Letting Lithium Live It Up

As stars evolve from the red giant branch to the red clump, they accumulate lithium on their surfaces. How does this accumulation happen?

The Red Giant Branch...
Stars with less than eight solar masses, or low mass stars, live fairly placid lives. A low mass star would start off burning hydrogen in its core like all main sequence stars. Once the core hydrogen has been exhausted, the star resorts to burning hydrogen in a shell surrounding its passive core, which is now mostly helium. This stage of life is called the red giant branch (RGB) stage.

While the helium core may not be burning any material, that doesn't mean it's not doing anything! The sheer mass of the core means that it collapses in on itself to the point that the only thing holding it up against gravity is something called electron degeneracy - you can't fit more than one electron in a space meant for only one electron.

...and the Red Clump
This standoff doesn't last forever though, as conditions become ideal for helium to ignite and start core burning again. This helium ignition is called a helium flash. The star is now at the horizontal branch stage, where it continues to burn hydrogen in a shell around the helium-burning core.

Multiple helium flashes can occur as the star transitions to the horizontal branch, but the first is the strongest. Cooler horizontal branch stars appear red and tend to cluster in a particular region in brightness-temperature space, aptly called the red clump (RC).

Observations of RGB and RC stars have found that RC stars have more lithium on their surface than RGB stars do. This suggests some enriching process - a process that results in more heavy elements being present in a region - occurs between the RGB and the RC stages. To investigate what could be behind this enriching, Josiah Schwab (University of California, Santa Cruz) used stellar evolution models combined with our knowledge of how material moves in stars.

Mixing Things Up
One way to create lithium in a star is to start with helium. Deep within the star, helium can fuse into beryllium. When the beryllium is transported to cooler regions closer to the star's surface, it can experience electron capture - when the nucleus of an atom absorbs one of the electrons orbiting it - and become lithium.

Schwab suggested the first helium flash that happens between the RGB and RC stages can trigger internal waves that mix material in the star. In some stars, this mixing would deplete lithium, but with simulations Schwab showed that the opposite happens as stars transition from the RGB to the RC, enhancing the amount of lithium present at the star's surface.

An observational check for this flash-induced mixing would be to determine lithium abundances for stars that are just beginning to evolve from the RGB to the RC, since the first helium flash occurs right at the start of this transition. More detailed stellar models will also be useful, but for now it seems the core of this mystery is solved!

Research Report: "A Helium-flash-induced Mixing Event Can Explain the Lithium Abundances of Red Clump Stars"

Related Links
AAS Nova
Stellar Chemistry, The Universe And All Within It

Tweet

Thanks for being there;
We need your help. The SpaceDaily news network continues to grow but revenues have never been harder to maintain.

With the rise of Ad Blockers, and Facebook - our traditional revenue sources via quality network advertising continues to decline. And unlike so many other news sites, we don't have a paywall - with those annoying usernames and passwords.

Our news coverage takes time and effort to publish 365 days a year.

If you find our news sites informative and useful then please consider becoming a regular supporter or for now make a one off contribution.

SpaceDaily Monthly Supporter
$5+ Billed Monthly

paypal only

SpaceDaily Contributor
$5 Billed Once

credit card or paypal

Newly discovered ghostly circles in the sky can't be explained by current theories
Sydney, Australia (The Conversation) Dec 03, 2020
In September 2019, my colleague Anna Kapinska gave a presentation showing interesting objects she'd found while browsing our new radio astronomical data. She had started noticing very weird shapes she couldn't fit easily to any known type of object. Among them, labelled by Anna as WTF?, was a picture of a ghostly circle of radio emission, hanging out in space like a cosmic smoke-ring. None of us had ever seen anything like it before, and we had no idea what it was. A few days later, our colleague ... read more