A forty-year-old puzzle about the stars is solved

Study led by Indian astrophysicists challenges the present understanding of nucleosynthesis in stars

July 08, 2020 08:09 pm | Updated July 09, 2020 11:07 am IST - Chennai:

About 40 years ago, a few stars were spotted in the Milky Way that were lithium-rich.

About 40 years ago, a few stars were spotted in the Milky Way that were lithium-rich.

A forty-year-old puzzle regarding the production of lithium in stars has been solved by Indian researchers. Stars, as per known mechanisms of evolution, actually destroy lithium as they evolve into red giants. Planets were known to have more lithium than their stars — as is the case with the Earth-Sun pair. However, leading to a contradiction, some stars were found that were lithium-rich. The new work by Bharat Kumar, currently a post doctoral fellow at the National Astronomical Observatories of China, Beijing, and an international team of co-workers shows that, in fact, when stars grow beyond their Red Giant stage into what is known as the Red Clump stage, they produce lithium in what is known as a Helium Flash and this is what enriches them with lithium. The study has been published in the journal Nature Astronomy on July 7.

Lithium’s interesting story

Lithium, a light element commonly used today in communication device technology, has an interesting story. It was first produced in the Big Bang, around 13.7 billion years ago when the universe came into being, along with other elements. While the abundance of other elements grew millions of times, the present abundance of lithium in the universe is only four times the original [Big Bang] value. It is actually destroyed in the stars. The Sun, for instance, has about a factor of 100 lower amount of lithium than the Earth. About 40 years ago, a few large stars were spotted that were lithium-rich. This was followed by further discoveries of lithium-rich stars, and that posed a puzzle — if stars do not produce lithium, how do some stars develop to become lithium rich?

“The planet engulfment theory was quite popular. For example, Earth-like planets may increase the star’s lithium content when they plunge into [their] star’s atmosphere when the latter become Red Giants. I was not comfortable with this idea,” said Professor Eswar Reddy, Director of India Thirty Meter Telescope Centre, Indian Institute of Astrophysics, Bengaluru, who led the study.

Prof. Reddy has been working on this puzzle for nearly 20 years now, and had, along with his students, devised a method of measuring lithium content using low-resolution spectra in a large number of stars, with facilities provided at the Indian Institute of Astrophysics.

Over 200,000 stars

For the present study, the group studied over 200,000 stars using the Galactic Archaeology survey of the Anglo-Australian Telescope, Australia. “This is a dedicated facility for obtaining high-resolution spectra for a large number of stars,” explains Prof. Reddy. This is the first study to demonstrate that lithium abundance enhancement among low mass giant stars is common. Until now, it was believed that only about 1% of giants are lithium rich. Secondly, the team has shown that as the star evolves beyond the Red Giant stage, and before it reaches the Red Clump stage, there is a helium flash which produces an abundance of lithium. Lastly, they set a lower limit for helium abundance which will classify the star as “lithium-rich”. This value is about 250 times lower than the previous limit.

The study challenges the present understanding of nucleosynthesis in stars. “Our next study may concentrate on helium-flash nucleosynthesis and how lithium escapes from destruction in the interior of stars and dredges-up to the surface,” said Prof. Reddy.

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