Angular Diameters of the G Subdwarf μ Cassiopeiae A and the K Dwarfs σ Draconis and HR 511 from Interferometric Measurements with the CHARA Array

Using the longest baselines of the CHARA Array, we have measured the angular diameter of the G5 V subdwarf $mu$ Cas A, the first such determination for a halo population star. We compare this result to new diameters for the higher metallicity K0 V stars, $sigma$ Dra and HR 511, and find that the metal-poor star, $mu$ Cas A, has an effective temperature ($T_{ m eff}=5297pm32$ K), radius ($R=0.791pm0.008 R_{ m odot}$), and absolute luminosity ($L=0.442pm0.014 L_{ m odot}$) comparable to the other two stars with later spectral types. We show that stellar models show a discrepancy in the predicted temperature and radius for $mu$ Cas A, and we discuss these results and how they provide a key to understanding the fundamental relationships for stars with low metallicity.