Three Nearby K-Giants with Planets: Accurate Determination of Basic Parameters, Including an Analysis of Metallicity Based on Fe I Lines

Fundamental parameters, including the effective temperature T eff , surface gravity log g , mass M , luminosity L , radius R , and age t , are determined for three bright, nearby K-giants, β Gem (K0 III), μ Leo (K2 III), and α Tau (K5 III), It is notable that around all three stars the giant planets have been found. Our results are compared with published high-precision data for benchmark stars included in the Gaia project. Very good agreement was obtained for all three giants with these data for the basic parameters T eff , log g , and M , although our technique of their determination was simpler. Special attention was devoted to analysis of the Fe I lines which are the basis of a simultaneous determination of the metallicity index [ Fe/H ] and microturbulent parameter V t . The equivalent widths W of the Fe I lines are automatically measured from published spectra for the benchmark stars. An analysis of Fe I lines from the list of “golden lines” selected in a study of benchmark stars led to the conclusion that the excitation potential E l of the low level of the lines plays an substantial role in determining [ Fe/H ] and V t . It is shown that in the case of the early K-giants β Gem and μ Leo with temperatures T eff between 4400 and 4900 K for lines in the range of W from 100 to 300 mÅ there is a dependence of the [ Fe/H ] and V t values on E l . This dependence was not taken into account earlier is studies of the K-giants, in particular in the Fe I lines analysis of the benchmark stars. It is shown that a correct accounting for it leads an ambiguity in the determination of [ Fe/H ] and V t for β Gem and μ Leo. In the case of the coolest K-giant α Tau (Aldebaran) with a temperature T eff = 3920K, this effect seems to be less pronounced. Recommendations are given for the Fe I lines selection for [ Fe/H ] and V t determination. It is sonfirmed for the example of the three stars studied here that the non-LTE effects in the Fe I lines for the K-giants with normal metallicity are very minor, so they cannot be the reason of the revealed ambiguity in [ Fe/H ] and V t . values. The low ratios of the carbon 12 C/ 13 C and oxygen 16 O/ 17 O isotopes confirm that all three giants passed the phase of deep convective mixing.