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Stellar Seismic Indices

Thanks to CoRoT (CNES) and Kepler (NASA) seismic observations, it is was possible to detect many solar-like oscillations in a very high number of stars. The observed oscillation spectra can be quite complicate to analyse and interpret. Nevertheless, these spectra show characteristic pattern, which can be more simply characterized with that we call stellar seismic indices. More specifically, the stellar seismic indices correspond to characteristic global seismic numbers that are extracted from the oscillation spectra of solar-like pulsating stars.
Among them we distinguish in particular:

  • the mean large separation (delta_nu)
  • the peak frequency (nu_max)

The first one corresponds to the mean frequency separation between two modes of same angular degree while the second one corresponds to the frequency at which the oscillation spectrum peaks. These seismic indices obey characteristic scaling relations that depend directly on the radius, mass and effective temperature of the star. From the knowledge of these quantities it is possible to estimate the mass and radius of a star, and subsequently the surface gravity of the star (log g).

Because they are relatively easy to measure thanks to CoRoT and Kepler, it has already been possible to measure seismic indices in several tens of thousands of redgiants and sub-giants, and subsequently to derive their mass,
radius and evolutionary status. Theses seismic indices are more and more used in the stellar physic but also for the study of Galactic population. These seismic indices have opened the way toward that we name now ensemble asteroseismology.

The light-curves of redgiants have also unveiled the presence of a signal characterizing the stellar granulation, which is a superficial signature of convection inside the star. As for the seismic indices, it is possible to extract in rather easy way the characteristic parameters of stellar granulation. These parameters also obey scaling relations, which in addition to seismic scaling relations, provide information about the surface layers of the star (see e.g. this short news).