Phase-resolved spectroscopy and Kepler photometry of the ultracompact AM CVn binary SDSS J190817.07+394036.4

Abstract

Kepler satellite photometry and phase-resolved spectroscopy of the ultracompact AM CVn type binary SDSS J190817.07+394036.4 are presented. The average spectra reveal a variety of weak metal lines of different species, including silicon, sulphur and magnesium as well as many lines of nitrogen, beside the strong absorption lines of neutral helium. The phasefolded spectra and the Doppler tomograms reveal an S-wave in emission in the core of the He I 4471 Å absorption line at a period of Porb = 1085.7 ± 2.8 s identifying this as the orbital period of the system. The Si II, MgII and the core of some He I lines show an S-wave in absorption with a phase offset of 170° ± 15° compared to the S-wave in emission. The NII, Si III and some helium lines do not show any phase variability at all. The spectroscopic orbital period is in excellent agreement with a period at Porb = 1085.108(9) s detected in the 3 yr Kepler light curve. A Fourier analysis of the Q6-Q17 short-cadence data obtained by Kepler revealed a large number of frequencies above the noise level where the majority shows a large variability in frequency and amplitude. In an Observed-minus-computed analysis, we measured a |P˙ | ~ 1.0×10-8 s s-1 for some of the strongest variations and set a limit for the orbital period to be |P˙ | < 10-10 s s-1. The shape of the phase-folded light curve on the orbital period indicates the motion of the bright-spot. Models of the system were constructed to see whether the phases of the radial velocity curves and the light-curve variation can be combined to a coherent picture. However, from the measured phases neither the absorption nor the emission can be explained to originate in the bright-spot.