Optical time series photometry of the short‐period magnetic white dwarf + probable brown dwarf binary SDSS J121209.31+013627.7 reveals pulse‐like variability in all bands from i′ to u′, increasing towards bluer wavelengths and peaking at u′. These modulations are most likely due to a self‐eclipsing accretion hot spot on the white dwarf, rotating into view every 88.43 min. This period is commensurate with the Hα radial velocity period of ≈90 min, and consistent with the rotation period of the accretor being equal to the binary orbital period. We combine our observations with other recently reported results to provide an accurate ephemeris. We also detect the system in X‐rays with Swift, and estimate the accretion rate at ≈10−13 M⊙ yr−1. We suggest that SDSS J121209.31+013627.7 is most likely a magnetic cataclysmic variable in an extended state of very low accretion, similar to the well‐studied polar EF Eri. Alternatively, the putative brown dwarf is not filling its Roche lobe and the system is a detached binary in which the white dwarf is efficiently accreting from the wind of the secondary. However, it is unclear whether an L dwarf wind is strong enough to provide the measured accretion rate. We suggest further observations to distinguish between the Roche lobe overflow and wind accretion scenarios.
We report the discovery of 76 new T dwarfs from the UKIDSS Large Area Survey (LAS). Near-infrared broad and narrow-band photometry and spectroscopy are presented for the new objects, along with WISE and warm-Spitzer photometry. Proper motions for 128 UKIDSS T dwarfs are presented from a new two epoch LAS proper motion catalogue. We use these motions to identify two new benchmark systems: LHS 6176AB, a T8+M4 pair and HD118865AB, a T5.5+F8 pair. Using age constraints from the primaries and evolutionary models to constrain the radii we have estimated their physical properties from their bolometric luminosity. We compare the colours and properties of known benchmark T dwarfs to the latest model atmospheres and draw two principal conclusions. Firstly, it appears that the H − [4.5] and J − W 2 colours are more sensitive to metallicity than has previously been recognised, such that differences in metallicity may dominate over differences in T eff when considering relative properties of cool objects using these colours. Secondly, the previously noted apparent dominance of young objects in the late-T dwarf sample is no longer apparent when using the new model grids and the expanded sample of late-T dwarfs and bench-
Presented are the results of a large and deep optical–near‐infrared multi‐epoch survey of the Praesepe open star cluster using data from the UKIRT Infrared Deep Sky Survey (UKIDSS) Galactic Clusters Survey. Multiple colour–magnitude diagrams were used to select potential members and proper motions were used to assign levels of membership probability. From our sample, 145 objects were designated as high probability members (p≥ 0.6) with most of these having been found by previous surveys although 14 new cluster members are also identified. Our membership assignment is restricted to the bright sample of objects (Z < 18). From the fainter sample, 39 candidates were found from an examination of multiple colour–magnitude plots. Of these, two have small but significant membership probabilities. Finally, using theoretical models, cluster luminosity and mass functions were plotted with the latter being fitted with a power law of α= 1.11 ± 0.37 for the mass range 0.6 to 0.125 M⊙ and an assumed cluster age of 500 Myr in the UKIDSS Z photometric band. Likewise taking an assumed cluster age of 1 Gyr we find α= 1.10 ± 0.37. Similar values were also found for the J and K bands. These results compare favourably with the result of Kraus & Hillenbrand (α= 1.4 ± 0.2) but are significantly lower than that of the more recent study conducted by Boudreault et al. (α= 1.8 ± 0.1).
The definitive version can be found at: http://onlinelibrary.wiley.com/ Copyright Royal Astronomical SocietyWe have carried out a search for late-type T dwarfs in the UKIRT Infrared Deep Sky Survey Galactic Plane Survey 6th Data Release. The search yielded two persuasive candidates, both of which have been confirmed as T dwarfs. The brightest, UGPS J0521+3640, has been assigned the spectral type T8.5 and appears to lie at a distance of 7-9 pc. The fainter of the two, UGPS J0652+0324, is classified as a T5.5 dwarf and lies at an estimated distance of 28-37 pc. Warm-Spitzer observations in IRAC channels 1 and 2, taken as part of the GLIMPSE360 Legacy Survey, are available for UGPS J0521+3640 and we used these data with the near-infrared spectroscopy to estimate its properties. We find best-fitting solar metallicity BT-Settl models for T-eff = 600 and 650 K and log g = 4.5 and 5.0. These parameters suggest a mass between 14 and 32M(J) for an age between 1 and 5 Gyr. The proximity of this very cool T dwarf, and its location in the Galactic plane, makes it an ideal candidate for high-resolution adaptive optics imaging to search for cool companions
We present the results of a photometric and astrometric study of the low‐mass stellar and substellar population of the young open cluster Blanco 1. We have exploited J‐band data, obtained recently with the Wide Field Camera (WFCAM) on the United Kingdom Infrared Telescope (UKIRT), and 10‐year‐old I‐ and z‐band optical imaging from CFH12k on the Canada–France–Hawaii Telescope (CFHT), to identify 44 candidate low‐mass stellar and substellar members, in an area of 2 deg2, on the basis of their colours and proper motions. This sample includes five sources which are newly discovered. We also confirm the lowest mass candidate member of Blanco 1 unearthed so far (29MJup). We determine the cluster mass function to have a slope of α = +0.93, assuming it to have a power‐law form. This is high, but nearly consistent with previous studies of the cluster (to within the errors), and also that of its much better studied Northern hemisphere analogue, the Pleiades.
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