Another change has been noticed on the boiling surface of Betelgeuse
Betelgeuse has gained a lot of attention recently, not only because variations in its brightness have led to speculation that an explosion could be imminent, but also because observations indicate that it is rotating much faster than expected.
This last interpretation, of Betelgeuse running faster than expected, is now called into question by an international team led by astronomers from the Max Planck Institute of Astrophysics, which proposes that the boiling surface of Betelgeuse can be confused with rotation even in the most advanced telescopes. Other astronomers are actively analysing new observation data to test these hypotheses.
Being one of the brightest stars in the Northern Hemisphere, Betelgeuse can be easily found with the naked eye in the constellation of Orion. This is one of the biggest stars known.
With a diameter of more than a billion km, it is almost 1000 times larger than the Sun. If it were in our Solar System, it would have swallowed the Earth with an atmosphere that would reach Jupiter.
Such a big star is not supposed to rotate this quickly
In their evolution, most stars expand and rotate downwards to preserve the angular momentum. However, recent observations suggest that Betelgeuse is rotating very fast (at 5 km/s), two orders of magnitude faster than an evolved star should rotate.
The most prominent proof of Betelgeuse rotation came from the Atacama Large Millimeter/submillimeter Array (ALMA). ALMA's 66 antennas work together as if they were a single giant telescope.
These use a technique known as interferometry, in which two or more antennas capture a signal from the Universe and join forces to analyse the signal and obtain information about its emission source.
Despite the techniques of ALMA, it is still difficult to clearly understand what is happening on the surface of Betelgeuse.
Using this technique, astronomers discovered a dipolar radial velocity map in the outer layer of Betelgeuse: half of the star seems to be approaching us and the other half seems to be moving away.
Thus, this observation, along with previous studies, led to the interpretation that Betelgeuse is rotating quickly. This interpretation would have been clear if Betelgeuse had been 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: boiling bubbles can be as large as the Earth's orbit around the Sun, covering a large fraction of the Betelgeuse surface. They go up and down at a speed of up to 30 km/s, faster than any manned spacecraft.
The boiling surface of Betelgeuse mimics rotation
Based on this physical framework, an international team led by Jing-Ze Ma, a doctoral student at the Max Planck Institute of Astrophysics, now offers an alternative explanation for the Betelgeuse dipolar velocity map: the boiling surface of Betelgeuse mimics rotation.
A group of boiling bubbles rises on one side of the star and another group of bubbles sinks on the other side. Due to the limited resolution of the ALMA telescope, these convective movements would be blurred in real observations, which would result in the dipolar speed map.
The team developed a new post-processing package to produce synthetic ALMA images and submillimetric spectra from their 3D hydrodynamic simulations of red supergiant stars without rotation.
In 90% of the simulations, the star would be interpreted as rotating at several km/s simply due to large-scale boiling movements on the surface that are not clearly seen in the ALMA telescope.
More observations are needed to better evaluate the rapid rotation of Betelgeuse, and the team made predictions for future observations with higher spatial resolution. Fortunately, other astronomers have already made observations of higher Betelgeuse resolution in 2022. The new data is being analysed at this time, which will put the forecasts to the test and help unravel the Betelgeuse mask.
News reference:
Ma J., Chiavassa A., Mink S., et al. Is Betelgeuse Really Rotating? Synthetic ALMA Observations of Large-scale Convection in 3D Simulations of Red Supergiants. The Astrophysical Journal Letters (2024).