Formation and tidal synchronization of sdB stars in binaries an asteroseismic investigation using Kepler Observations

Abstract

<p>Subdwarf B (sdB) stars are low mass (0.5 M_sun) helium burning stars with thin hydrogen envelopes and T_eff 22000-40000 K. Many of these stars are found in binary systems. One common proposed formation mechanism is common envelope (CE) ejection, where the companion spirals deep into the star's envelope ejecting the outer layers and forming a close binary system. In this dissertation, we use short cadence (t_int=58.86 s) <em>Kepler</em> photometric time-series data to study three close sdB binaries with P $\approx$ 10 hours and g-mode pulsations. Asteroseismic analysis finds that each system has a constant period spacing of &sim 250 s consistent with single sdB stars. This analysis also shows the presence of rotational multiplets which we used to find the rotation period. In all three cases the binary system is far from tidal synchronization with a rotation period an order of magnitude longer than the orbital period. These observations agree with predictions using the Zahn formulation of tidal evolution which predicts a synchronization time longer than the sdB lifetime ($10⁸ yr). We use this synchronization time to backtrack the sdB's rotation history and find its initial rotation period as it is first exiting the CE. This is one of the only observationally based constraints that has been placed on CE evolution. Preliminary investigations of single sdB stars show similar rotation periods, indicating that the rotation period may be independent of the formation channel.</p>