New Delhi: Researchers from the Indian Institute of Astrophysics have investigated the chemical abundances in ageing stars using the ground-based Himalayan
Chandra Telescope, as well as archival data captured by telescopes in Chile, France and USA. For determining the chemical inventories of stars, it is essential to measure the abundance of helium, which cannot be measured directly, as cooler stars such our own Sun do not get hot enough to excite helium to produce observable spectral lines. The Indian astronomers used a complex method to determine the helium abundance of the stars based on the observable spectral lines.
First the atomic and molecular spectral lines of magnesium were observed, which was then used to measure the hydrogen abundance. Hydrogen and helium have a relative abundance, with a reduction on hydrogen enhancing the helium. The departure from the hydrogen abundance in the expected standard value was then used to derive the helium abundance. The researchers made the unexpected discovery that the red giants that were rich in helium were also rich in lithium. This is the first time that photospheric helium abundance in normal and lithium-rich red giants has been studied. The lithium enrichment was observed in helium-rich red giants in various phases of stellar evolution.
Unexpected chemical inventories of ageing stars
A paper describing the findings has been published in The Astrophysical Journal. Lead author of the study BP Hema said, “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 2 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”.
Coauthor of the study, Gajendra Pandey says, “The key result of this work is that all the helium enhanced red giants were found to be super-lithium rich, the exception being the super-giant star. But all the lithium rich giants are not helium enhanced. This indicates that the photospheric helium enrichment is accompanied by the lithium enrichment in giants, as we had envisaged.”
