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

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.

💡 Research Summary

This paper presents the third installment in a series devoted to the orbital properties of binary systems containing M‑type giant stars. By exploiting a newly derived set of orbital solutions for 135 Hipparcos‑identified M‑giant binaries, the authors construct, for the first time, a statistically robust eccentricity–log period (e‑log P) diagram for this class. The diagram reveals a clear empirical boundary defined by e < 0.4 log P − 1 (with P expressed in days). Systems that fall below this line are overwhelmingly those known to have undergone mass transfer – namely barium stars, Tc‑poor S‑type stars, and a subset of open‑cluster K‑giant binaries.

A key result is that the orbital elements of post‑AGB binaries are virtually indistinguishable from those of the M‑giant sample. This suggests that, for the objects considered, the transition from the asymptotic giant branch (AGB) to the post‑AGB phase occurred early, around spectral type M4, before the stars expanded to the extreme radii typical of later AGB evolution. Consequently, the mass‑loss episode that produced the post‑AGB star did not dramatically reshape the orbit.

Equally striking is the similarity between the orbital distributions of M‑giants and post‑mass‑transfer systems such as barium stars. Classical binary‑evolution theory predicts that Roche‑lobe overflow or common‑envelope evolution should substantially circularize or shrink the orbit. The data, however, show that the eccentricities and periods of barium and S‑type systems occupy the same region of the e‑log P plane as the unevolved M‑giants, implying that the mass‑transfer process was either relatively gentle (e.g., wind‑accretion with modest angular‑momentum loss) or highly non‑conservative, allowing the orbit to retain its original shape.

The authors also analyse the mass‑function distribution f(M) for the whole sample. For systems that satisfy the e < 0.4 log P − 1 condition, the derived companion masses cluster around 0.6 M☉, precisely the expected mass range for white dwarfs. This peak is reproduced in the open‑cluster K‑giant binaries, where a lower limit on the companion mass can be set from the cluster’s turn‑off mass. Hence, the combination of the e‑log P boundary and the mass‑function peak provides a powerful diagnostic for identifying post‑mass‑transfer binaries.

From an evolutionary standpoint, the findings challenge the prevailing view that mass transfer dramatically reshapes binary orbits. Instead, the data support a scenario in which many M‑giant binaries evolve into post‑AGB, barium, or S‑type systems with only modest orbital alteration. The early termination of AGB evolution, perhaps triggered by binary interaction, appears to be a common pathway.

In summary, the paper delivers three major contributions: (1) a comprehensive e‑log P diagram for M‑giant binaries, (2) evidence that post‑AGB and post‑mass‑transfer binaries share orbital characteristics with their unevolved counterparts, and (3) an empirical criterion (e < 0.4 log P − 1) that reliably isolates systems whose companions are likely white dwarfs. These results provide new constraints for binary‑evolution models, especially concerning the efficiency of angular‑momentum loss during wind‑driven or Roche‑lobe mass transfer, and they lay the groundwork for future spectroscopic and astrometric studies aimed at directly confirming the nature of the unseen companions.