A new look at Betelgeuse’s boiling surface

A new look at Betelgeuse’s boiling surface
Betelgeuse is a well-known red supergiant star in the constellation Orion. Recently, it has attracted a lot of attention, not only because variations in its brightness have led to speculation about an imminent explosion, but also because observations have indicated that it is spinning much faster than expected. This latter interpretation is now being questioned by an international team led by astronomers from the Max Planck Institute for Astrophysics, the Lagrange laboratory (Observatoire de la Côte d’Azur-Université Côte d’Azur-CNRS) and the Université from Uppsala. The study suggests that Betelgeuse’s bubbling surface could be mistaken for a rotation, even with the most advanced telescopes. Other astronomers are actively analyzing new observational data to test such hypotheses.

As one of the brightest stars in the northern hemisphere, Betelgeuse can be easily spotted with the naked eye in the constellation Orion. Betelgeuse is one of the largest known stars. With a diameter of more than a billion kilometers, it is almost 1000 times larger than the Sun. If it had been in our solar system, it would have engulfed the Earth with an atmosphere that reached the orbit of Jupiter. A star this size is not expected to rotate quickly. As they evolve, most stars expand and slow down to maintain their angular momentum. However, recent observations seem to suggest that Betelgeuse rotates at speeds close to 5 km/s, or two orders of magnitude faster than a single evolved star would be expected to rotate.

The most obvious evidence of Betelgeuse’s rotation was provided by the large radio interferometer ALMA (Atacama Large Millimeter/submillimeter Array). ALMA’s 66 antennas work together as if it were a single giant telescope. They use a technique known as interferometry, in which two or more antennas pick up a signal and join forces to analyze the signal and obtain information about its transmitting source. Thanks to this technique, astronomers have highlighted
a dipole map of radial velocities emanating from the outer layers of Betelgeuse: half of the star appears to be moving closer to us, while the other half appears to be moving away. This observation, along with previous studies, led to the interpretation that Betelgeuse is rapidly rotating.

This interpretation would have been obvious if Betelgeuse were a perfectly round sphere. However, the surface of Betelgeuse is a vibrant world, governed by a physical process called convection. We can observe convection in our daily lives when we boil water, but in Betelgeuse this process is much more violent: bubbles of boiling gas can be as large as Earth’s orbit around the Sun and cover a large part of the surface of Betelgeuse. They rise and fall at speeds of up to 30 km/s, faster than any crewed spacecraft.
An international team led by Jing-Ze Ma, doctoral student at the Max Planck Institute for Astrophysics, and of which Andrea Chiavassa, CNRS researcher, is a member, offers another explanation for the ALMA observations: the bubbling surface of Betelgeuse imitates the rotation . Due to the limited resolution of the ALMA telescope, such convective motions would be blurred in observations, resulting in a map of dipole velocities.

The team developed new post-processing software to produce synthetic ALMA images and submillimeter spectra from their hydrodynamic simulations of red supergiant stars. In 90% of simulations, observations of the star would be interpretable as rotating at several km/s due to large-scale convective motions of the surface. Movements that are not clearly visible with the ALMA telescope. More observations are needed to better assess Betelgeuse’s rapid rotation, and the team made predictions for future observations with higher spatial resolution. Fortunately, other astronomers have already made higher-resolution observations of Betelgeuse in 2022. The new data is being analyzed, which will help verify the predictions and reveal Betelgeuse’s mask.

source: https://www.mpa-garching.mpg.de/1094283/hl202403