Angular Diameters of the G Subdwarf $\hat{I}¼$ Cassiopeiae A and the K Dwarfs $\ddot{I}ƒ$ Draconis and HR 511 from Interferometric Measurements with the CHARA Array

Abstract

Using the longest baselines of the CHARA Array, we have measured the angular diameter of the G5 V subdwarf $\hat{I}¼$ 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, $\ddot{I}ƒ$ Dra and HR 511, and find that the metal-poor star, $\hat{I}¼$ Cas A, has an effective temperature ($T_{\mathrm{eff}}=5297\pm 32$ K), radius ($R=0.791\pm 0.008R_{\odot }$), and absolute luminosity ($L=0.442\pm 0.014L_{\odot }$) 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 $\hat{I}¼$ Cas A, and we discuss these results and how they provide a key to understanding the fundamental relationships for stars with low metallicity.

Accepted for publication in The Astrophysical Journal