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

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.

• The paper proposes that a previously undetected binary companion, $\alpha$ Orionis B, is the origin of Betelgeuse's 2100-day Long Secondary Period.
• The study estimates the companion $\alpha$ Ori B has a mass around 1.17 $M_\odot$ and influences Betelgeuse's light curve by affecting its circumstellar dust environment.
• The existence of this companion could significantly impact Betelgeuse's mass loss, evolutionary path towards supernova, and provides targets for future observational confirmation.

Binarity and the Long Secondary Period of Betelgeuse

The research paper titled "A Buddy for Betelgeuse: Binarity as the Origin of the Long Secondary Period in $\alpha$ Orionis," authored by Jared A. Goldberg and collaborators, explores the intriguing possibility that Betelgeuse's Long Secondary Period (LSP) is caused by a binary companion. This paper presents a compelling case for the presence of a previously undetected companion star, referred to as $\alpha$ Orionis B, and investigates the implications this has for understanding the variability and ultimate fate of Betelgeuse, one of the most well-studied red supergiants.

Overview of Long Secondary Periods (LSPs)

LSPs are a class of variability observed in several types of cool, evolved stars, including red giants and red supergiants. These periods are typically a few times longer than the primary pulsation periods of the stars. For Betelgeuse, the LSP is approximately 2100 days, juxtaposed against its more commonly understood fundamental radial pulsation period of about 400 days. The cause of LSPs has been a longstanding mystery, with various hypotheses including giant convective cells, non-radial oscillations, and the presence of binary companions.

The Case for a Binary Companion

The notion of a binary companion offers an enticing explanation for Betelgeuse's LSP. The authors argue that a low-mass companion star, $\alpha$ Ori B, influences the circumstellar environment of Betelgeuse, particularly dust, which modulates the star's light curve. This perspective diverges from earlier theories that required the companion itself to cause dimming by dragging a dust cloud in front of Betelgeuse from the observer's perspective.

The authors derive several properties of $\alpha$ Ori B using available RV data and lightcurve analysis, estimating its mass to be around $1.17 \, M_\odot$ and its orbital separation to be about $1850 \pm 70 \, R_\odot$, roughly 2.43 times Betelgeuse's radius. These parameters provide a framework for understanding the orbital dynamics and potential impact of such a companion on Betelgeuse's circumstellar environment.

Implications of the Binary Hypothesis

The existence of a binary companion could significantly affect Betelgeuse's future evolution. The gravitational interaction with a companion could influence the mass loss rates and geometry of mass loss, potentially affecting Betelgeuse's path towards its eventual supernova phase. Additionally, the findings imply that the frequent variability phenomena like the Great Dimming of 2020 could be tied to phases in the binary orbit, providing deeper insights into the circumstellar dynamics at play.

Methodological and Observational Considerations

The study uses an array of observational techniques, including radial velocity (RV) measurements and photometric light curves, to substantiate the binary hypothesis. The paper highlights the importance of future observations, particularly targeting radio interferometry and UV spectroscopic data, to further tangle out the presence and properties of $\alpha$ Ori B. Upcoming epochs when $\alpha$ Ori B might be most separable from Betelgeuse have been projected to facilitate targeted observation campaigns.

Conclusion

The prospect of Betelgeuse being in a binary system fundamentally shifts our understanding of its behavior and lifecycle. While the existence and properties of $\alpha$ Ori B require further confirmation, the hypothesis provides a coherent framework explaining Betelgeuse's LSP, with far-reaching implications for studying other long-period variables and predicting the supernova characteristics of red supergiants. The study lays a foundation for future observational strategies and theoretical models, challenging researchers to refine our understanding of stellar evolution in complex systems.