The formation of the UV OH spectral lines has been investigated for a range of stellar parameters in the light of 3D hydrodynamical model atmospheres .
The low atmospheric temperatures encountered at low metallicities compared with the radiative equilibrium values enforced in classical 1D hydrostatic model atmospheres have a profound impact on the OH line strengths .
As a consequence , the derived O abundances using 3D models are found to be systematically lower by more than 0.6 dex at [ Fe/H ] = -3.0 compared with previous 1D analyses , casting doubts on the recent claims for a monotonic increase in [ O/Fe ] towards lower metallicities .
In fact , taken at face value the resulting 3D LTE trend is in rough agreement with the conventional [ O/Fe ] plateau .
Caution must , however , be exercised in view of the remaining assumptions in the 3D calculations .
We have verified that the stellar parameters remain essentially unchanged with 3D model atmospheres provided that the infrared flux method ( \Delta T _ { eff } \la 20 K ) , Hipparcos parallaxes ( \Delta { log } g \la 0.05 ) and Fe ii lines ( \Delta [ { Fe / H } ] \la 0.1 dex ) are utilised , leaving the 3D O abundances from OH lines largely intact ( \Delta [ { O / H } ] \la 0.05 dex ) .
Greater concern stems from possible departures from LTE in both the line formation and the molecular equilibrium , which , if present , would increase the derived O abundances again .
Non-LTE line formation calculations with 1D model atmospheres suggest no significant steepening of the [ O/Fe ] trend even if the abundance corrections amount to about 0.2 dex for all investigated stellar parameters .
We note , however , that the 3D case may not necessarily be as metallicity-independent .
The apparent lack of laboratory or theoretical rate coefficients at the relevant temperatures for the involved molecular reactions unfortunately prevents a quantitative discussion on the possible effects of non-equilibrium chemistry .