An asteroseismic signature of helium ionization |
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Authors: | G Houdek D O Gough |
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Institution: | Institute of Astronomy, Madingley Road, Cambridge CB3 0HA;Department of Applied Mathematics and Theoretical Physics, Wilberforce Road, Cambridge CB3 0WA;South African Astronomical Observatory, PO Box 9, Observatory 7935, South Africa |
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Abstract: | We investigate the influence of the ionization of helium on the low-degree acoustic oscillation frequencies in model solar-type stars. The signature in the oscillation frequencies characterizing the ionization-induced depression of the first adiabatic exponent γ is a superposition of two decaying periodic functions of frequency ν, with 'frequencies' that are approximately twice the acoustic depths of the centres of the He i and He ii ionization regions. That variation is probably best exhibited in the second frequency difference Δ2ν n , l ≡ν n ?1, l ? 2ν n , l +ν n +1, l . We show how an analytic approximation to the variation of γ leads to a simple representation of this oscillatory contribution to Δ2ν which can be used to characterize the γ variation, our intention being to use it as a seismic diagnostic of the helium abundance of the star. We emphasize that the objective is to characterize γ, not merely to find a formula for Δ2ν that reproduces the data. |
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Keywords: | Sun: abundances Sun: oscillations stars: abundances stars: interiors stars: oscillations |
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