Fundamental parameters and abundances of metal-poor stars: the SDSS standard BD +17 4708

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 $T_{{\rm eff}}=6141$ ± 50 K, $\log g=3.87$ ± 0.08, and ${\rm [Fe/H]}=-1.74$ ± 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: $E(B-V)=0.010$ ± 0.003, $f_{{\rm bol}}=(4.89\pm0.10)$ $\times$ 10-9 erg cm-2 s-1, $\theta=0.1016$ ± 0.0023 mas, $M=0.91$ ± $0.06~M_\odot$, ${\rm [Mg/Fe]}=0.40$ ± 0.10, ${\rm [Si/Fe]}=0.35$ ± 0.11, ${\rm [Ca/Fe]}=0.36$ ± 0.11. This star is a unique example of a moderately metal-poor star for which the effective temperature ($T_{{\rm eff}}$) 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 (~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 $\ion{Mg}{i}$ 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 $\ion{Fe}{i}$ and $\ion{Fe}{ii}$ lines. We find that the ionization balance of Fe lines is satisfied only if a low $T_{{\rm eff}}$ (~5950 K) is adopted. The mean iron abundance we obtain from the $\ion{Fe}{ii}$ lines corresponds to $A_{\rm Fe}=5.77$ ± 0.09 (${\rm [Fe/H]}=-1.74$ for our derived $A_{{\rm Fe},\odot}=7.51$) while that from the $\ion{Fe}{i}$ lines is $A_{\rm Fe}=5.92$ ± 0.11, and therefore with our preferred $T_{{\rm eff}}$ (6141 K), the discrepancy between the mean iron abundance from $\ion{Fe}{i}$ and $\ion{Fe}{ii}$ lines cannot be explained by overionization by UV photons as the main non-LTE effect. Interestingly, the $\ion{Fe}{i}$ excitation balance is satisfied with a $T_{{\rm eff}}$ 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 $\ion{O}{i}$ triplet