On the Eccentricity Excitation in Post-Main Sequence Binaries

Abstract

Several classes of stellar binaries with post-main sequence (post-MS) components - millisecond pulsars with the white dwarf companions (MSP+WD) and periods P_b~30 d, binaries hosting post-asymptotic giant branch (post-AGB) stars or barium stars with P_b~several yr - feature high eccentricities (up to 0.4) despite the expectation of their efficient tidal circularization during the AGB phase. It was suggested that the eccentricities of these binaries can be naturally excited by their tidal coupling to the circumbinary disk, formed by the material ejected from the binary. Here we critically reassess this idea using simple arguments rooted in the global angular momentum conservation of the disk+binary system. Compared to previous studies we (1) fully account for the viscous spreading of the circumbinary disk, (2) consider the possibility of reaccretion from the disk onto the binary (in agreement with simulations and empirical evidence), and (3) allow for the reduced viscosity after the disk expands, cools, and forms dust. These ingredients conspire to significantly lower the efficiency of eccentricity excitation by the disk tides. We find that explaining eccentricities of the post-AGB binaries is difficult and requires massive ($> 10^{-2}M_\odot$), long-lived ($> 10^5$ yr) circumbinary disks, that do not reaccrete. Reaccretion is detrimental for the eccentricity growth also in the MSP+WD systems. Reduced efficiency of the disk-driven excitation motivates study of alternative mechanisms for producing the peculiar eccentricities of the post-MS binaries.