Does Betelgeuse even rotate? Probably not

Betelgeuse is a well-known red giant star at the corner of Orion’s constellation. The name translated in some languages ​​means 'armpit of the elder’ among all star names, which is great! Betelgeuse has been captivating observers of late only because it unexpectedly dimmed a few years ago, but a recent study shows its fastest rotation speed, compared to other supergiants, has never been seen before.

One of the brightest stars in the Northern Hemisphere sky, actually the tenth brightest, Betelgeuse has a stunning red hue. It is a semi-regular variable star, meaning that there is some regularity in its variable light output, but there are occasions when the variable is interrupted between 20 and 2000 days. If Betelgeuse were placed in the position of the Sun, its visible surface would extend beyond the orbit of Mars and swallow everything in between.

1998/9 UV HST images of Betelgeuse showing asymmetric pulsations with associated spectral line profiles (Credit: STScI, NASA, ESA)

Like all stars, Betelgeuse rotates but a recent study using the Atacama Large Millimeter Array (ALMA) shows that Betelgeuse rotates faster than expected. Cool stars like Betelgeuse expand as they form and must rotate slowly to conserve momentum. Mass loss from the interstellar wind can further reduce the rotation speed. Current theory predicts that red giants rotate at 1 km/s, while red supergiants rotate at less than 0.1 km/s.

Image showing the two receivers of the ALMA array in the Atacama Desert.
Two Atacama Large Millimeter/Submillimeter Array (ALMA) 12-meter antennas (Credit: Iztok Bon?ina/ESO)

Current theory aside, it appears that there have been many observations of at least a few hundred giant stars rotating rapidly. Betelgeuse in particular has a faster than expected rotation. Somewhat useful, it is so close to Earth that its surface can be resolved and accurate measurements can be taken. Measurements show that half of the visible hemisphere turns blue and the other half turns red. This information can be used to accurately calculate a rotational speed.

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When arriving at Betelgeuse, the radial velocity was measured with ALMA to be 5.47 km/s. This value was compared with previous observations using the Hubble Space Telescope and fortunately it agreed. A leading theory takes binary stellar evolution as a possible cause and specifically merges with a low-mass companion star. This is not an uncommon process, as about a third of red supergiants experience interstellar mergers before their core collapses, marking the end of their lives. For red giants, the team considered the effect of merging with planetary systems on rotational speed.

There were problems with getting enough data, however, but the team designed 3D radiative hydrodynamic simulations of red supergiants with properties similar to those of Betelgeuse. Throwing a proverbial spanner in the works, the team suggests that the observations may be wrong and the false signals may have been picked up from the scattering of convective plasma at the surface rather than the rotation of the star!

In an effort to determine whether they could accurately measure the rotation speeds of red giants and supergiants, they had to develop new processing techniques to establish predictions comparable to observations of Betelgeuse. The team ultimately concludes that to establish beyond doubt that Betelgeuse and other red supergiants are rapidly rotating, higher resolution observations are needed than current technology can reliably provide.

Source: Does Betelgeuse Really Spin? Synthetic ALMA observations of large-scale convection in 3D simulations of red supergiants

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