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Discovery of Li-rich giant stars

  • Category
    Science & Technology
  • Published
    29th Apr, 2020

Researchers at the Indian Institute of Astrophysics (IIA) have discovered hundreds of Li-rich giant stars indicating that Li is being produced in the stars and accounts for its abundance in the interstellar medium.

Context

Researchers at the Indian Institute of Astrophysics (IIA) have discovered hundreds of Li-rich giant stars indicating that Li is being produced in the stars and accounts for its abundance in the interstellar medium.

About

Lithium (Li), is one of the three primordial elements, apart from Hydrogen and Helium (He), produced in the big bang nucleosynthesis (BBN) whose models predict primordial Li abundance (A(Li) ~2.7~dex).

Big Bang Nucleosynthesis:

  • The theory of big-bang nucleosynthesis (BBN) predicts rather successfully the primordial abundances of light elements.
  • It gives a detailed mathematical description of the production of the light "elements" deuterium, helium-3, helium-4, and lithium-7.
  • Specifically, the theory yields precise quantitative predictions for the mixture of these elements, that is, the primordial abundances at the end of the big-bang.
  • It predicts that roughly 25% the mass of the Universe consists of Helium. It also predicts about 0.01% deuterium, and even smaller quantities of lithium.
  • Natural lithium is a mixture of two stable isotopes, lithium-6 and lithium-7. Lithium-7 accounts for over 92% of the natural abundance of the element.
  • Lithium is an alkali metal. It's silver-white in pure form and is so soft it can be cut with a butter knife. It has one of the lowest melting points and a high boiling point for a metal.

The discovery:

  • Scientists have discovered a number of super Li-rich giants which have Li quantity which is equal to or in some cases, more than 10 times the present value, A(Li) =3.2 dex ( measured in logarithmic scale relative to hydrogen).
  • They have also associated such Li enhancement with central He-burning stars, also known as red clump giants, thereby opening up new vistas in the evolution of the red giant stars.
  • However, the present measurement of Li in the interstellar medium and very young stars is about 4 times more than the primordial value.
  • Apart from reactions, in which high energy cosmic ray particles bombard with heavier nuclei such as carbon and oxygen-producing lighter particles such as Li, stars are also proposed as likely Li source in the Galaxy.
  • In general, stars are considered as Li sinks. This means that the original Li, with which stars are born, only gets depleted over stars’ life-time as Li burns at relatively very low temperatures of about 2.5X106 K – a range which is easily encountered in stars.

How they did it?

  • Scientists followed a two-fold strategy:
    • by increasing the sample by systematically searching for high Li among low mass evolved stars in the Galaxy and determining the exact evolutionary phase of these high Li abundance stars.
    • by employing data from large scale ground and space missions, they discovered hundreds of Li-rich giants.
  • Though their study increased the number of Li-rich giants by many-fold Li-rich giants still accounts for only about 1 in 100 in the Galaxy.

Significance of the finding:

  • This is an important discovery that will help to eliminate many proposed theories such as planet engulfment or nucleosynthesis during the red giant evolution in which helium at the center is not burning.

Moreover, identification of sources of Li enrichment in our Galaxy has been a great interest to researchers to validate Big Bang Nucleosynthesis as well as a stellar mixing process.

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