Walter E. van Rossem
Asteroseismology is a powerful tool with which we can characterize stars. In the last decade space telescope missions such as Kepler, TESS, and CoRoT have allowed for huge advances in our understanding of the interior workings of stars. A special type of oscillation, called a mixed mode, can bring information about the more central regions of a star to light. By looking at how strongly the outer and inner oscillations couple together we can infer some structural properties such as the convective envelope mass of Red Clump stars. Red clump stars are low mass stars which are burning helium in their core quiescently. Combining this with other methods to determine structural properties we aim to be able to create a model which will give us structural properties such as the convective core mass, the helium core mass, and the convective envelope mass. In the future we will use this model to search for binary interaction products where stars have either gained or lost mass from a companion star and so would have atypical structures.
Asteroseismology is the study of oscillations in stars. Different oscillation modes travel through different layers of a star, allowing for the internal structure of a star to be inferred, which is otherwise not directly observable through global properties like luminosity or temperature.
There are two main types of oscillation modes; pressure modes (p-modes) and gravity modes (g-modes). P-modes are acoustic waves where the restoring force is pressure and tend to propagate in the outer layers of a star. For g-modes the restoring force is buoyancy and they tend to propagate in the inner regions of a star. These typical propagation regions can be seen in the figure to the right.
Mass transfer can have significant effects a star’s structure. The schematic to the left shows how the more massive star (left) expands and then donates almost all its hydrogen envelope (red), leaving a star consisting mostly of its helium core (yellow). The less massive companion (right) gains this mass and now consists of a very large hydrogen envelope and a relatively small helium core compared to its total mass.
Slide 2:
Mixed modes are a mixture of the two, they exhibit g-like properties in the central regions of a star and p-like properties in the outer regions. The coupling coefficient q is a measure of how strong this characteristic mixing is and is a highly useful diagnostic parameter.
The right figure shows the propagation diagram of a 1 solar mass star on Red Clump (RC). The Lamb and Brunt–Väisälä frequencies are shown as the blue and orange lines respectively. Their reduced counterparts, which include effects due to the gravitational potential, are shown as the green and red lines. The horizontal black line shows the frequency of maximum power (ν_max). The boundaries of the evanescent zone at ν_max, r1 and r2 are shown as the blue and red dots and are the edges of the evanescent region. The hydrogen burning shell is shown as the cyan shaded region. The p- and g-mode cavities are shown as the orange and brown hatched regions respectively.
Smaller evanescent regions cause strong coupling, and various effects determine the size of this region. One of the strongest effects is the metallicity, with higher metallicities having smaller q. This is shown in the left figure.
Slide 3:
The left figure compares the stellar evolution models to the data. The trends in metallicity and mass agree quite well, showing similar behaviour for both in the coupling strength. The stellar evolution models are then used to calculate the mass ratio of the convective core to the helium core from observed stellar quantities such as the mode density, coupling, metallicity and surface gravity. Deviations from this relationship would then indicate the effects of MT.