Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) and NSF’s Karl G. Jansky Very Large Array (VLA) have created the most detailed map yet of the atmosphere of Antares, one of the largest and brightest stars visible to naked eye in the night sky.
Radio images of Antares with ALMA and the VLA. ALMA observed Antares close to its surface in shorter wavelengths, and the longer wavelengths observed by the VLA revealed the star’s atmosphere further out. In the VLA image a huge wind is visible on the right, ejected from Antares and lit up by its smaller but hotter companion star Antares B. Image credit: ALMA / ESO / NAOJ / NRAO / E. O’Gorman / AUI / NSF / S. Dagnello.
Antares is located approximately 554 light-years away in the constellation of Scorpius.
Also known as Alpha Scorpii, it appears as a single star when viewed with the naked eye, but it is actually a binary star system.
The brighter of the pair, Antares A (Alpha Scorpii A), is the red supergiant, while the fainter, Antares B (Alpha Scorpii B), is a hot main sequence star.
The exact size of Antares A remains uncertain, but if placed at the center of the Solar System, it would reach to somewhere between the orbits of Mars and Jupiter. Its mass is calculated to be around 12 times that of the Sun.
In a new study, Dublin Institute for Advanced Studies astronomer Eamon O’Gorman and colleagues used ALMA and VLA to create a radio map of the binary system.
ALMA observed Antares close to its surface — its optical photosphere — in shorter wavelengths; the longer wavelengths observed by the VLA revealed the star’s atmosphere further out.
As seen in visible light, Antares A’s diameter is about 700 times larger than the Sun.
But when ALMA and the VLA revealed its atmosphere in radio light, the supergiant turned out to be even more gigantic.
“The size of a star can vary dramatically depending on what wavelength of light it is observed with,” Dr. O’Gorman said.
“The longer wavelengths of the VLA revealed the supergiant’s atmosphere out to nearly 12 times the star’s radius.”
An artist’s impression of the atmosphere of Antares. As seen with the naked eye (up until the photosphere), Antares is around 700 times larger than our sun, big enough to fill the Solar System beyond the orbit of Mars (Solar System scale shown for comparison). But ALMA and VLA showed that its atmosphere, including the lower and upper chromosphere and wind zones, reaches out 12 times farther than that. Image credit: NRAO / AUI / NSF / S. Dagnello.
The astronomers also measured the temperature of most of the gas and plasma in the atmosphere of Antares A. Most noticeable was the temperature in its chromosphere.
Thanks to ALMA and the VLA, they discovered that the star’s chromosphere extends out to 2.5 times the star’s radius.
They also found that the temperature of the chromosphere is lower than previous optical and ultraviolet observations have suggested.
The temperature peaks at 3,500 degrees Celsius (6,400 degrees Fahrenheit), after which it gradually decreases. As a comparison, the Sun’s chromosphere reaches temperatures of almost 20,000 degrees Celsius (36,032 degrees Fahrenheit).
“We found that the chromosphere is ‘lukewarm’ rather than hot, in stellar temperatures,” Dr. O’Gorman said.
“The difference can be explained because our radio measurements are a sensitive thermometer for most of the gas and plasma in the star’s atmosphere, whereas past optical and ultraviolet observations were only sensitive to very hot gas and plasma.”
“We think that red supergiant stars, such as Antares and Betelgeuse, have an inhomogeneous atmosphere,” said Dr. Keiichi Ohnaka, an astronomer at the Universidad Católica del Norte.
“Imagine that their atmospheres are a painting made out of many dots of different colors, representing different temperatures.”
“Most of the painting contains dots of the lukewarm gas that radio telescopes can see, but there are also cold dots that only infrared telescopes can see, and hot dots that UV telescopes see. At the moment we can’t observe these dots individually, but we want to try that in future studies.”
In the ALMA and VLA data, astronomers for the first time saw a clear distinction between the chromosphere and the region where winds start to form.
In the VLA image, a huge wind is visible, ejected from Antares A and lit up by its smaller but hotter companion star Antares B.
“When I was a student, I dreamt of having data like this,” said Dr. Graham Harper, an astronomer at the University of Colorado, Boulder.
“Knowing the actual sizes and temperatures of the atmospheric zones gives us a clue of how these huge winds start to form and how much mass is being ejected.”
The findings were published in the journal Astronomy & Astrophysics.
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E. O’Gorman et al. 2020. ALMA and VLA reveal the lukewarm chromospheres of the nearby red supergiants Antares and Betelgeuse. A&A 638, A65; doi: 10.1051/0004-6361/202037756
