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

Abstract

Abstract Most relatively warm, unevolved, metal-poor stars (Teff ≳ 5800 K and [Fe/H] ≲ −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; [Fe/H] < −4.0) have significantly lower lithium abundances than those of the plateau. Since most of the UMP stars are carbon-enhanced metal-poor stars with no excess of neutron-capture elements (CEMP-no stars), 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 [Fe/H] ∼ −3.0, where there are numerous carbon-normal stars with available lithium abundances that can be considered. Our 1D local thermodynamic equilibrium 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 of depleted lithium in UMP stars.