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Evolutionary and pulsational models of metal-poor subdwarfs Shkolnik, Evgenya

Abstract

Metal-poor subdwarfs (MPSDs) are probably the Sun's oldest neighbours and are almost certainly low-amplitude p-mode pulsators, like the Sun. One of the goals of the MOST Space Telescope project (due for launch in 2002 - 2003) is to detect MPSD oscillations and apply asteroseismology to refine the calibration of globular cluster isochrone fitting and possibly set a lower limit to the age of the Universe. To be able to interpret the eigenfrequency data, a comprehensive grid of nonadiabatic, nonradial pulsation models for MPSDs was generated for the first time using a code developed by Guenther (1994). Each pulsation model was calculated from a structural evolutionary model using the Yale Stellar Evolution Code with Rotation (YREC) which included up-to-date physics such as both He and heavy element diffusion and the latest EOS and opacity tables. The grid spanned the following ranges: helium abundance Y = 0.235 to 0.255 (consistent with estimates of primordial Y); heavy element abundance Z = 0.0002 to 0.01 (consistent with MPSD spectra); mass M = 0.7 to 1.0 M[sub o] ; and age A = 5 to 16 Gyr. From the calculated eigenfrequencies of each model, the large and small frequency spacings (Δ v, δV[sub0,2]) were derived as defined by asymptotic pulsation theory (Tassoul 1980). These spacings are sensitive diagnostics of the mass and mainsequence age of the star. Combining MOST's resolution with the uncertainties in the input parameters, MOST data could refine MPSD ages to better than ± 0.056 Gyr (an average error assuming the input physics are correct).

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