WTS-2 b: a hot Jupiter orbiting near its tidal destruction radius around a K dwarf

Abstract

We report the discovery of WTS-2 b, an unusually close-in 1.02-day hot Jupiter (M P = 1.12M J , R P = 1.363R J ) orbiting a K2V star, which has a possible gravitationally-bound M-dwarf companion at 0.6 arcsec separation contributing ∼ 20 percent of the total flux in the observed J-band light curve. The planet is only 1.5 times the separation from its host star at which it would be destroyed by Roche lobe overflow, and has a predicted remaining lifetime of just ∼ 40 Myr, assuming a tidal dissipation quality factor of Q ′ ⋆ = 10 6 . Q ′ ⋆ is a key factor in determining how frictional processes within a host star affect the orbital evolution of its companion giant planets, but it is currently poorly constrained by observations. We calculate that the orbital decay of WTS-2 b would correspond to a shift in its transit arrival time of T shift ∼ 17 seconds after 15 years assuming Q ′ ⋆ = 10 6 . A shift less than this would place a direct observational constraint on the lower limit of Q ′ ⋆ in this system. We also report a correction to the previously published expected T shift for WASP-18 b, finding that T shift = 356 seconds after 10 years for Q ′ ⋆ = 10 6 , which is much larger than the estimated 28 seconds quoted in WASP-18 b discovery paper. We attempted to constrain Q ′ ⋆ via a study of the entire population of known transiting hot Jupiters, but our results were inconclusive, requiring a more detailed treatment of transit survey sensitivities at long periods. We conclude that the most informative and straight-forward constraints on Q ′ ⋆ will be obtained by direct observational measurements of the shift in transit arrival times in individual hot Jupiter systems. We show that this is achievable across the mass spectrum of exoplanet host stars within a decade, and will directly probe the effects of stellar interior structure on tidal dissipation.

WASP-121 b: a hot Jupiter close to tidal disruption transiting an active F star

“…Fig. 12 (left) shows the distribution of the orbital distance to the Roche limit ratio (a/a R ) as a function of the planetary mass for the known transiting planets with 0.2 M Jup < M p < 13 M Jup and P < 12 d. WASP-121 b is one of the closest systems to tidal disruption, its direct competitors being WASP-12 b (Hebb et al 2009), WASP-19 b (Hebb et al 2010), WASP-103 b (Gillon et al 2014), OGLE-TR-56 b (Konacki et al 2003;Adams et al 2011), and WTS-2 b (Birkby et al 2014). According to Matsumura et al (2010), these extreme planets are now expected to undergo tidal orbital decay through tidal dissipation inside the star only (we will elaborate on this in Section 5.2 in the case of WASP-121 b).…”

“…Combining the Matsumura et al's expression for da/dt and the Kepler's third law, we can calculate the current rate of orbital period change: (dP/dt) 0 = −0.0035 (10 6 /Q ,0 ) s yr −1 for P , 0 = 0.89 d and (dP/dt) 0 = −0.0118 (10 6 /Q ,0 ) s yr −1 for P , 0 = 1.13 d. We can then estimate how long it would take to observe significant transit timing variations (TTVs) due to the orbital decay of the planet using, e.g. the equation 7 of Birkby et al (2014). We note that this estimation assumes a constant rate of orbital period change dP/dt = (dP/dt) 0 .…”

“…The finding of several hot Jupiters with orbital separations a lower than 2 a R , such as WASP-12 b (a/a R ∼ 1.09; Hebb et al 2009), WASP-19 b (a/a R ∼ 1.08; Hebb et al 2010), WASP-103 b (a/a R ∼ 1.16; Gillon et al 2014), OGLE-TR-56 b (a/a R ∼ 1.23; Konacki et al 2003;Adams et al 2011), and WTS-2 b (a/a R ∼ 1.27; Birkby et al 2014), challenged the scattering scenario as these planets would have been destroyed or completely ejected from their systems if they had been directly scattered to such short pericentre distances (see e.g. Guillochon, Ramirez-Ruiz & Lin 2011).…”

Departure from the constant-period ephemeris for the transiting exoplanet WASP-12 b

“…The decaying orbital period is expected to be observed through transit timing. For some planets, the cumulative shift in transit times may be about 10 2 s after ten years (Birkby et al 2014). Tentative detections of the orbital decay were reported for the systems OGLE-TR-113 and WASP-43 (Adams et al 2010;Blecic et al 2014), but have not been confirmed by further observations (Hoyer et al 2016;Jiang et al 2016).…”

The two-colour EMCCD instrument for the Danish 1.54 m telescope and SONG

“…This object consists of a gas giant (mass 1.4 M Jup , radius 1.7 R Jup ) transiting an F-type star (mass 1.3 M , radius 1.7 R ) every 1.21 days (Southworth 2012). The comparatively large component radii and short orbital period make OGLE-TR-56 a good candidate for the detection of tidally-induced orbital decay (Birkby et al 2014). This is potentially detectable by measuring the progressively earlier occurrence of transits compared to a linear ephemeris, by an amount which is predicted to be roughly 30 s over a decade (Birkby et al 2014).…”