Spectroscopic binaries among Hipparcos M giants III. The eccentricity-period diagram and mass-transfer signatures

TL;DR

This study analyzes the orbital properties of M giant binaries, revealing insights into their evolutionary stages, mass transfer effects, and the characteristics of post-mass-transfer systems like barium stars.

Contribution

It constructs the first (e-logP) diagram for M giant binaries and compares their properties with related binary systems, providing new evolutionary constraints.

Findings

Post-AGB and M giant binaries have similar orbital elements.

Mass transfer does not significantly alter orbital elements.

Systems with e < 0.4 log P are mainly post-mass-transfer, with white dwarf companions.

Abstract

This paper is the third one in a series devoted to studying the properties of binaries involving M giants. We use a new set of orbits to construct the first (e-logP) diagram of an extensive sample of M giant binaries, to obtain their mass-function distribution, and to derive evolutionary constraints for this class of binaries and related systems. The orbital properties of binaries involving M giants were analysed and compared with those of related families of binaries (K giants, post-AGB stars, barium stars, Tc-poor S stars). The orbital elements of post-AGB stars and M giants are not different, which may very indicate that, for the considered sample of post-AGB binaries, the post-AGB star left the AGB at quite an early stage (M4 or so). Neither are the orbital elements of post-mass-transfer binaries like barium stars very different from those of M giants, suggesting that the mass transfer did not alter the orbital elements much, contrary to current belief. Finally, we show that binary systems with e < 0.4 log P - 1 (with periods expressed in days) are predominantly post-mass-transfer systems, because (i) the vast majority of barium and S systems match this condition, and (ii) these systems have companion masses peaking around 0.6 solar mass, as expected for white dwarfs. The latter property has been shown to hold as well for open-cluster binaries involving K giants, for which a lower bound on the companion mass may easily be set.