Scientists spot elusive mini red giant stars, victims of stellar stealing

Two strange types of red giant stars were formed after a tug-of-war between neighboring stars.

40 examples of two different types of red giant stars were found by the astronomer. The dense core of a white dwarf can sometimes be a greedy neighbor, so scientists expected that such objects existed. Red giants form together and then, late in their lives, each sheds its layers of gas to become a white dwarf.

Yaguang Li, the study lead author and a PhD candidate at the University of Sydney in Australia, said in a statement that they were extremely lucky to find about 40 slimmer red giants.

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The slimmed-down stars popped up in archival data gathered by NASA's exoplanet-hunting Kepler space telescope. After that, Kepler did an extended mission and retired. The same category of stars that our sun will become when it ages was measured by the brightness of tens of thousands of red giant stars, thanks to the constant pointing of the telescope at a spot in the constellation Cygnus.

The team found two unusual red giant stars, one with a lower mass and one that shines less brightly.

The researchers said the mass loss could only be explained by the size of a red giant star. One would be very slow and gradual loss, but that process would take longer than the age of the universe and thus be impossible.

That left what the team referred to as a greedy neighbor, meaning a white dwarf pulling mass away from the red giant, as the only explanation for low-mass red giants.

Underluminous stars have normal mass up to twice that of the sun, which was the second unusual type. The stars seem to be smaller and fainter than modeling would suggest. Only seven of the stars were found in the data.

The rareness of the stars, along with the lack of apparent explanation for their weight loss by normal physical processes, caused researchers to conclude that a hidden companion must be stealing away mass from the underluminous stars.

The stars were probed using asteroseismology to get more information about their properties.

The team said that the astronomer discovered that some red giants have tiny amounts relative to the entire population, inspiring the investigations about why some red giants are very small or underluminous.

The team wrote in their paper that their results open up new possibilities to study the evolution of post-mass-transfer binaries.

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