A Buddy for Betelgeuse: Binarity as the Origin of the Long Secondary Period in $\alpha$ Orionis

TL;DR

This paper proposes that Betelgeuse's long secondary period is caused by a low-mass companion, predicting its existence and characterizing its properties to explain observed lightcurve and radial velocity variations.

Contribution

It introduces a binary companion hypothesis for Betelgeuse's LSP, providing detailed predictions of its mass, orbit, and observational signatures, challenging previous explanations.

Findings

Predicted a low-mass companion, Betelgeuse B, with specific orbital parameters.

Demonstrated consistency of the binary model with existing observational data.

Identified observational tests to confirm the companion's existence.

Abstract

We predict the existence of $\alpha$ Ori B, a low-mass companion orbiting Betelgeuse. This is motivated by the presence of a 2170-day Long Secondary Period (LSP) in Betelgeuse's lightcurve, a periodicity $\approx5$ times longer than the star's 416 day fundamental radial pulsation mode. While binarity is currently the leading hypothesis for LSPs in general, the LSP and the radial velocity variation observed in Betelgeuse, taken together, necessitate a revision of the prevailing physical picture. The lightcurve-RV phase difference requires a companion to be behind Betelgeuse at the LSP luminosity minimum, 180 degrees out of phase with the system orientation associated with occultation. We demonstrate the consistency of this model with available observational constraints and identify tensions in all other proposed LSP hypotheses. Within this framework, we calculate a mass for $\alpha$ Ori B of $M\sin i =1.17\pm0.7\,M_\odot$ and an orbital separation of $1850\pm70\,R_\odot$, or $2.43^{+0.21}_{-0.32}$ times the radius of Betelgeuse. We then describe the features of the companion as constrained by the fundamental parameters of Betelgeuse and its orbital system, and discuss what would be required to confirm the companion's existence observationally.