Lithium in CEMP-no stars: A new constraint on the lithium depletion mechanism in the early universe

TL;DR

This study investigates lithium abundances in ultra metal-poor CEMP-no stars to understand lithium depletion mechanisms in the early universe, finding that low iron abundance, not carbon enhancement, correlates with lithium depletion.

Contribution

The paper provides new lithium abundance measurements for CEMP-no stars at [Fe/H] ≈ -3.0, offering insights into lithium depletion origins in the early universe.

Findings

CEMP-no stars at [Fe/H] ≈ -3.0 have lithium levels near the plateau.

Lithium depletion in UMP stars is more likely linked to low iron abundance than carbon enhancement.

Results suggest iron deficiency plays a key role in lithium depletion in the early universe.

Abstract

Most of relatively warm, unevolved metal-poor stars ($T_{\rm eff}\gtrsim 5800\,\mathrm{K}$ and $[{\rm Fe/H}]\lesssim -1.5$) exhibit almost constant lithium abundances, irrespective of metallicity or effective temperature, and thus form the so-called Spite plateau. This was originally interpreted as arising from lithium created by the Big Bang nucleosynthesis. Recent observations, however, have revealed that ultra metal-poor stars (UMP stars; $[\mathrm{Fe/H}]<-4.0$) have significantly lower lithium abundances than that of the plateau. Since most of the UMP stars are CEMP-no stars, carbon-enhanced metal-poor stars with no excess of neutron-capture elements, a connection between the carbon enhancement and lithium depletion is suspected. A straightforward approach to this question is to investigate carbon-normal UMP stars. However only one object is known in this class. As an alternative, we have determined lithium abundances for two CEMP-no main-sequence turn-off stars with metallicities $[{\rm Fe/H}]\sim -3.0$, where there are numerous carbon-normal stars with available lithium abundances that can be considered. Our 1D LTE analysis indicates that the two CEMP-no stars have lithium abundances that are consistent with values near the plateau, which suggests that carbon enhancement and lithium depletion are not directly related. Instead, our results suggest that extremely low iron abundance is a fundamental cause to depleted lithium in UMP stars.