No abstract

We report the discovery of 16 detached M dwarf eclipsing binaries with J < 16 mag and provide a detailed characterization of three of them, using high-precision infrared light curves from the WFCAM Transit Survey (WTS). Such systems provide the most accurate and modelindependent method for measuring the fundamental parameters of these poorly understood yet numerous stars, which currently lack sufficient observations to precisely calibrate stellar evolution models. We fully solve for the masses and radii of three of the systems, finding orbital periods in the range 1.5 < P < 4.9 d, with masses spanning 0.35-0.50 M and radii between 0.38 and 0.50 R , with uncertainties of ∼3.5-6.4 per cent in mass and ∼2.7-5.5 per cent in radius. Close companions in short-period binaries are expected to be tidally locked into fast rotational velocities, resulting in high levels of magnetic activity. This is predicted to inflate their radii by inhibiting convective flow and increasing starspot coverage. The radii of the WTS systems are inflated above model predictions by ∼3-12 per cent, in agreement with the observed trend, despite an expected lower systematic contribution from starspot signals at infrared wavelengths. We searched for correlation between the orbital period and radius inflation by combining our results with all existing M dwarf radius measurements of comparable precision, but we found no statistically significant evidence for a decrease in radius inflation for longer period, less active systems. Radius inflation continues to exists in non-synchronized systems, indicating that the problem remains even for very low activity M dwarfs. Resolving this issue is vital not only for understanding the most populous stars in the Universe, but also for characterizing their planetary companions, which hold the best prospects for finding Earth-like planets in the traditional habitable zone.

We report on the discovery of four ultra‐short‐period (P ≤ 0.18 d) eclipsing M‐dwarf binaries in the Wide‐Field Camera (WFCAM) Transit Survey. Their orbital periods are significantly shorter than that of any other known main‐sequence binary system, and are all significantly below the sharp period cut‐off at P ∼ 0.22  d as seen in binaries of earlier‐type stars. The shortest‐period binary consists of two M4‐type stars in a P = 0.112  d orbit. The binaries are discovered as part of an extensive search for short‐period eclipsing systems in over 260 000 stellar light curves, including over 10 000 M‐dwarfs down to J = 18 mag, yielding 25 binaries with P ≤ 0.23 d. In a popular paradigm, the evolution of short‐period binaries of cool main‐sequence stars is driven by the loss of angular momentum through magnetized winds. In this scheme, the observed P ∼ 0.22 d period cut‐off is explained as being due to time‐scales that are too long for lower‐mass binaries to decay into tighter orbits. Our discovery of low‐mass binaries with significantly shorter orbits implies that either these time‐scales have been overestimated for M‐dwarfs, e.g. due to a higher effective magnetic activity, or the mechanism for forming these tight M‐dwarf binaries is different from that of earlier‐type main‐sequence stars.

Using the Position and Proper Motion Extended-L (PPMXL) catalogue, we have used optical and near-infrared colour cuts together with a reduced proper motion cut to find bright M dwarfs for future exoplanet transit studies. PPMXL's low proper motion uncertainties allow us to probe down to smaller proper motions than previous similar studies. We have combined unique objects found with this method to that of previous work to produce 8479 K < 9 M dwarfs. Low resolution spectroscopy was obtained of a sample of the objects found using this selection method to gain statistics on their spectral type and physical properties. Results show a spectral type range of K7-M4V. This catalogue is the most complete collection of K < 9 M dwarfs currently available and is made available here.

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.