Coronal activity may be ’buried alive’ in red giant stars, say Colorado researchers

When Earth’s sun expands into a red giant star in roughly five billion years, long after Earth has become uninhabitable, the hydrogen core will be burned out and the bloated outer shell will be cool and murky.

But according to new research by the University of Colorado at Boulder, such red giants still retain surface magnetic storms and coronas — the very hot and patchy outer atmosphere of the sun and sun-like stars — at temperatures of millions of degrees Fahrenheit that often signal stellar youth.

The red giant coronal regions, however, appear to be submerged in the extended outer shell known as the chromosphere, “buried alive” in these red giants.

Low-mass stars like the sun begin their lives as fast-spinning dwarfs exhibiting high levels of magnetic activity including giant solar flares that could affect early life on planets like Earth, said Professor Tom Ayres of the astrophysical and planetary sciences department. As sun-like stars age and ultimately expand into red giants, the rotation slows or even stops, he said.

“Rotation is thought to be a key ingredient in the coronal magnetic activity,” said Ayres. “Once the rotation halts, the surface magnetic storms should cease.”

In 1990, scientists proposed these red giant stars were a coronal graveyard, said Ayres. “But new evidence, surprisingly, indicates that there probably still is coronal activity buried beneath the murky atmospheres of these dying stars.”

The submerged violent activity may be tied to the remarkable winds that blow off red giant stars, said Ayres. “These mysterious winds travel at a mind-boggling 100,000 miles per hour and are so strong they literally blow away much of the stellar ’outer envelope’ during the terminal phases of the star’s life.”

The team imaged two bright stars, Arcturus and Aldebaran, thought to have been very sun-like until their evolution into red giants millions of years ago. They used NASA’s powerful Chandra X-Ray Orbiting Observatory, ultraviolet instruments on the Hubble Space Telescope and the Far Ultraviolet Spectroscopic Explorer, or FUSE.

“We indirectly detected the presence of hot coronal gas through UV emissions of highly charged carbon and oxygen detected by FUSE,” he said. “Apparently magnetic storms still occur on the surfaces of red giants but do not rise very high into the thick chromosphere of warm gas – about 10,000 F — that surrounds the cooler layers that measure about 5,000 F.”

Ayres said the chromosphere is opaque to X-rays but is more transparent in the UV, allowing the hot emissions from the surface magnetic storms to escape. “With only the X-ray view, we might have thought the magnetic activity in the “coronal graveyard had entirely faded away,” he said. “Now, the UV perspective has revealed that the activity is merely ’buried alive’ and that we will have to dig deeper to fully understand the inner workings of these dying stars.”

A paper on the subject by Ayres, Alexander Brown and Graham Harper, all research associates at CU-Boulder’s Center for Astrophysics and Space Astronomy, or CASA, was presented at the 201st annual meeting of the American Astronomical Society held in Seattle Jan. 6 to Jan. 12.

The data were obtained from NASA’s Chandra X-Ray Observatory and several other instruments, including FUSE — which was partly assembled and built at CU-Boulder’s CASA — and the Hubble Space Telescope. The work was supported by in part by grants from the Smithsonian Astronomical Observatory, the Space Telescope Science Institute in Baltimore and NASA.

Additional Contact:
Jim Scott, (303) 492-3114