Post-AGB stars with hot circumstellar dust: binarityof the low-amplitude pulsators

摘要

The influence of binarity on the late stages of stellar evolution remains an open issue.Aims. While the first binary post-AGB stars were serendipitously discovered, the distinct characteristics of their spectral energydistribution (SED) allowed us to launch a more systematic search for binaries. We selected post-AGB objects, which exhibit a broaddust excess starting either at H or K, pointing to the presence of a gravitationally bound dusty disc in the system. We initiated anextensive multiwavelength study of those systems and here report on our radial velocity and photometric monitoring results for sixstars of early F type, which are pulsators of small amplitude.Methods. To determine the radial velocity of low signal-to-noise ratio time-series data, we constructed dedicated autocorrelationmasks based on high signal-to-noise ratio spectra, used in our published chemical studies. The radial velocity variations were analysedin detail to differentiate between pulsational variability and variability caused by orbital motion. When available, the photometricmonitoring data were used to complement the time series of radial velocity data and to establish the nature of the pulsation. Finally,orbital minimalisation was performed to constrain the orbital elements.Results. All of the six objects are binaries with orbital periods ranging from 120 to 1800 days. Five systems have non-circular orbits.The mass functions range from 0.004 to 0.57 Mo and the companions are probably unevolved objects of (very) low initial mass. Weargue that these binaries must have evolved through a phase of strong binary interaction when the primary was a cool supergiant.Although the origin of the circumstellar disc is not well understood, the disc is generally believed to have formed during this stronginteraction phase. The eccentric orbits of these highly evolved objects remain poorly understood. In one object, the line-of-sight grazesthe edge of the puffed-up inner rim of the disc.Conclusions. These results corroborate our earlier statement that evolved objects in binary stars create a Keplerian dusty circumbinarydisc. With the measured orbits and mass functions, we conclude that the circumbinary discs seem to have a major impact on theevolution of a significant fraction of binary systems.