A study of the O/Fe ratio in metal-poor main sequence and subgiant stars is presented using the [ \ion Oi ] 6300 Å line , the \ion Oi 7774 Å triplet , and a selection of weak \ion Feii lines observed on high-resolution spectra acquired with the VLT UVES spectrograph .
The [ \ion Oi ] line is detected in the spectra of 18 stars with -0.5 < \mbox { [ Fe / H ] } -2.4 , and the triplet is observed for 15 stars with [ Fe/H ] ranging from -1.0 to -2.7 .
The abundance analysis was made first using standard model atmospheres taking into account non-LTE effects on the triplet : the [ \ion Oi ] line and the triplet give consistent results with [ O/Fe ] increasing quasi-linearly with decreasing [ Fe/H ] reaching [ O/Fe ] \simeq +0.7 at [ Fe/H ] = -2.5 .
This trend is in reasonable agreement with other results for [ O/Fe ] in metal-poor dwarfs obtained using standard atmospheres and both ultraviolet and infrared OH lines .
There is also broad agreement with published results for [ O/Fe ] for giants obtained using standard model atmospheres and the [ \ion Oi ] line , and the OH infrared lines , but the \ion Oi lines give higher [ O/Fe ] values which may , however , fall into place when non-LTE effects are considered .
When hydrodynamical model atmospheres representing stellar granulation in dwarf and subgiant stars replace standard models , the [ O/Fe ] from the [ \ion Oi ] and \ion Feii lines is decreased by an amount which increases with decreasing [ Fe/H ] .
These 3D effects on [ O/Fe ] is compounded by the opposite behaviour of the [ \ion Oi ] ( continuous opacity effect ) and \ion Feii lines ( excitation effect ) .
The [ O/Fe ] vs [ Fe/H ] relation remains quasi-linear extending to [ O/Fe ] \simeq +0.5 at [ Fe/H ] = -2.5 , but with a tendency of a plateau with [ O/Fe ] \simeq +0.3 for -2.0 < [ Fe/H ] < -1.0 , and a hint of cosmic scatter in [ O/Fe ] at \mbox { [ Fe / H ] } \simeq - 1.0 .
Use of the hydrodynamical models disturbs the broad agreement between the oxygen abundances from the [ \ion Oi ] , \ion Oi and OH lines , but 3D non-LTE effects may serve to erase these differences .
The [ O/Fe ] values from the [ \ion Oi ] line and the hydrodynamical model atmospheres for dwarfs and subgiant stars are lower than the values for giants using standard model atmospheres and the [ \ion Oi ] and \ion Oi lines .