From ALMA: “Supergiant Atmosphere of Antares Revealed by Radio Telescopes”

ESO/NRAO/NAOJ ALMA Array in Chile in the Atacama at Chajnantor plateau, at 5,000 metres

From ALMA

Nicolás Lira
Education and Public Outreach Coordinator
Joint ALMA Observatory, Santiago – Chile
Phone: +56 2 2467 6519
Cell phone: +56 9 9445 7726
Email: nicolas.lira@alma.cl

Masaaki Hiramatsu
Education and Public Outreach Officer, NAOJ Chile
Observatory
, Tokyo – Japan
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Email: hiramatsu.masaaki@nao.ac.jp

Bárbara Ferreira
ESO Public Information Officer
Garching bei München, Germany
Phone: +49 89 3200 6670
Email: pio@eso.org

Iris Nijman
Public Information Officer
National Radio Astronomy Observatory Charlottesville, Virginia – USA
Cell phone: +1 (434) 249 3423
Email: alma-pr@nrao.edu

From NRAO.

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Radio images of Antares with ALMA [above] and the VLA.

NRAO/Karl V Jansky Expanded Very Large Array, on the Plains of San Agustin fifty miles west of Socorro, NM, USA, at an elevation of 6970 ft (2124 m)

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. Credit: ALMA (ESO/NAOJ/NRAO), E. O’Gorman; NRAO/AUI/NSF, S. Dagnello.

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Artist 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. Credit: NRAO/AUI/NSF, S. Dagnello.

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Artist impression of red supergiant star Antares Credit: NRAO/AUI/NSF, S. Dagnello.

An international team of astronomers has created the most detailed map yet of the atmosphere of the red supergiant star Antares. The unprecedented sensitivity and resolution of both the Atacama Large Millimeter/submillimeter Array (ALMA) and the National Science Foundation’s Karl G. Jansky Very Large Array (VLA) revealed the size and temperature of Antares’ atmosphere from just above the star’s surface, throughout its chromosphere, and all the way out to the wind region.

Red supergiant stars, like Antares and its more well-known cousin Betelgeuse, are huge, relatively cold stars at the end of their lifetime. They are on their way to run out of fuel, collapse, and become supernovae. Through their vast stellar winds, they launch heavy elements into space, thereby playing an important role in providing the essential building blocks for life in the universe. But it is a mystery how these enormous winds are launched. A detailed study of the atmosphere of Antares, the closest supergiant star to Earth, provides a crucial step towards an answer.

The ALMA and VLA map of Antares is the most detailed radio map yet of any star, other than the Sun. ALMA observed Antares close to its surface (its optical photosphere) in shorter wavelengths, and the longer wavelengths observed by the VLA revealed the star’s atmosphere further out. As seen in visible light, Antares’ diameter is approximately 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,” explained Eamon O’Gorman of the Dublin Institute for Advanced Studies in Ireland and lead author of the study published in the June 16 edition of the journal Astronomy & Astrophysics. “The longer wavelengths of the VLA revealed the supergiant’s atmosphere out to nearly 12 times the star’s radius.”

The radio telescopes measured the temperature of most of the gas and plasma in Antares’ atmosphere. Most noticeable was the temperature in the chromosphere. This is the region above the star’s surface that is heated up by magnetic fields and shock waves created by the vigorous roiling convection at the stellar surface – much like the bubbling motion in a pot of boiling water. Not much is known about chromospheres, and this is the first time that this region has been detected in radio waves.

Thanks to ALMA and the VLA, the scientists discovered that the star’s chromosphere extends out to 2.5 times the star’s radius (our Sun’s chromosphere is only 1/200th of its 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.

“We found that the chromosphere is ‘lukewarm’ rather than hot, in stellar temperatures,” said O’Gorman. “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 co-author Keiichi Ohnaka of the Universidad Católica del Norte in Chile who previously observed Antares’ atmosphere in infrared light. “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 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 co-author Graham Harper of 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.”

“Our innate understanding of the night sky is that stars are just points of light. The fact we can map the atmospheres of these supergiant stars in detail, is a true testament to technological advances in interferometry. These tour de force observations bring the universe close, right into our own backyard,” said Chris Carilli of the National Radio Astronomy Observatory, who was involved in the first observations of Betelgeuse at multiple radio wavelengths with the VLA in 1998.

See the full article here .

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The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of Europe, North America and East Asia in cooperation with the Republic of Chile. ALMA is funded in Europe by the European Organization for Astronomical Research in the Southern Hemisphere (ESO), in North America by the U.S. National Science Foundation (NSF) in cooperation with the National Research Council of Canada (NRC) and the National Science Council of Taiwan (NSC) and in East Asia by the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Academia Sinica (AS) in Taiwan.

ALMA construction and operations are led on behalf of Europe by ESO, on behalf of North America by the National Radio Astronomy Observatory (NRAO), which is managed by Associated Universities, Inc. (AUI) and on behalf of East Asia by the National Astronomical Observatory of Japan (NAOJ). The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA.

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