Lithium Abundance in Interstellar Space

• 20 Apr 2020

Why in News

Recently, the researchers at the Indian Institute of Astrophysics (IIA) have discovered hundreds of Lithium (Li) rich giant stars which indicate that lithium is being produced in the stars and accounts for its abundance in the interstellar (between stars) medium.

• The study was published in the Astrophysical Journal Letters and Monthly Notices of Royal Astronomical Society (MNRAS).

Key Points

• The scientists have discovered a number of super Li-rich giants with the Li quantity 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).
• Scientists followed a two-fold strategy of 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.
  
  • Hundreds of Li-rich giants were discovered by employing data from large scale ground and space missions.
  • However, Li-rich giants still account for only about 1 in 100 in the Galaxy.
• The evolutionary phase of these giants was determined by analyzing relative positions of thousands of stars using their temperature and luminosity and also subjecting their independent data set to atmospheric oscillations analysis using data from Kepler Space Telescope.
  
  • For the first time, it was shown that the Li enhancement in giants is associated only with central He-burning stars (also known as the Red Clump Giants)
    
    • This discovery will help to eliminate or validate many proposed theories such as planet engulfment or Big Bang Nucleosynthesis (BBN) during the red giant evolution in which helium at the center is not burning.
• Lithium (Li), is one of the three primordial elements, apart from Hydrogen (H) and Helium (He), produced in the BBN.
  
  • This model predicts primordial Li abundance [A(Li) ~2.7~dex].
• Stars are also proposed as a likely Li source in the Galaxy and are considered as Li sinks.
  
  • 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 Kelvin (a range which is easily encountered in stars).

Planetary Engulfment

• In the universe, planets accompany host stars (like the Sun is the host star for the planets of the Solar system).
• As the host star evolves off the main sequence to become a white dwarf, the planets with sufficiently close orbits can be engulfed during the giant phase.
• Planetary engulfment events involve the chemical assimilation of a planet into a star's external layer.
  
  • This can cause a change in the chemical pattern of the stellar atmosphere in a way that mirrors the composition of the rocky object engulfed.

Big Bang Nucleosynthesis

• It is the leading explanation about how the universe began. At its simplest, it says that the universe started with a small singularity and then inflated over the next 13.8 billion years to the cosmos currently observed.
• The Universe's light-element abundance is another important criterion by which this theory is verified.
  
  • It is now known that the elements observed in the Universe were created in either of two ways.
    
    • Light elements (namely deuterium, helium, and lithium) were produced in the first few minutes of the Big Bang, while elements heavier than helium are thought to have their origins in the interiors of stars which formed much later in the history of the Universe.
• The theory 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.

Source: PIB