismology as a way to derive stellar fundamental parameters.

Type of Work: Thesis (Doctorates > Ph.D.)
Award Type: Doctorates > Ph.D.
Licence: All rights reserved
College/Faculty: Colleges (2008 onwards) > College of Engineering & Physical Sciences
School or Department: School of Physics and Astronomy
Funders: Other
Other Funders: University of Birmingham
Subjects: Q Science > QB Astronomy
Q Science > QC Physics


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