This paper presents the results of a detailed theoretical investigation of the impact of non-LTE effects and of granulation inhomogeneities on the derived iron and oxygen abundances in the metal-poor halo subgiant HD140283 .
Our analysis is based on both the ‘ classical ’ one-dimensional ( 1D ) stellar atmosphere models and on the new generation of three-dimensional ( 3D ) hydrodynamical models .
The non-LTE calculations presented here have been carried out without inelastic collisions with neutral hydrogen atoms .
We find that if non-LTE effects are taken into account when synthetizing the Fe i spectrum in both type of models , then the derived iron abundance turns out to be very similar in both cases .
The emergent spectral line profiles in both models are very much weaker in non-LTE than in LTE because the UV overionization mechanism produces a very strong underpopulation of the Fe i levels , in particular in the granular regions of the 3D model .
As a result , the non-LTE effects on the derived iron abundance are very important , amounting to { \sim } 0.9 dex and to { \sim } 0.6 dex in the 3D and 1D cases , respectively .
On the other hand , we find that non-LTE and 3D effects have to be taken into account for a reliable determination of the iron abundance from weak Fe ii lines , because the significant overexcitation of their upper levels in the granular regions tend to produce emission features .
As a result such Fe ii lines are weaker than in LTE and the abundance correction amounts to { \sim } 0.4 dex for the 3D case .

We derive also the oxygen-to-iron abundance ratio in the metal-poor star HD140283 by using the O i triplet at 7772–5 Å and the forbidden [ O i ] line at 6300 Å .
Our 3D results for the oxygen abundance confirm the values reported in some recent investigations .
While the oxygen abundance derived from the O i IR triplet is not very sensitive to the presence of granulation inhomogeneities , such 3D effects amount to { \sim } -0.2 dex for the [ O i ] line .
The non-LTE abundance correction for the O i IR triplet turns out to be -0.2 dex , approximately .

Interestingly , when both non-LTE and 3D effects are taken into account there still remains significant discrepancies in the iron abundances derived from Fe i and Fe ii lines , as well as in the oxygen abundances inferred from the O i and [ O i ] lines .
We conclude that the discrepancies could be due to uncertainties in the stellar parameters of this metal poor star .
We argue that adopting T _ { eff } { \approx } 5600 K ( instead of T _ { eff } { \approx } 5700 K ) and [ Fe/H ] { \approx } -2.0 ( instead of [ Fe/H ] { \approx } -2.5 ) substantially reduces the discrepancies in the abundances of iron and oxygen inferred from several spectral lines .
Under such circumstances we find [ O/Fe ] { \approx } 0.5 at [ Fe/H ] = -2 .
Obviously , our tentative conclusion that the metalicity of this type of metal-poor stars is significantly larger than previously thought may have far-reaching implications in stellar astrophysics .