IIA traces link between Lithium-rich red giant stars and their enhanced Helium abundance in a new study

Astronomers at the Indian Institute of Astrophysics (IIA) in a new study have traced a link between Lithium-rich red giant stars and their enhanced Helium abundance.

The IIA team has been able to trace this link using data from the Himalayan Chandra Telescope and other archival data which also gives a new lead to the evolution of cool giant stars along the red giant phase.

According to the Department of Science and Technology, Helium, the most abundant element next to hydrogen, plays a crucial role in measuring the abundances of other key elements accurately, and also to understand the structure and evolutionary history of stars.  

The helium abundance in stars is measured using indirect methods as it is not directly observable from the star’s surface. Particularly in cool stars, like our Sun and other cooler giants, the surface temperature is not sufficient to excite helium to produce observable spectral lines.

Hence, helium abundance is inferred by analysing the effects on a star’s structure, evolution and other observable elements and molecules.

“The change in hydrogen and helium abundance is relative. If there is reduction in hydrogen, helium is enhanced proportionally,” the department said.

In the study, the researchers measured the Hydrogen abundance, and any departure from its standard value, by comparing the magnesium abundances derived from its atomic versus molecular spectral lines.

This departure in hydrogen abundance was then translated to the corresponding helium abundance using the model atmospheres computed for differing values of hydrogen to helium ratio (He/H ratio).

This method was also effectively used by the team for determining the Helium abundance of the Sun in an earlier work.

“We applied this method to a sample of cool giant stars to study the correlation between the lithium and the helium abundances. This sample, consisting of 18 red giant stars and two super giant stars, was studied using high-resolution spectra obtained primarily from the Himalayan Chandra Telescope in Hanle, Ladakh, operated by IIA, as well as from the archives of various telescopes worldwide”, said B.P. Hema, lead author of the study.

The effective temperature, surface gravity and the abundances of 23 different elements were calculated by analysing both atomic lines and molecular bands. These analyses were carried out by adopting the models constructed with appropriate He/H ratios that return very similar Mg abundances from the lines of MgH and Mg I.

Of the 20 programme stars, six were determined to have higher He/H ratio (enhancement in helium) than the standard value of 0.1. Among these six giants, five are red giants, and one is a super-giant.