Scientists spot elusive red giant mini stars, victims of stellar theft
A tug-of-war between nearby stars has resulted in the formation of two strange types of red giant stars, as seen in the eyes of a lost telescope.
Astronomers reported that they found 40 examples of two different varieties of minimized red giant stars. Scientists expected such objects to exist, since red giants are often found in binary systems near the dense core of a dead star, called a white dwarf, which can sometimes be a greedy neighbor. (These mismatched pairs arise because red giants form together; then, in old age, each loses its gas layers to become a white dwarf.)
“It’s like finding Waldo,” Yaguang Li, the study’s lead author and Ph.D. candidate at the University of Sydney in Australia, he said in a statement. “We were extremely lucky to find about 40 thinnest red giants hidden in a sea of giants.”
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The minimized stars emerged in archival data collected by NASA’s Kepler Space Telescope, whose main mission ran from 2009 to 2013. (Kepler conducted an extended mission thereafter and was retired in late 2018. ) During that time, Kepler was continually pointed at one point in the constellation of Cygnus, allowing him to measure changes in brightness in tens of thousands of red giant stars, the same category that our sun will become as it ages.
Revisiting these observations, the team found two unusual types of red giant stars: one with lower masses and one that shines less brightly.
Very low-mass stars contain only about half the mass of the sun. Given the typical size of a red giant star, the researchers said this mass loss can only be explained in two ways. One would be a very slow and gradual loss, but that process would take longer than the age of the universe (13.8 billion years) and therefore would be impossible.
This left what the team called a “greedy neighbor,” meaning a white dwarf pulling mass away from the red giant, as the only possible explanation for low-mass red giants.
The second unusual type were the “sub-bright stars”, which have normal masses up to twice that of the sun. However, these stars appear to be smaller and fainter than modeling would suggest. Stars are also rare, as only seven of this type have been found in Kepler’s data.
The rarity of the stars, coupled with the lack of an apparent explanation for their weight loss by normal physical processes, has led researchers to conclude that, once again, a hidden companion must steal mass from the sub-bright stars.
The researchers probed the stars using asteroseismology, or the study of stellar vibrations, to provide more information on their properties, including evolutionary stage, mass and size.
It was through this larger survey that astronomers found that some red giants have tiny masses compared to the entire population, inspiring investigations into why some red giants are very small or dim, the team said.
‘Our findings open up new possibilities for studying the evolution of post-mass transfer binary systems,’ the team wrote in their paper, which further delves into the mechanisms of how white dwarfs remove mass from a companion red giant and is was published in Nature Astronomia on Thursday (April 14).