Stellar model analysis of the oscillation spectrum of [eta] Bootis obtained from MOST

Abstract

Eight consecutive low-frequency radial p-modes are identified in the G0 IV star [eta] Bootis based on 27 days of ultraprecise rapid photometry obtained by the MOST (Microvariability and Oscillations of Stars) satellite. The MOST data extend smoothly, to lower overtones, the sequence of radial p-modes reported in earlier ground-based spectroscopy by other groups. The sampling is nearly continuous; hence, the ambiguities in p-mode identifications due to aliases, such as the cycle day 1 alias found in ground observations, are not an issue. The lower overtone modes from the MOST data constrain the interior structure of the model of [eta] Boo, giving a best fit on a grid of ~ 300,000 stellar models for a composition of (X, Z ) = (0.71, 0.04), a mass of M = 1.71 [plus or minus] 0.05 M[circle dot] , and an age of t = 2.40 [plus or minus] 0.03 Gyr. The surface temperature and luminosity of this model, which were constrained only by using the oscillation modes, are close (1 [sigma]) to current best estimates of [eta] Boo’s surface temperature and luminosity. With the interior fit anchored by the lower overtone modes seen by MOST, standard models are not able to fit the higher overtone modes with the same level of accuracy. The discrepancy, model minus observed frequency, increases from 0.5 [mu]Hz at 250 [mu]Hz to 5 [mu]Hz at 1000 [mu]Hz and is similar to the discrepancy that exists between the Sun’s observed p-mode frequencies and the p-mode frequencies of the standard solar model. This discrepancy promises to be a powerful constraint on models of three-dimensional convection.