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Astronomers observe a dying red giant star's final act

An international team of astronomers has observed a striking spiral pattern in the gas surrounding a red giant star named LL Pegasi and its companion star 3,400 light-years from Earth, using a powerful telescope in northern Chile called Atacama Large Millimeter/submillimeter Array, or ALMA.

Astronomers observe a dying red giant star's final act
Molecular gas forms a spiral pattern around the red giant star LL Pegasi 
[Credit: ALMA, Hyosun Kim]
"What we are seeing in splendid detail with these observations is the final act of a dying red giant star, as it sheds most of its gaseous bulk in a strong, outflowing wind," said study co-author Mark Morris, UCLA professor of physics and astronomy.

After comparing their telescopic observations with computer simulations, the astronomers concluded that a highly elliptical orbit is responsible for the shape of the gaseous emissions surrounding this system.

"Because of the orbital motion of the mass-losing red giant, the cold molecular gas constituting the wind from that star is being spun out like the sprays of water from a rotating garden sprinkler, forming the outflowing pattern of spiral shells," Morris said.

3-D visualization of the molecular gas material surrounding LL Pegasi. Image of LL Pegasi, first as
 it appears to the Hubble Space Telescope, and then as it appears in the emission from molecules, 
as observed with ALMA. The numerical model appears beside the nebula, and both the model 
and the image are rotated to display the excellent three-dimensional agreement 
[Credit: Hyosun Kim et al./I-Ta Hsieh (ASIAA) and Mark Morris (UCLA)]

Visualizing the ALMA image cube of LL Pegasi. Each frame of the video shows the molecular gas 
emission from LL Pegasi for a different line-of-sight velocity. It starts with the gas moving 
towards us, and ends with the gas moving away from us. The field size is 20,000 times 
the distance between the Sun and the Earth [Credit: ALMA (ESO/NAOJ/NRAO)/
Hyosun Kim et al. in Nature Astronomy]

ALMA, a powerful international facility operated cooperatively by many countries around the world including the United States, measures extremely short wavelength radio emission. Using this unique instrument, the scientists were able to create a three-dimensional image of the emission from molecules ejected from LL Pegasi and which then form a spiral pattern caused by the presence of the orbiting companion star.

The images, which show many complete revolutions of the spiral pattern, offer clues about the dynamics of the binary system over a period of 5,000 years.

"This unusually ordered system opens the door to understanding how the orbits of such systems evolve with time as one of the stars loses most of its mass," Morris said.

The research appears in the journal Nature Astronomy.

Source: University of California, Los Angeles [March 16, 2017]

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