The atmospheric parameters and iron abundance of the Sloan Digital Sky Survey ( SDSS ) spectrophotometric standard star BD +17 4708 are critically examined using up-to-date Kurucz model atmospheres , LTE line formation calculations , and reliable atomic data .
We find T _ { \mathrm { eff } } = 6141 \pm 50 K , \log g = 3.87 \pm 0.08 , and \mathrm { [ Fe / H ] } = -1.74 \pm 0.09 .
The line-of-sight interstellar reddening , bolometric flux , limb-darkened angular diameter , stellar mass , and the abundances of Mg , Si , and Ca are also obtained : E ( B - V ) = 0.010 \pm 0.003 , f _ { \mathrm { bol } } = ( 4.89 \pm 0.10 ) \times 10 ^ { -9 } erg cm ^ { -2 } s ^ { -1 } , \theta = 0.1016 \pm 0.0023 mas , M = 0.91 \pm 0.06 M _ { \odot } , \mathrm { [ Mg / Fe ] } = 0.40 \pm 0.10 , \mathrm { [ Si / Fe ] } = 0.35 \pm 0.11 , \mathrm { [ Ca / Fe ] } = 0.36 \pm 0.11 .
This star is a unique example of a moderately metal-poor star for which the effective temperature ( T _ { \mathrm { eff } } ) can be accurately constrained from the observed spectral energy distribution ( corrected for reddening ) .
Such analysis leads to a value that is higher than most spectroscopic results previously reported in the literature ( \sim 5950 K ) .
Interstellar reddening was estimated using various prescriptions , including an analysis of interstellar lines .
The surface gravity of the star was inferred from the fitting of the wings of the Mg i b lines .
We used transition probabilities measured in the laboratory and reliable damping constants for unblended Fe lines to derive the iron abundance using both Fe i and Fe ii lines .
We find that the ionization balance of Fe lines is satisfied only if a low T _ { \mathrm { eff } } ( \sim 5950 K ) is adopted .
The mean iron abundance we obtain from the Fe ii lines corresponds to A _ { \mathrm { Fe } } = 5.77 \pm 0.09 ( \mathrm { [ Fe / H ] } = -1.74 for our derived A _ { \mathrm { Fe } , \odot } = 7.51 ) while that from the Fe i lines is A _ { \mathrm { Fe } } = 5.92 \pm 0.11 , and therefore with our preferred T _ { \mathrm { eff } } ( 6141 K ) , the discrepancy between the mean iron abundance from Fe i and Fe ii lines can not be explained by overionization by UV photons as the main non-LTE effect .
Interestingly , the Fe i excitation balance is satisfied with a T _ { \mathrm { eff } } only slightly warmer than our preferred solution and not with the lower value of 5950 K. We also comment on non-LTE effects and the importance of inelastic collisions with neutral H atoms in the determination of oxygen abundances in metal-poor stars from the 7774 Å O i triplet .