The TRENDS High-contrast Imaging Survey. V. Discovery of an Old and Cold Benchmark T-dwarf Orbiting the Nearby G-star HD 19467

The nearby Sun-like star HD 19467 shows a subtle radial velocity (RV) acceleration of -1.37 ± 0.09 m s^-1 yr^-1 over a 16.9 yr time baseline (an RV trend), hinting at the existence of a distant orbiting companion. We have obtained high-contrast images of the star using NIRC2 at Keck Observatory and report the direct detection of the body that causes the acceleration. The companion, HD 19467 B, is ΔK_s = 12.57 ± 0.09 mag fainter than its parent star (contrast ratio of 9.4 × 10^-6), has blue colors J - K_s = -0.36 ± 0.14 (J - H = -0.29 ± 0.15), and is separated by ρ = 1.''653 ± 0.''004 (51.1 ± 1.0 AU). Follow-up astrometric measurements obtained over a 1.1 yr time baseline demonstrate physical association through common parallactic and proper motion. We calculate a firm lower-limit of m\ge 51.9^{+3.6}_{-4.3}M_J for the companion mass from orbital dynamics using a combination of Doppler observations and imaging. We estimate a model-dependent mass of m=56.7^{+4.6}_{-7.2}M_{Jup} from a gyrochronological age of 4.3^{+1.0}_{-1.2} Gyr. Isochronal analysis suggests a much older age of 9 ± 1 Gyr, which corresponds to a mass of m=67.4^{+0.9}_{-1.5}M_J. HD 19467 B's measured colors and absolute magnitude are consistent with a late T dwarf [≈T5-T7]. We may infer a low metallicity of [Fe/H] =-0.15 ± 0.04 for the companion from its G3V parent star. HD 19467 B is the first directly imaged benchmark T dwarf found orbiting a Sun-like star with a measured RV acceleration.