COol Companions ON Ultrawide orbiTS (COCONUTS). I. A High-Gravity T4 Benchmark around an Old White Dwarf and A Re-Examination of the Surface-Gravity Dependence of the L/T Transition

Zhoujian Zhang; Michael C. Liu; J. J. Hermes; Eugene A. Magnier; Mark S. Marley; Pier-Emmanuel Tremblay; Michael A. Tucker; Aaron Do; Anna V. Payne; Benjamin J. Shappee

doi:10.3847/1538-4357/ab765c

paper

COol Companions ON Ultrawide orbiTS (COCONUTS). I. A High-Gravity T4 Benchmark around an Old White Dwarf and A Re-Examination of the Surface-Gravity Dependence of the L/T Transition

arXiv:2002.05723 · doi:10.3847/1538-4357/ab765c

Abstract

We present the first discovery from the COol Companions ON Ultrawide orbiTS (COCONUTS) program, a large-scale survey for wide-orbit planetary and substellar companions. We have discovered a co-moving system COCONUTS-1, composed of a hydrogen-dominated white dwarf (PSO J058.9855+45.4184; $d = 31.5$ pc) and a T4 companion (PSO J058.9869+45.4296) at a $40. 6^{''}$ (1280 au) projected separation. We derive physical properties for COCONUTS-1B from (1) its near-infrared spectrum using cloudless Sonora atmospheric models, and (2) its luminosity and the white dwarf's age ( $7. 3_{- 1.6}^{+ 2.8}$ Gyr) using Sonora evolutionary models. The two methods give consistent temperatures and radii, but atmospheric models infer a lower surface gravity and therefore an unphysically young age. Assuming evolutionary model parameters ( $T_{eff} = 125 5_{- 8}^{+ 6}$ K, $lo g g = 5.4 4_{- 0.03}^{+ 0.02}$ dex, $R = 0.78 9_{- 0.005}^{+ 0.011}$ R $_{Jup}$ ), we find cloudless model atmospheres have brighter Y- and J-band fluxes than the data, suggesting condensate clouds have not fully dispersed around 1300 K. The W2 flux (4.6 $μ$ m) of COCONUTS-1B is fainter than models, suggesting non-equilibrium mixing of CO. To investigate the gravity dependence of the L/T transition, we compile all 60 known L6-T6 benchmarks and derive a homogeneous set of temperatures, surface gravities, and masses. As is well-known, young, low-gravity late-L dwarfs have significantly fainter, redder near-infrared photometry and $\approx 200 - 300$ K cooler temperatures than old, high-gravity objects. Our sample now reveals such gravity dependence becomes weaker for T dwarfs, with young objects having comparable near-infrared photometry and $\approx 100$ K cooler temperatures compared to old objects. Finally, we find that young objects have a larger amplitude J-band brightening than old objects, and also brighten at H band as they cross the L/T transition.

ApJ, in press. 54 pages including 16 figures and 5 tables

COol Companions ON Ultrawide orbiTS (COCONUTS). I. A High-Gravity T4 Benchmark around an Old White Dwarf and A Re-Examination of the Surface-Gravity Dependence of the L/T Transition

Abstract

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