WASP-128b: a transiting brown dwarf in the dynamical-tide regime

Abstract

Massive companions in close orbits around G dwarfs are thought to undergo rapid orbital decay due to runaway tidal dissipation. We report here the discovery of WASP-128b, a brown dwarf discovered by the WASP survey transiting a G0V host on a $2.2\mathrm{d}$ orbit, where the measured stellar rotation rate places the companion in a regime where tidal interaction is dominated by dynamical tides. Under the assumption of dynamical equilibrium, we derive a value of the stellar tidal quality factor $\log Q_{\star }^{\prime }=6.96\pm 0.19$. A combined analysis of ground-based photometry and high-resolution spectroscopy reveals a mass and radius of the host, $M_{\star }=1.16\pm 0.04M_{\odot }$, $R_{\star }=1.16\pm 0.02R_{\odot }$, and for the companion, $M_{\mathrm{b}}=37.5\pm 0.8M_{\mathrm{Jup}}$, $R_{\mathrm{b}}=0.94\pm 0.02R_{\mathrm{Jup}}$, placing WASP-128b in the driest parts of the brown dwarf desert, and suggesting a mild inflation for its age. We estimate a remaining lifetime for WASP-128b similar to that of some ultra-short period massive hot Jupiters, and note it may be a propitious candidate for measuring orbital decay and testing tidal theories.

7 pages, 3 figures. Accepted in MNRAS. Added references, fixed typos, and clarified some parts of the text

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