A fine analysis of calcium and iron lines in the spectrum of the K0 giant Pollux: accurate fundamental parameters

Yellow and red spectral regions containing calcium and iron lines have been observed for the bright K0 III standard Pollux (β Gem). A fine and detailed model atmosphere analysis of the resulting spectra, which are of high quality, has yielded the following fundamental parameters. Effective temperature: $$T_\text{eff} = 4865 \pm 50\enspace\text K$$. Surface gravity: $$\text {log}\enspace g = 2.75 \pm 0.15\enspace\text{dex} \enspace(\text {g in cm s}^{-2}) $$. Logarithmic iron abundance (relative to the Sun): $$\text{[Fe/H]} = - 0.04 \pm 0.05\enspace \text {dex}$$. Logarithmic calcium abundance (relative to H = 12.00): $$\text{Ca/H} = 6.25\pm 0.1\enspace\text {dex}$$. Microturbulence:$$\xi = 1.4 \pm 0.1 \enspace\text{km}\enspace\text s^{-1}$$. The results of detailed radiative transfer and statistical equilibrium calculations have been used to account for departures from LTE in the calcium spectrum. Large departures from LTE have been confirmed in low-excitation and strong, high-excitation lines of Fe I. Weak, high-excitation Fe I lines appear to be formed in conditions close to LTE. The temperature–depth relations of various model atmospheres have been tested against the wings of the extremely strong ionized calcium line at 8542 Å and an optimum model atmosphere, based on this line profile, has been determined. The spectroscopic mass of Pollux, derived using the above physical parameters, is 1.7 M_⊙. This mass is consistent with the position of Pollux on the theoretical H–R diagram and suggests that the star is either on the first ascent of the red-giant branch or in the post-helium-flash clump-giant stage.