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Aims. We try to identify ultra-cool dwarfs from the seventh Data Release of the Sloan Digital Sky Survey (SDSS DR7) with SDSS i-zand r-zcolors. We also obtain proper motion data from SDSS, 2MASS, and UKIDSS and improve spectral typing from SDSS and 2MASS photometric colors.

Aims.We present an analysis of high resolution spectra in the Jband of five ultra cool dwarfs from M6 to L0.

Context.We report on the first ultracool dwarf discoveries from the UKIRT Infrared Deep Sky Survey (UKIDSS) Large Area Survey Early Data Release (LAS EDR), in particular the discovery of T dwarfs which are fainter and more distant than those found using the 2MASS and SDSS surveys.

We present results of modelling the bulk of the spectral energy distribution (0.35–5 μm) for GJ406 (M6V). Synthetic spectra were calculated using the NextGen, Dusty and Cond model atmospheres and incorporate line lists for H2O, TiO, CrH, FeH, CO, MgH molecules as well as the VALD line list of atomic lines. A comparison of synthetic and observed spectra gives Teff= $2800\pm 100$K. We determine $M_{\rm bol} = 12.13 \pm 0.10$for which evolutionary models by Baraffe et al. (2003, A&A, 402, 711) suggest an age of around 0.1–0.35 Gyr consistent with its high activity. The age and luminosity of GJ406 correspond to a wide range of plausible masses (0.07–0.1  $M_{\odot}$).

We provide a set of atomic lines which are suitable for the description of ultracool dwarf spectra from 10 000 to 25 000 Å. This atomic linelist was made using both synthetic spectra calculations and existing atlases of infrared spectra of Arcturus and Sunspot umbra. We present plots which show the comparison of synthetic spectra and observed Arcturus and Sunspot umbral spectra for all atomic lines likely to be observable in high resolution infrared spectra.

We present a catalogue of 6206 stars which have proper motions exceeding $0.18\arcsec$yr-1with an R-band faint magnitude limit of 19.5 mag. This catalogue has been produced using SuperCOSMOS digitized R-band ESO and UK Schmidt Plates in 131 Schmidt fields covering more than 3 000 square degrees (>7.5% of the whole sky) at the South Galactic Cap. The survey is ≥90% complete within the nominal limits of the Luyten Two Tenths Catalogue of $m_R\le18.5$mag and $0.2\le\mu\le2.5\arcsec$yr-1, and is ≥80% complete for $m_R\le19.5$mag and $\mu\le2.5\arcsec$yr-1.

New radial velocities of αCen A & B obtained in the framework the Anglo-Australian Planet Search programme as well as in the CORALIE programme are added to those by Endl et al. ([CITE]) to improve the precision of the orbital parameters. The resulting masses are $1.105\pm0.0070$$M_{\odot}$and $0.934\pm0.0061$$M_{\odot}$for A and B respectively. The factors limiting how accurately these masses can be derived from a combined visual-spectroscopic solution are investigated. The total effect of the convective blueshift and the gravitational redshift is also investigated and estimated to differ by $215\,\pm\,8$m s-1between the components. This suggests that the difference in convective blueshift between the components is much smaller than predicted from current hydrodynamical model atmosphere calculations.

We present a machine selected catalogue of 11 289 objects with proper motions exceeding 0.18$\arcsec$ ${\rm yr}^{-1}$ and an $R$-band faint magnitude limit of 19.5 mag. The catalogue was produced using SuperCOSMOS digitized $R$-Band ESO and UK Schmidt Plates in 287 Schmidt fields covering almost 7000 square degrees (~17% of the whole sky) at the South Galactic Cap. The catalogue includes UK Schmidt $B_J$ and $I$ magnitudes for all of the stars as well as 2MASS magnitudes for 10 447 of the catalogue stars. We also show that the NLTT is ~95% complete for Dec $>$ -32.5$^{\circ}$.

We compare the observational and theoretical spectra of the $\Delta v = 2$ CO bands in a range of M dwarfs. We investigate the dependence of theoretical spectra on effective temperatures as well as carbon abundance. In general we find that the synthetic CO bands fit the observed data extremely well and are excellent diagnostics. In particular the synthetic spectra reasonably match observations and the best fit temperatures are similar to those found by empirical methods. We also examine the $^{12}$C/$^{13}$C isotopic ratio. We find that fundamental $^{13}$CO bands around 2.345 and 2.375 $\mu$m are good discriminators for the $^{12}$C/$^{13}$C ratio in M dwarfs. The 2.375 $\mu$m band is more useful because it doesn't suffer such serious contamination by water vapour transitions. Our current dataset does not quite have the wavelength coverage to perform a reliable determination of the $^{12}$C/$^{13}$C ratio in M dwarfs. For this we recommend observing the region 2.31–2.40 $\mu$m at a resolution of better than 1000. Alternatively the observational problems of contamination by water vapour at 2.345 $\mu$m maybe solved by observing at resolutions of around 50 000. We also investigated the possibility of using the $\Delta v$ = 1 CO bands around 4.5 $\mu$m. We find that the contamination due to water vapour is even more of a problem at these wavelengths.

We present a simple mathematical criterion for determining whether a given statistical model does not describe several independent sets of measurements, or data modes, adequately. We derive this criterion for two data sets and generalise it to several sets by using the Bayesian updating of the posterior probability density. To demonstrate the usage of the criterion, we apply it to observations of exoplanet host stars by re-analysing the radial velocities of HD 217107, Gliese 581, and υAndromedae and show that the currently used models are not necessarily adequate in describing the properties of these measurements. We show that while the two data sets of Gliese 581 can be modelled reasonably well, the noise model of HD 217107 needs to be revised. We also reveal some biases in the radial velocities of υAndromedae and report updated orbital parameters for the recently proposed 4-planet model. Because of the generality of our criterion, no assumptions are needed on the nature of the measurements, models, or model parameters. The method we propose can be applied to any astronomical problems, as well as outside the field of astronomy, because it is a simple consequence of the Bayes’ rule of conditional probabilities.

Aims.In this work we present chromospheric activity indices, kinematics, radial-velocities, and rotational velocities for more than 850 FGK-type dwarfs and subgiant stars in the southern hemisphere and test how best to calibrate and measure S-indices from echelle spectra.

Aims.The aim of our new laboratory measurements is to measure accurate absolute oscillator strengths for neutral manganese transitions in the infrared needed for the study of late-type stars and ultracool dwarfs.

Aims.We present parallaxes of 11 mid-to-late T dwarfs observed in the UKIRT Infrared Deep Sky Survey. We use these results to test the reliability of model predictions in magnitude–color space, determine a magnitude-spectral type calibration, and, estimate a bolometric luminosity and effective temperature range for the targets.