Why in the News?

A recent study conducted by Indian Institute of Astrophysics (IIA) has discovered a link between Lithium-rich red giant stars and their enhanced helium abundance.

What are Red Giant Stars?

• Overview: Evolved stars that have exhausted core hydrogen, causing the core to contract and the outer layers to expand into a large, cool, reddish envelope.
• Formation Process: Core contraction increases temperature while the outer shell expands and cools, triggering hydrogen shell burning.
• Temperature and Luminosity: Surface temperature drops to 2,000–5,000 K, but luminosity rises sharply due to vastly increased radius.
• Internal Fusion: Helium fusion begins in the core, producing heavier elements like carbon and oxygen.
• Evolutionary Stage: Represents the late life cycle of medium-mass stars; the Sun will enter this phase in about 5 billion years.
• End Stage: Outer layers are shed into a planetary nebula, leaving a white dwarf remnant that cools over time.

Key Findings of the Study:

• New Discovery: IIA established the first spectroscopic link between helium enhancement and lithium enrichment in red giant stars.
• Data Source: Based on Himalayan Chandra Telescope observations and archival global spectroscopic datasets.
• Sample Profile: 20 cool giants studied- 18 red giants and 2 supergiants.
• Helium-Enriched Stars: Six stars showed high helium-to-hydrogen ratios (He/H > 0.1).
• Distribution: Five were red giants and one a supergiant, showing a trend toward helium enhancement in lithium-rich giants.
• Scientific Insight: Offers direct evidence of deep internal mixing and nucleosynthesis shaping surface chemical composition.

What is the correlation between Lithium and Helium?

• Coupled Enrichment: All helium-enhanced giants were lithium-rich, suggesting a shared internal mixing mechanism.
• Asymmetry: Not all lithium-rich giants showed helium enhancement, implying lithium can rise without parallel helium increase.
• Internal Mixing Role: Deep convection likely dredges up newly formed helium and lithium from the interior to the photosphere.
• Photospheric Evidence: Confirms mixing-driven changes detectable on the stellar surface during the red giant stage.

Significance of the Findings:

• First Measurement: Provides the first direct spectroscopic photospheric helium estimates for normal and lithium-rich red giants.
• Astrophysical Value: Refines understanding of mixing, nucleosynthesis, and energy transport inside red giant branch (RGB) stars.
• Galactic Evolution: Improves models of how stars contribute heavier elements to the interstellar medium.
• Methodological Advance: Strengthens indirect helium-measurement techniques for cool stars where helium lines are not visible.
• Evolutionary Insight: Shows helium enrichment is integral to changes in luminosity, temperature evolution, and mass-loss pathways.

Get an IAS/IPS ranker as your 1: 1 personal mentor for UPSC 2024

Attend Now