Asteroseismic determination of helium abundance in stellar envelopes

Abstract

Intermediate degree modes of the solar oscillations have previously been used to determine the solar helium abundance to a high degree of precision. However, we cannot expect to observe such modes in other stars. In this work we investigate whether low degree modes that should be available from space-based asteroseismology missions can be used to determine the helium abundance, Y, in stellar envelopes with sufficient precision. We find that the oscillatory signal in the frequencies caused by the depression in Γ₁ in the second helium ionization zone can be used to determine the envelope helium abundance of low-mass main-sequence stars. For frequency errors of one part in 10⁴, we expect errors σ_Y in the estimated helium abundance to range from 0.03 for 0.8-M_⊙ stars to 0.01 for 1.2-M_⊙ stars. The task is more complicated in evolved stars, such as subgiants, but is still feasible if the relative errors in the frequencies are less than 10^-4.