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

TL;DR

This study systematically investigates binary post-AGB stars with hot circumstellar dust, revealing their orbital characteristics and suggesting significant binary interaction influences their evolution and disc formation.

Contribution

It provides the first detailed orbital parameters for a sample of six pulsating post-AGB binaries with dusty discs, highlighting their eccentric orbits and low-mass companions.

Findings

All six stars are confirmed binaries with periods 120-1800 days.

Most systems exhibit non-circular, eccentric orbits.

Companions are likely low-mass, unevolved objects.

Abstract

While the first binary post-AGB stars were serendipitously discovered, the distinct characteristics of their Spectral Energy Distribution (SED) allowed us to launch a more systematic search for binaries. We selected post-AGB objects which show a broad dust excess often starting already at H or K, pointing to the presence of a gravitationally bound dusty disc in the system. We started a very extensive multi-wavelength study of those systems and here we report on our radial velocity and photometric monitoring results for six stars of early F type, which are pulsators of small amplitude. To determine the radial velocity of low signal-to-noise time-series, we constructed dedicated auto-correlation masks. The radial velocity variations were subjected to detailed analysis to differentiate between pulsational variability and variability due to orbital motion. Finally orbital minimalisation was performed to constrain the orbital elements. 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 solar mass and the companions are likely unevolved objects of (very) low initial mass. We argue that these binaries must have been subject to severe 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 be formed during this strong interaction phase. The eccentric orbits of these highly evolved objects remain poorly understood. With the measured orbits and mass functions we conclude that the circumbinary discs seem to have a major impact on the evolution of a significant fraction of binary systems.