Astronomy and Astrophysics, volume 459, 613-625 (2006/11-4)
Fundamental parameters and abundances of metal-poor stars: the SDSS standard BD +17 4708.
RAMIREZ I., ALLENDE PRIETO C., REDFIELD S. and LAMBERT D.L.
Abstract (from CDS):
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 Teff=6141±50K, logg=3.87±0.08, and [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, fbol=(4.89±0.10)x10–9 erg/cm2/s, θ=0.1016±0.0023 mas, M=0.91±0.06M☉, [Mg/Fe]=0.40±0.10, [Si/Fe]=0.35±0.11, [Ca/Fe]=0.36±0.11. This star is a unique example of a moderately metal-poor star for which the effective temperature (Teff) 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 (∼5950K). 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 MgIb lines. We used transition probabilities measured in the laboratory and reliable damping constants for unblended Fe lines to derive the iron abundance using both FeI and FeII lines. We find that the ionization balance of Fe lines is satisfied only if a low Teff (∼5950K) is adopted. The mean iron abundance we obtain from the FeII lines corresponds to AFe=5.77±0.09 ([Fe/H]=-1.74 for our derived AFe,☉=7.51) while that from the FeI lines is AFe=5.92±0.11, and therefore with our preferred Teff (6141K), the discrepancy between the mean iron abundance from FeI and FeII lines cannot be explained by overionization by UV photons as the main non-LTE effect. Interestingly, the FeI excitation balance is satisfied with a Teff only slightly warmer than our preferred solution and not with the lower value of 5950K. 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 7774Å OI triplet.