Evidence for the Keplerian orbit of a close companion around a giant star

TL;DR

This paper provides direct observational evidence of a close-in companion orbiting an AGB star using (sub)millimeter spectroscopy, revealing Keplerian motion and offering insights into binary evolution and tidal interactions.

Contribution

It presents the first direct detection of a close companion around an AGB star through multi-epoch (sub)millimeter interferometry, demonstrating the method's effectiveness.

Findings

Detected Keplerian orbit of a companion around pi1 Gruis

Companion likely a main-sequence star with a circular orbit

Highlights potential underestimation of circularization rates in models

Abstract

Close companions influence stellar evolution through tidal interactions, mass transfer, and mass loss effects. While such companions are detected around young stellar objects, main-sequence stars, red giants, and compact objects, direct observational evidence of close-in companions around asymptotic giant branch (AGB) stars has remained elusive. Here, we present (sub)millimeter time-domain imaging spectroscopy revealing the Keplerian motion of a close-in companion around the AGB star pi1 Gruis. The companion, slightly more massive than the AGB star, is likely a main-sequence star. Unlike more evolved stars with companions at comparable distances, pi1 Gru's companion follows a circular orbit, suggesting an eccentricity-generating mechanism late- or post-AGB. Our analysis suggests that model-predicted circularization rates may be underestimated. Our results highlight the potential of multi-epoch (sub)millimeter interferometry in detecting the Keplerian motion of close companions to giant stars and open avenues for our understanding of tidal interaction physics and binary evolution.