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Register a new userStructural and Evolutionary Diagnostics from Asteroseismic Phase Functions
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In the asymptotic parameterisation of mode frequencies, the phase function $epsilon( u)$ completely specifies the detailed structure of the frequency eigenvalues. In practice, however, this function of frequency is reduced to a single scalar $epsilon$, defined, particularly by observers, as the intercept of a least-squares fit to the frequencies against radial order, or via the central value of this function. The procedure by which this is done is not unique. We derive a few simple expressions relating various observational estimators of $epsilon$ for radial modes to each other, and to the underlying theoretical object. In particular we demonstrate that a ``reduced functional parameterisation is both insensitive to mis-estimations of $Delta u$, and easy to evaluate locally in terms of both observational and theoretical quantities. It has been shown previously that such a local definition of $epsilon$ can distinguish between stars on the ascending part of the red giant branch and those in the red clump. We find that this sensitivity to evolutionary stage arises from differences in the local frequency derivative of the underlying phase function, a consequence of differences in internal structure. By constructing an HR-like diagram out of purely seismic observables, we provide a unified view of the textit{Kepler} asteroseismic sample, as well as the initial results from textit{TESS}. We investigate how various astrophysical quantities and modelling parameters affect the morphology of isochrones on this seismic diagram. We also show that $epsilon$ can be used as an independent input when deriving stellar parameters from global asteroseismic quantities.
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Asteroseismology of solar-type stars has entered a new era of large surveys with the success of the NASA textit{Kepler} mission, which is providing exquisite data on oscillations of stars across the Hertzprung-Russell (HR) diagram. From the time-series photometry, the two seismic parameters that can be most readily extracted are the large frequency separation ($Delta u$) and the frequency of maximum oscillation power ($ u_mathrm{max}$). After the survey phase, these quantities are available for hundreds of solar-type stars. By scaling from solar values, we use these two asteroseismic observables to identify for the first time an evolutionary sequence of 1-M$_odot$ field stars, without the need for further information from stellar models. Comparison of our determinations with the few available spectroscopic results shows an excellent level of agreement. We discuss the potential of the method for differential analysis throughout the main-sequence evolution, and the possibility of detecting twins of very well-known stars.
With the observations of an unprecedented number of oscillating subgiant stars expected from NASAs TESS mission, the asteroseismic characterization of subgiant stars will be a vital task for stellar population studies and for testing our theories of stellar evolution. To determine the fundamental properties of a large sample of subgiant stars efficiently, we developed a deep learning method that estimates distributions of fundamental parameters like age and mass over a wide range of input physics by learning from a grid of stellar models varied in eight physical parameters. We applied our method to four Kepler subgiant stars and compare our results with previously determined estimates. Our results show good agreement with previous estimates for three of them (KIC 11026764, KIC 10920273, KIC 11395018). With the ability to explore a vast range of stellar parameters, we determine that the remaining star, KIC 10005473, is likely to have an age 1 Gyr younger than its previously determined estimate. Our method also estimates the efficiency of overshooting, undershooting, and microscopic diffusion processes, from which we determined that the parameters governing such processes are generally poorly-constrained in subgiant models. We further demonstrate our methods utility for ensemble asteroseismology by characterizing a sample of 30 Kepler subgiant stars, where we find a majority of our age, mass, and radius estimates agree within uncertainties from more computationally expensive grid-based modelling techniques.
We briefly review the main physical and structural properties of Very Low-Mass stars. The most important improvements in the physical inputs required for the stellar models computations are also discussed. We show some comparisons with observational measurements concerning both the Color-Magnitude diagrams, mass-luminosity relations and mass-radius one, in order to disclose the level of agreement between the present theoretical framework and observations.
Sudden changes in the internal structure of stars, placed at the interface between convective and radiative regions, regions of partial ionisation, or between layers that have acquired different chemical composition as a result of nuclear burning, often produce specific signatures in the stars oscillation spectra. Through the study of these signatures one may gain information on the physical processes that shape the regions that produce them, including diffusion and chemical mixing beyond the convectively unstable regions, as well as information about the helium content of stars. In this talk, I will review important theoretical and observational efforts conducted over the years towards this goal. I will emphasise the potential offered by the study of acoustic, gravity, and mixed modes observed in stars of different mass and evolutionary stages, at a time when space-based data is allowing us to build on the knowledge gained from the study of the sun and white dwarfs, where these efforts have long been undertaken, extending the methods developed to stars across the HR diagramme.
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