Is Betelgeuse Really Rotating? Synthetic ALMA Observations of Large-scale Convection in 3D Simulations of Red Supergiants

Abstract: The evolved stages of massive stars are poorly understood, but invaluable constraints can be derived from spatially resolved observations of nearby red supergiants, such as Betelgeuse. Atacama Large Millimeter/submillimeter Array (ALMA) observations of Betelgeuse showing a dipolar velocity field have been interpreted as evidence for a projected rotation rate of about $5\, \mathrm{km\, s^{-1}}$. This is 2 orders of magnitude larger than predicted by single-star evolution, which led to suggestions that Betelgeuse is a binary merger. We propose instead that large-scale convective motions can mimic rotation, especially if they are only partially resolved. We support this claim with 3D CO5BOLD simulations of nonrotating red supergiants that we postprocessed to predict ALMA images and SiO spectra. We show that our synthetic radial velocity maps have a 90% chance of being falsely interpreted as evidence for a projected rotation rate of $\mathrm{2\, km\, s^{-1}}$ or larger for our fiducial simulation. We conclude that we need at least another ALMA observation to firmly establish whether Betelgeuse is indeed rapidly rotating. Such observations would also provide insight into the role of angular momentum and binary interaction in the late evolutionary stages. The data will further probe the structure and complex physical processes in the atmospheres of red supergiants, which are immediate progenitors of supernovae and are believed to be essential in the formation of gravitational-wave sources.

• The paper shows that large-scale convective motions can mimic rotational signals in Betelgeuse’s atmosphere, challenging prior binary merger interpretations.
• It employs 3D CO5BOLD simulations to create synthetic ALMA observations, revealing a 90% chance that convective cells may be mistaken for rotation.
• The findings underscore the need for higher-resolution ALMA imaging to accurately differentiate between convective dynamics and genuine stellar rotation.

Synthetic ALMA Observations of Red Supergiants: Betelgeuse's Illusion of Rotation

The paper "Is Betelgeuse Really Rotating? Synthetic ALMA Observations of Large-Scale Convection in 3D Simulations of Red Supergiants," published in The Astrophysical Journal Letters, addresses the intriguing discrepancy between the observed rotational velocities of Betelgeuse, as seen through ALMA, and the predicted rotational velocities from single-star evolutionary models. This study, involving researchers like Jing-Ze Ma and Andrea Chiavassa, provides a compelling argument about the role of large-scale convection in red supergiants (RSGs) that could mimic rotational signatures.

Background and Motivation

Betelgeuse, a well-known red supergiant, has been at the forefront of astronomical investigations due to its proximity and peculiar behaviors, notably the recent "Great Dimming" event. Historic observations have indicated a higher-than-expected rotation rate of approximately 5 km/s, which starkly contrasts with the predictions of single-star evolutionary models which suggest much lower values for RSGs. Previous interpretations posited a binary merger as a possible cause for this anomaly, which suggests Betelgeuse could be the result of stellar binary interaction.

However, this paper offers a hypothesis that large-scale convective motions can produce similar rotational signatures when observed with instruments like ALMA. These motions create a dipolar velocity field that might be misinterpreted as rotational velocity in spatially resolved images.

Methodology and Simulations

The research takes advantage of 3D CO5BOLD simulations to mimic the atmospheric dynamics of non-rotating red supergiants, which are subsequently processed to predict ALMA observations and SiO spectra. The study effectively employs a synthetic observation technique, wherein the simulated atmospheric data is manipulated to create maps comparable to ALMA's observations.

A notable finding through these simulations is that, in 90% of cases, the synthetic radial velocity maps could be erroneously construed as indicative of a high rotation rate of at least 2 km/s due to convective motions. This probability highlights the potential for significant observational misinterpretations.

Key Findings

The study demonstrates a plausible scenario where large-scale convective cells—and not rotational dynamics—account for the observed dipolar velocity fields in Betelgeuse’s atmosphere. Convective cells can span a significant fraction of the stellar surface and, if only partially resolved, mimic the rotational motions erroneously inferred from observational data.

Implications and Future Directions

The results underscore the need for additional epochs of precise ALMA observations, including higher spatial resolution imaging, to disentangle the complex interplay of convection and rotation in RSGs accurately. Such advancements will improve our understanding of angular momentum transfer processes in these evolved stars and their evolutionary histories.

These findings also have broader implications for our comprehension of red supergiants as precursors to supernovae and gravitational wave sources. The characteristics of the convective motions affecting rotational interpretations may prove crucial in forecasting the fate of such massive stellar bodies.

Future developments in ALMA's capability, such as enhanced resolution anticipated from extending baseline arrays, could yield more discernible insights into Betelgeuse’s atmospheric dynamics. If the findings of this study are corroborated with more refined observations, it may necessitate a reevaluation of the assumptions underlying the rotational dynamics of red supergiants. This could further stimulate theoretical refinement in stellar evolution models, particularly those accounting for convective behaviors.

In summary, this paper presents a strong case for reconsidering the interpretation of rotational signatures in red supergiant observations by emphasizing the impact of convective processes, paving the way for richer, more nuanced understandings of these celestial phenomena.