INDIA: The abundance of heavy metals in the oldest metal-poor stars that are born from the expelled remnants of first stars has intrigued astronomers for a long time as the already known process of reaction of chemical elements by nuclear fusion within stars (nucleosynthesis) didn’t reap the desired answers. Scientists have made a breakthrough about this large quantity of heavy metals in a nucleosynthetic process called the i-process.
Earlier Scientists discovered that heavier elements are created mainly by two processes of nucleosynthesis– slow and rapid neutron-capture processes which are termed as s and r-process respectively. The s-process elements were thought to be produced in low and intermediate-mass stars towards the final stage of their stellar evolution. The proposed sites of the r-process are exotic proceedings such as supernovae and neutron star mergers.
The CEMP stars presenting enhancements of s-process and r-process elements are known as CEMP-s and CEMP-r stars respectively. However, there is another amazing subclass of CEMP stars, known as CEMP-r/s stars which exhibit enhancement of both s- and r-process elements whose creation had remained a puzzle.
A group of scientists comprising of Prof. Aruna Goswami, from the Indian Institute of Astrophysics (IIA), an autonomous institute under the Department of Science & Technology, Govt. of India, her doctoral student Partha Goswami and Master’s student Rajeev Rathour has achieved a significant advancement in unscrambling this puzzle. In a recent study published by the team in the journal, ‘Astronomy & Astrophysics (A&A), they have found a process which they called i-process operating at neutron densities intermediate to the densities for s-process and r-process is responsible for the peculiar abundance pattern of CEMP-r/s stars. They also proposed new stellar classification criteria based on the abundances of barium, lanthanum, and europium to distinguish between the CEMP-s and CEMP-r/s stars.
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The team analyzed high quality, high-resolution spectra obtained from the five CEMP stars acquired from the 2-m Himalayan Chandra Telescope (HCT) at the Indian Astronomical Observatory, 1.52-m Telescope at the European Southern Observatory at La Silla, Chile, and the 8.2-m SUBARU Telescope at the summit of Mauna Kea, Hawaii, operated by the National Astronomical Observatory of Japan.
With the help of a large sample of CEMP-s and CEMP-r/s stars from the literature, the IIA team has significantly analyzed the different criteria used by various authors for CEMP-s and CEMP-r/s . They found that none of the existing classification criteria was efficient enough in distinguishing the CEMP-s and CEMP-r/s stars and hence their new proposed criteria can successfully fill this gap.
Partha Goswami said, “This scheme of classification is based on the abundance ratios of three very crucial neutron-capture elements barium, lanthanum, and europium and can be effectively used to distinguish the CEMP-s and CEMP-r/s stars”.