Kepler observations of the beaming binary KPD 1946+4340

Abstract

The Kepler Mission has acquired 33.5d of continuous one-minute photometry of\nKPD 1946+4340, a short-period binary system that consists of an sdB and a white\ndwarf. In the light curve, eclipses are clearly seen, with the deepest\noccurring when the compact white dwarf crosses the disc of the sdB (0.4%) and\nthe more shallow ones (0.1%) when the sdB eclipses the white dwarf. As\nexpected, the sdB is deformed by the gravitational field of the white dwarf,\nwhich produces an ellipsoidal modulation of the light curve. Spectacularly, a\nvery strong Doppler beaming (aka Doppler boosting) effect is also clearly\nevident at the 0.1% level. This originates from the sdB's orbital velocity,\nwhich we measure to be 164.0\\pm1.9 km/s from supporting spectroscopy. We\npresent light curve models that account for all these effects, as well as\ngravitational lensing. We derive system parameters and uncertainties from the\nlight curve using Markov Chain Monte Carlo simulations. Adopting a theoretical\nwhite dwarf mass-radius relation, the mass of the subdwarf is found to be\n0.47\\pm0.03 Msun and the mass of the white dwarf 0.59\\pm0.02 Msun. The\neffective temperature of the white dwarf is 15 900\\pm300K. With a spectroscopic\neffective temperature of Teff = 34 730\\pm250K and a surface gravity of log g =\n5.43\\pm0.04, the sdB is in a shell He burning stage. The detection of Doppler\nbeaming in Kepler light curves potentially allows one to measure radial\nvelocities without the need of spectroscopic data. For the first time, a\nphotometrically observed Doppler beaming amplitude is compared to a\nspectroscopically established value. The sdB's radial velocity amplitude\nderived from the photometry 168\\pm4 km/s is in perfect agreement with the\nspectroscopic value. After subtracting our best model for the orbital effects,\nwe searched the residuals for stellar oscillations but did not find any\nsignificant pulsation frequencies.\n