Molecular absorption lines of OH ( 99 lines ) and CH ( 105 lines ) are measured for the carbon-enhanced metal-poor star BD+44 ^ { \circ } 493 with [ Fe/H ] = -3.8 .
The abundances of oxygen and carbon determined from individual lines based on an 1D-LTE analysis exhibit significant dependence on excitation potentials of the lines ; d \log \epsilon / d \chi \sim - 0.15 – -0.2 dex/eV , where \epsilon and \chi are elemental abundances from individual spectral lines and their excitation potentials , respectively .
The dependence is not explained by the uncertainties of stellar parameters , but suggests that the atmosphere of this object possesses a cool layer that is not reproduced by the 1D model atmosphere .
This result agrees with the predictions by 3D model calculations .
Although absorption lines of neutral iron exhibit similar trend , it is much weaker than found in molecular lines and that predicted by 3D LTE models .