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."± 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 ⩾ 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