2M1938+4603: A rich, multimode pulsating sdB star with an eclipsing dM companion observed with Kepler

TL;DR

This study analyzes the pulsation spectrum and system parameters of the rare eclipsing sdB+dM binary star 2M1938+4603 using Kepler data, ground-based photometry, and spectroscopy, revealing complex pulsations and precise component masses.

Contribution

First detailed analysis of a pulsating eclipsing sdB+dM star with complex pulsation modes and well-constrained system parameters from combined observational data.

Findings

Rich pulsation spectrum with p-mode and g-mode oscillations

Derived component masses: M1=0.48 M_sun, M2=0.12 M_sun

Orbital period of 0.126 days, longest among similar systems

Abstract

2M1938+4603 (KIC 9472174) displays a spectacular light curve dominated by a strong reflection effect and rather shallow, grazing eclipses. The orbital period is 0.126 days, the second longest period yet found for an eclipsing sdB+dM, but still close to the minimum 0.1-d period among such systems. The phase-folded light curve was used to detrend the orbital effects from the dataset, and the resulting amplitude spectrum shows a rich collection of pulsation peaks spanning frequencies from ~50 to 4500 uHz. The presence of a complex pulsation spectrum in both the p-mode and the g-mode regions has never been seen before in a compact pulsator. Eclipsing sdB+dM stars are very rare, with only seven systems known and only one with a pulsating primary. Pulsating stars in eclipsing binaries are especially important since they permit masses derived from seismological model fits to be cross checked with orbital mass constraints. We present a first analysis of this star based on the Kepler 9.7-day commissioning light curve and extensive ground-based photometry and spectroscopy that allow us to set useful bounds on the system parameters. We derive a radial-velocity amplitude K_1 = 65.7 +/- 0.6 km/s, inclination angle i = 69.45 +/- 0.20 degrees, and find that the masses of the components are M_1 = 0.48 +/- 0.03 and M_2 = 0.12 +/- 0.01 solar masses.