Indian scientists pin down mechanism behind lithium production in low mass Red Clump Stars

Indian scientists pin down mechanism behind lithium production in low mass Red Clump Stars
Lithium is produced during the helium-flashing phase of 2 million years.

New Delhi: Scientists from the Indian Institute of Astrophysics have pinned down the mechanism behind the lithium production in low mass Red Clump Stars. The discrepancy between the abundance of lithium as inferred from observations of stars and the theoretically predicted amount has intrigued astronomers for a long time.

Having found lithium excess to be common among the low mass Red Clump Giants, they have now traced helium-flashing phase of the star’s evolution as the site for high lithium production. This transition phase lasts for about 2 million years, during which Red Giant Branch (RGB) giantstars with inert helium-core at the centre become Red Clump Giant stars of helium-core burning.

Recently, a group of astronomers at the Indian Institute of Astrophysics (IIA), Bengaluru, an autonomous institute of the Department of Science & Technology, Government of India, found observational evidence for lithium enhancement during the helium-flashing phase of 2 million years, followed by a rapid decrease in lithium abundances of such stars. According to their work, it seems the lithium excess in giants is a transient phenomenon.

Indian scientists pin down mechanism behind lithium production in low mass Red Clump Stars
Variation of lithium abundance with gravity mode period spacing in RC stars tracks the evolution of the stars and simulations of helium-flashing phase starting at the tip of RGB to RC phase

The researchers led by Mr. Raghubar Singh and Prof. Eswar Reddy of IIA along with Simon Campbell of Monash University, Australia; Bharat Kumar of CAS, China;and MathieuVrard of Ohio State University, USA used asteroseismology (seismic study of stars using time-resolved photometry from Kepler Space Telescope) combined with spectroscopic abundances of elements to track the evolution of lithium in a sample of giant stars. In addition to the evidence for lithium production site, a first-of-its-kind correlation between the two independent observed quantities – lithium abundance and stellar oscillations (gravity mode period spacing) – will serve to track the helium-flashing phase of converting RGB giant of an inert, electron-degenerate helium-core into a fully convective helium-burning core by a series of core helium-flashes, a theory developed in the 1960s. This work is published in the ‘Astrophysical Journal Letters’.

Lead author Mr. Raghubar Singh said, “We do not have much insight into the helium-flash phase, let alone understanding of lithium production. These new results will inspire further observations as well as theoretical models.”

Talking about the importance of this result Prof. Reddy said, “These results will be of great interest to a larger community of theoreticians and observers. This is because of lithium’s broader implications to cosmological models, which predict Big Bang lithium abundance, which is a factor of four less than the presently observed values in the interstellar medium or very young stars, indicating lithium is increasing. Identification of production sites is important for accounting for lithium enhancement in the Universe and provides excellent insights into the internal working of stars.”(India Science Wire)

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