Radial Velocity and Astrometric Evidence for a Close Companion to Betelgeuse

TL;DR

This study presents evidence suggesting Betelgeuse has a low-mass, nearly hidden companion star in a close orbit, which influences its observed variability and rapid rotation, indicating a binary system.

Contribution

It provides the first combined radial velocity and astrometric evidence for a close, low-mass companion to Betelgeuse, explaining its spin and variability.

Findings

Betelgeuse likely has a low-mass companion less than a solar mass.

The companion orbits at a 2,110-day period just over twice Betelgeuse's radius.

Future orbit decay will lead to the companion being swallowed within 10,000 years.

Abstract

We examine a century of radial velocity, visual magnitude, and astrometric observations of the nearest red supergiant, Betelgeuse, in order to reexamine the century-old assertion that Betelgeuse might be a spectroscopic binary. These data reveal Betelgeuse varying stochastically over years and decades due to its boiling, convective envelope, periodically with a $ 5.78$~yr long secondary period, and quasi-periodically from pulsations with periods of several hundred days. We show that the long secondary period is consistent between astrometric and RV datasets, and argue that it indicates a low-mass companion to Betelgeuse, less than a solar mass, orbiting in a 2,110 day period at a separation of just over twice Betelgeuse's radius. The companion star would be nearly twenty times less massive and a million times fainter than Betelgeuse, with similar effective temperature, effectively hiding it in plain sight near one of the best-studied stars in the night sky. The astrometric data favor an edge-on binary with orbital plane aligned with Betelgeuse's measured spin axis. Tidal spin-orbit interaction drains angular momentum from the orbit and spins up Betelgeuse, explaining the spin--orbit alignment and Betelgeuse's anomalously rapid spin. In the future, the orbit will decay until the companion is swallowed by Betelgeuse in the next 10,000 years.