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Two New Classes of Low-Mass Red Giants Discovered


Mass loss in red giant stars is one of the major uncertainties in the astrophysics.

Red giant stars. Image credit: NASA’s Goddard Space Flight Center / Chris Smith, KBRwyle.

Red giant stars. Image credit: NASA’s Goddard Space Flight Center / Chris Smith, KBRwyle.

Red giants are evolved stars that have burned all the hydrogen in their cores into helium through a process of nuclear fusion and instead burn hydrogen in a shell. Towards the ends of their lives, these stars also start burning the helium in their cores.

There are millions of red giants found in our Milky Way Galaxy. These very bright stars are what our Sun will become in about 4 billion years.

For some time, astronomers have predicted the existence of slimmer red giants.

After finding a smattering of them, University of Sydney astronomer Yaguang Li and colleagues can finally confirm their existence.

“It’s like finding Waldo. We were extremely lucky to find 39 slimmer red giants, hidden in a sea of normal ones. The slimmer red giants are either smaller in size or less massive than normal red giants,” Li said.

“How and why did they slim down? Most stars in the sky are in binary systems — two stars that are gravitationally bound to each other.”

“When the stars in close binaries expand, as stars do as they age, some material can reach the gravitational sphere of their companion and be sucked away.”

“In the case of relatively tiny red giants, we think a companion could possibly be present.”

Using a technique called asteroseismology, Li and co-authors analyzed archival data of 7,538 helium-burning red giants from NASA’s Kepler mission.

Two types of unusual stars were revealed: very low-mass red giants, and underluminous (dimmer) red giants.

The very low-mass stars weigh only 0.5 to 0.7 solar mass — around half the weight of our Sun. If the very low-mass stars had not suddenly lost weight, their masses would indicate they were older than the age of the Universe — an impossibility.

“So, when we first obtained the masses of these stars, we thought there was something wrong with the measurement. But it turns out there wasn’t,” Li said.

The underluminous stars, on the other hand, have normal masses, ranging from 0.8 to 2 solar mass.

“However, they are much less ‘giant’ than we expect,” said Dr. Simon Murphy, an astronomer at the University of Southern Queensland.

“They’ve slimmed down somewhat and because they’re smaller, they’re also fainter, hence ‘underluminous’ compared to normal red giants.”

Only seven such underluminous stars were found, and the researchers suspect many more are hiding in the sample.

“The problem is that most of them are very good at blending in. It was a real treasure hunt to find them,” Dr. Murphy said.

These unusual data points could not be explained by simple expectations from stellar evolution.

This led the scientists to conclude that another mechanism must be at work, forcing these stars to undergo dramatic weight loss: theft of mass by nearby stars.

Their paper was published in the journal Nature Astronomy.

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Y. Li et al. Discovery of post-mass-transfer helium-burning red giants using asteroseismology. Nat Astron, published online April 14, 2022; doi: 10.1038/s41550-022-01648-5



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