Astronomers have used the Atacama Large Millimeter/submillimeter Array (ALMA) to observe the nearby supergiant star Betelgeuse. Beyond just delivering a spectacularly detailed image, ALMA has provided unique information about Betelgeuse’s extended atmosphere.
This image from ALMA shows the M2 spectral-type supergiant star Betelgeuse. Image credit: ALMA / ESO / NAOJ / NRAO / E. O’Gorman / P. Kervella.
Betelgeuse, the second brightest star in the constellation Orion, is a red supergiant located approximately 650 light-years from Earth.
With a radius around 1,400 times larger than the Sun’s in the millimeter continuum, Betelgeuse is one of the biggest stars known. It is also one of the most luminous stars known, emitting more light than 100,000 Suns.
Such extreme properties foretell the demise of a short-lived stellar king.
With an age of only 8 million years, Betelgeuse is already nearing the end of its life and is soon doomed to explode as a supernova. When it does, the supernova should be seen easily from Earth, even in broad daylight.
The relative proximity of Betelgeuse allows scientists to study its atmosphere in great detail.
In 2009, two teams of astronomers discovered a vast plume of gas almost as large as our Solar System.
The researchers also found a gigantic bubble that boils away on Betelgeuse’s surface.
These features help to explain how the star is shedding gas and dust at tremendous rates.
In the millimeter continuum Betelgeuse is around 1,400 times larger than our Sun. The overlaid annotation shows how large the star is compared to the Solar System. Betelgeuse would engulf all four terrestrial planets — Mercury, Venus, Earth and Mars — and even the gas giant Jupiter; only Saturn would be beyond its surface. Image credit: ALMA / ESO / NAOJ / NRAO / E. O’Gorman / P. Kervella.
Now Dublin Institute for Advanced Studies astronomer Dr. Eamon O’Gorman and co-authors have used ALMA to observe the hot gas of Betelgeuse’s lower chromosphere at sub-millimeter wavelengths — where localized increased temperatures explain why it is not symmetric.
“The chromosphere of Betelgeuse has a lower opacity than our Sun and so our ALMA observations are most likely probing very close to the temperature minimum,” the researchers said.
The ALMA observations show that Betelgeuse’s atmosphere has a mean temperature of 2,760 degrees Kelvin at 1.3 stellar radii, which is below both the photospheric effective temperature (3,690 degrees Kelvin) and the temperatures at 2 stellar radii.
“This is unambiguous proof for the existence of an inversion of the mean temperature in the atmosphere of a red supergiant,” Dr. O’Gorman and colleagues said.
“The emission in the star’s atmosphere is clearly not spherically symmetric with two notable deviations from a uniform disk detected in both the images and visibilities.”
“The most prominent asymmetry is located in the north-east quadrant of the disk and is spatially resolved showing it to be highly elongated with an axis-ratio of 2.4 and occupying 5% of the disk. Its temperature is approximately 1,000 degrees Kelvin above the measured mean temperature at 1.3 stellar radii.”
“The other main asymmetry is located on the disk limb almost due east of the disk center and occupies 3% of the disk projected area.”
“Both emission asymmetries are clear evidence for localized heating taking place in the atmosphere of Betelgeuse,” the astronomers said.
They added: “given that Betelgeuse has a longitudinal magnetic field strength of 1G, which is similar to the solar value, and it is predicted to harbor local regions of strong magnetic fields, it is possible that the sub-mm asymmetries are related to magnetic activity.”
“Near-infrared interferometric observations suggest there are large convection cells on the surface of Betelgeuse. These cells could sustain a local magnetic field.”
“Indeed, a bright spot linked to a large convection cell has been reported in the northeast quadrant of Betelgeuse’s photosphere less than one month prior to our ALMA observations, which is tantalizing evidence for a link between photospheric convection and heating in the atmosphere of a red supergiant,” the authors concluded.
The research is published in the journal Astronomy & Astrophysics (arXiv.org preprint).
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E. O’Gorman et al. 2017. The inhomogeneous submillimeter atmosphere of Betelgeuse. A&A 602, L10; doi: 10.1051/0004-6361/201731171
