Saniya Khan
Because stars are opaque, studies of their internal structure have, until recently, been restricted to indirect inferences based on observations of their outer layers. That is, until the “space photometry revolution” led by telescopes CoRoT, Kepler/K2 and TESS. Global, resonant oscillations were then detected in tens of thousands of stars in the Milky Way, allowing us to perform seismic investigations of stellar interiors, in a comparable way as studies of our planet’s interior with Earth seismology.
One long-standing and crucial deficiency in our understanding of the evolution of low-mass stars (M ≤ 2.3 solar masses) is the treatment of convection in stellar interiors. Convection involves macroscopic motions of matter that carry energy and constitute a very efficient mixing mechanism. However, the occurrence and efficiency of mixing processes beyond formal convective boundaries is poorly known, despite having significant implications on the predicted evolutionary path, lifetime, and chemical composition of stars.
The red-giant branch provides an observational means to constrain these processes, with the red-giant branch bump (RGBb). The latter corresponds to a temporary drop in luminosity as the star evolves on the red-giant branch, leading to a local maximum in the luminosity distribution. The occurrence of the bump is related to the hydrogen-burning shell approaching and eventually advancing through the chemical composition gradient left by the convective envelope as it penetrates into deeper layers of the star and brings to the surface the nucleosynthetic products resulting from main-sequence burning. Thus, one can use the luminosity of the RGBb as a calibrator for extra-mixing processes.
My research consists in gathering observational evidence on the RGBb from asteroseismology, astrometry, photometry, and spectroscopy to offer additional answers regarding the physical nature and efficiency of convective boundary mixing (Khan et al. 2018, 2019), and work towards a better understanding of stellar evolution theory.
Asteroseismology, the study of stellar oscillations, allows us to probe the interior of stars, that was previously inaccessible from outer layers properties alone. The left graph illustrates the strong potential of asteroseismology, with different modes probing different parts of the star. The plot on the right shows an example of a red-giant power spectrum. Each colour is associated to mode frequencies of a certain harmonic degree. Several features are highlighted on the diagram: 1) the frequency of maximum oscillation power, where the gaussian envelope peaks at; 2) the average large separation, which is the spacing between two radial modes; 3) individual dipole mode frequencies. The first two are average spectral parameters, which can be related to fundamental stellar parameters, and the last one can provide direct constraints on variations in chemical composition in the stellar interior.
The red-giant branch bump (RGBb) corresponds to a temporary drop in luminosity as the star evolves on the red-giant branch. Its occurrence is related to the hydrogen-burning shell approaching and eventually advancing through the chemical discontinuity, left by the convective envelope. This is illustrated by the diagram on the left, which shows the chemical composition profile within a red giant: the hydrogen-burning shell moves away from the helium core and approaches the discontinuity. The latter is located deeper into the star when overshooting is included: as a result, the discontinuity is met earlier by the shell and the RGBb has a fainter luminosity. In the middle, we have a Hertzsprung-Russell diagram (luminosity as a function of effective temperature), which illustrates theoretically-predicted stellar evolution trajectories. The hexagonal binning plot shows the observations while the lines correspond to modelled evolutionary tracks. The red shape indicates the location of the RGBb, seen as an overdensity of stars in the data and as a zig-zag shape in the models. Consequently, one can use the luminosity of the RGBb as a calibrator for extra-mixing processes.
During my PhD, I have developed a novel approach to study the RGBb, combining asteroseismic and spectroscopic constraints (Khan et al. 2018). The plot shows a Hertzsprung-Russell diagram (using an asteroseismic parameter as a proxy for the luminosity), zoomed near the location of the RGBb. The coloured points correspond to the data, in a specific mass interval and for different metallicity bins; while the lines are standard models not including overshooting. It is clear that the observed RGBb is fainter than the predicted one, hence the inclusion of overshooting in the models helps reproducing the observations. We also note that the discrepancy is larger as we move towards lower metallicities, which suggests that, for low-mass stars, the extra-mixing efficiency increases with decreasing metallicity. Following this study, I aim at further constraining the RGBb using independent measurements, as well as detailed asteroseismic diagnostics.