Radio measurements reveal 'weird' atmosphere of fast-spinning star

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Altair is the brightest star in the constellation Aquila "The Eagle." (Shutterstock)

The nearby blue-white star Altair is surrounded by an ultraviolet-emitting layer known as a chromosphere, according to first-of-their-kind radio-wave observations. The feature, unusual for large and hot stars such as Altair, is likely generated because the star rotates more than 100 times faster than the sun does.

The new stellar measurements, published April 29 in The Astrophysical Journal Letters, are the latest entry for a catalog of the radio waves emitted from stars, which are time-intensive to observe and poorly documented. Understanding them can inform how stars evolve over time, what material orbits around them and whether their planets can harbor life.

The observation of Altair's chromosphere also contradicts preexisting scientific models of the structures of A-type main-sequence stars, a category of stars somewhat bigger and hotter than the sun that also includes Sirius A, the brightest star in the sky.

"That's a really interesting new result for these types of stars," said lead author Jacob White, an astronomy fellow at the National Radio Astronomy Observatory in West Virginia. "We haven't really seen it with any of them because the physics says that they shouldn't really exist."

The low-frequency radio waves that stars "shine" are much more difficult to detect than the stars' visible light. Even the best telescopes need up to several hours to obtain useful radio-wave observations of some stars, according to White, and given the tight competition for telescope use among astronomers, the radio spectra of few stars have ultimately been gathered. (In astronomy, the term "radio waves" also includes microwave radiation.)

To address these gaps in knowledge, White and other researchers have been measuring the radio waves of nearby stars as part of his MESAS Project; the title is an acronym for "measuring the emission of stellar atmospheres at submillimeter/millimeter wavelengths." 

Following measurements of Sirius A and three other stars, the new paper focused on the radio spectra of Altair, which is the 12th-brightest star in the sky and 1.8 times as massive as the sun. The A-type main-sequence star spins 120 times faster than the sun and completes a revolution every nine hours, an unusually fast rotational speed that makes the star significantly bulge outward at its equator.

White said Altair's relatively close proximity to Earth — 16.8 light-years away — makes it less difficult to measure, and its rapid rotation could provide a new insight into the behavior of A-type stars.

"We wanted to know … how can this type of star that happens to be rapidly rotating, what impact could that have on the radio spectrum?" White said. "Because if you don't take that into consideration, you may have an incorrect model of what radio emission could look like."

The authors of the paper used two telescope arrays — the Very Large Array in New Mexico and the Northern Extended Millimeter Array in France — to collect radio-wave frequencies emitted by Altair. The observations were used to produce the first known radio spectrum of a fast-spinning A-type star.

Combining the radio data with new models of Altair's atmosphere, the researchers determined that the star probably has a chromosphere, which is a "really weird thing in A-type stars," said Francisco Tapia-Vàzquez, an author of the paper who helped develop the models. Very few of these stars have been found to have a chromosphere, a region above a star's surface that emits ultraviolet light. A chromosphere is commonly present in smaller stars, including the sun.

The results put to rest earlier speculation that Altair's ultraviolet radiation stemmed from a source other than a chromosphere. The astronomers said it is probably the result of the heat created by Altair's unusually strong magnetic field, which in turn is generated by charged particles in the star rotating at high speeds.

Because chromospheres emit ultraviolet radiation, which can be harmful to life and is largely blocked on Earth by its atmosphere as well as sunscreen, the presence and nature of a star's chromosphere can determine whether its planets can harbor life, according to Tapia-Vàzquez. 

"The impact of the chromosphere on the planet has many applications," said Tapia-Vàzquez, an astrophysics Ph.D. student at the National Autonomous University of Mexico. "If we study the main-sequence stars, we can model these conditions for the life and other things, like space weather."

Although no known exoplanets orbit Altair, these radio measurements and others in the MESAS Project could be used by other astronomers to extrapolate the radio spectrum of a similar unmeasured star, according to White. To expand the catalog, he and Tapia-Vázquez have applied for the telescope time to measure about 20 more stars, a total that White said would stretch modern telescope capabilities. Upcoming telescopes, such as the Next Generation Very Large Array, could provide the the ability to detect and analyze many more stars near our solar system.

"Realistically, our telescopes can't observe more than about 30 of them, so we're really pushing the current telescopes to their absolute limits with this project," White said. "The only thing we can do to improve upon that is build bigger and better telescopes — or find more hours in the day."

The study "The first radio spectrum of a rapidly rotating A-type star," published April 29 in The Astrophysical Journal Letters, was authored by J. White, National Radio Astronomy Observatory; F. Tapia-Vázquez, L. Zapata and V. De la Luz, National Autonomous University of Mexico; A. Hughes, University of British Columbia; A. Moór, Konkoly Observatory and ELTE Eötvös Lorànd University; B. Matthews, NRC Herzberg Astronomy and Astrophysics Research Centre; D. Wilner, Center for Astrophysics | Harvard & Smithsonian; J. Aufdenberg, Embry-Riddle Aeronautical University; O. Fehèr, Institute of Millimetric Radioastronomy; A. M. Hughes, Wesleyan University; and A. McNaughton, Embry-Riddle Aeronautical University and Pacific Northwest National Laboratory.

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