Stars with too much lithium may have stolen it

Something is giving small, pristine stars extra lithium. A dozen newly discovered stars contain more of the element than astronomers can explain.

Some of the newfound stars are earlier in their life cycles than stars previously found with too much lithium, researchers report in the Jan. 10 Astrophysical Journal Letters. Finding young lithium-rich stars could help explain where the extra material comes from without having to tinker with well-accepted stellar evolution rules.

The first stars in the Milky Way formed from the hydrogen, helium and small amounts of lithium that were produced in the Big Bang, so most of this ancient cohort have low lithium levels at the surface (SN: 11/14/15, p. 12). As the stars age, they usually lose even more.
Mysteriously, some aging stars have unusually high amounts of lithium. About a dozen red giant stars — the end-of-life stage for a sunlike star — have been spotted over the last few decades with extra lithium at their surfaces. It’s not enough lithium to explain a different cosmic conundrum, in which the universe overall seems to have a lot less lithium than it should (SN: 10/18/14, p. 15). But it’s enough to confuse astronomers. Red giants usually dredge up material that is light on lithium from their cores, making their surfaces look even more depleted in the element.

Finding lithium-enriched red giants “is not expected from standard models of low-mass star evolution, which is usually regarded as a relatively well-established field in astrophysics,” says astronomer Wako Aoki of the National Astronomical Observatory of Japan in Tokyo. A red giant with lots of lithium must have had a huge amount of lithium in its former life, or imply a tweak is needed to some fundamental rule of stellar evolution.

Aoki and his colleagues used the Subaru Telescope in Hawaii to find the 12 new lithium-rich stars, all about 0.8 times the mass of the sun. Five of the stars seem to be relatively early in their life cycles — a little older than regular sunlike stars but a little younger than red giants.
That suggests lithium-rich stars somehow picked up the extra lithium early in their lives, though it’s not clear from where. The stars could have stolen material from companion stars, or eaten unfortunate planets (SN: 5/19/01, p. 310). But there are reasons to think they did neither — for one thing, they don’t have an excess of any other elements.

“This is a mystery,” Aoki says.

Further complicating the picture is the possibility that the five youngish stars could be red giants after all, thanks to uncertainties in the measurements of their sizes, says astronomer Evan Kirby of Caltech. Future surveys should check stars that are definitely in the same stage of life as the sun.

Still, the new results are “tantalizing,” Kirby says. “It’s a puzzle that’s been around for almost 40 years now, and so far there’s no explanation that has satisfying observational support.”

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