Most of relatively warm , unevolved metal-poor stars ( T _ { eff } \gtrsim 5800 \mathrm { K } and [ { 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 [ { 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 .