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. Methods. We measured our parameters using the high-resolution and high-S/N FEROS echelle spectra acquired for this purpose. Results. We confirm the bimodal distribution of chromospheric activities for such stars and highlight the role that the more active K-dwarfs play in biasing the number of active stars. We show that the age-activity relationship does appear to continue to ages older than the Sun if we simply compare main sequence stars and subgiant stars with an offset of around 2.5 Gyr between the peaks of both distributions. Also we show evidence of an increased spin-down timescale for cool K dwarfs compared with earlier F and G type stars. We highlight that activities drawn from low-resolution spectra (R < 2500) significantly increase the rms scatter when calibrating onto common systems of measurements like the Mt. Wilson system. Also we show that older and widely used catalogues of activities in the south appear to be offset compared to more recent works at the ~0.1 dex level in \hbox{$\log R'_{\rm{HK}}$}logRHK′ through calibrator drift. In addition, we show how kinematics can be used to preselect inactive stars for future planet search projects. We see the well known trend between projected rotational velocity and activity, however we also find a correlation between kinematic space velocity and chromospheric activity. It appears that after the Vaughan-Preston gap there is a quick step function in the kinematic space motion towards a significantly broader spread in velocities. We speculate on reasons for this correlation and provide some model scenarios to describe the bimodal activity distribution through magnetic saturation, residual low level gas accretion, or accretion by the star of planets or planetesimals. Finally, we provide a new empirical measurement for the disk-heating law, using the latest age-activity relationships to reconstruct the age-velocity distribution for local disk stars. We find a value of 0.337 ± 0.045 for the exponent of this power law (i.e. σtot ∝ t0.337), in excellent agreement with those found using isochrone fitting methods and with theoretical disk-heating models.
We present the spectroscopic analysis of a large sample of late-M, L, and T dwarfs from the United Kingdom Deep Infrared Sky Survey. Using the YJHK photometry from the Large Area Survey and the ...red-optical photometry from the Sloan Digital Sky Survey we selected a sample of 262 brown dwarf candidates and we have followed-up 196 of them using the echelle spectrograph X-shooter on the Very Large Telescope. The large wavelength coverage (0.30–2.48 μm) and moderate resolution (R ∼ 5000–9000) of X-shooter allowed us to identify peculiar objects including 22 blue L dwarfs, 2 blue T dwarfs, and 2 low-gravity M dwarfs. Using a spectral indices-based technique, we identified 27 unresolved binary candidates, for which we have determined the spectral type of the potential components via spectral deconvolution. The spectra allowed us to measure the equivalent width of the prominent absorption features and to compare them to atmospheric models. Cross-correlating the spectra with a radial velocity standard, we measured the radial velocity of our targets, and we determined the distribution of the sample, which is centred at −1.7 ± 1.2 km s−1 with a dispersion of 31.5 km s−1. Using our results, we estimated the space density of field brown dwarfs and compared it with the results of numerical simulations. Depending on the binary fraction, we found that there are (0.85 ± 0.55) × 10−3 to (1.00 ± 0.64) × 10−3 objects per cubic parsec in the L4–L6.5 range, (0.73 ± 0.47) × 10−3 to (0.85 ± 0.55) × 10−3 objects per cubic parsec in the L7–T0.5 range, and (0.74 ± 0.48) × 10−3 to (0.88 ± 0.56) × 10−3 objects per cubic parsec in the T1–T4.5 range. We notice that there seems to be an excess of objects in the L–T transition with respect to the late-T dwarfs, a discrepancy that could be explained assuming a higher binary fraction than expected for the L–T transition, or that objects in the high-mass end and low-mass end of this regime form in different environments, i.e. following different initial mass functions.
Nanoporous molecular frameworks are important in applications such as separation, storage and catalysis. Empirical rules exist for their assembly but it is still challenging to place and segregate ...functionality in three-dimensional porous solids in a predictable way. Indeed, recent studies of mixed crystalline frameworks suggest a preference for the statistical distribution of functionalities throughout the pores rather than, for example, the functional group localization found in the reactive sites of enzymes. This is a potential limitation for 'one-pot' chemical syntheses of porous frameworks from simple starting materials. An alternative strategy is to prepare porous solids from synthetically preorganized molecular pores. In principle, functional organic pore modules could be covalently prefabricated and then assembled to produce materials with specific properties. However, this vision of mix-and-match assembly is far from being realized, not least because of the challenge in reliably predicting three-dimensional structures for molecular crystals, which lack the strong directional bonding found in networks. Here we show that highly porous crystalline solids can be produced by mixing different organic cage modules that self-assemble by means of chiral recognition. The structures of the resulting materials can be predicted computationally, allowing in silico materials design strategies. The constituent pore modules are synthesized in high yields on gram scales in a one-step reaction. Assembly of the porous co-crystals is as simple as combining the modules in solution and removing the solvent. In some cases, the chiral recognition between modules can be exploited to produce porous organic nanoparticles. We show that the method is valid for four different cage modules and can in principle be generalized in a computationally predictable manner based on a lock-and-key assembly between modules.
We report the discovery of a peculiar L dwarf from the United Kingdom Infrared Deep Sky Survey Large Area Survey, ULAS J222711−004547. The very red infrared photometry (MKO J − K = 2.79 ± 0.06, WISE ...W1−W2 = 0.65 ± 0.05) of ULAS J222711−004547 makes it one of the reddest brown dwarfs discovered so far. We obtained a moderate resolution spectrum of this target using the XSHOOTER spectrograph on the Very Large Telescope, and we classify it as L7pec, confirming its very red nature. Comparison to theoretical models suggests that the object could be a low-gravity L dwarf with a solar or higher than solar metallicity. Nonetheless, the match of such fits to the spectral energy distribution is rather poor, and this and other less red peculiar L dwarfs pose new challenges for the modelling of ultracool atmospheres, especially to the understanding of the effects of condensates and their sensitivity to gravity and metallicity. We determined the proper motion of ULAS J222711−004547 using the data available in the literature, and we find that its kinematics do not suggest membership of any of the known young associations. We show that applying a simple de-reddening curve to its spectrum allows it to resemble the spectra of the L7 spectroscopic standards without any spectral features that distinguish it as a low-metallicity or low-gravity dwarf. Given the negligible interstellar reddening of the field containing our target, we conclude that the reddening of the spectrum is mostly due to an excess of dust in the photosphere of the target. De-reddening the spectrum using extinction curves for different dust species gives surprisingly good results and suggests a characteristic grain size of ∼0.5 μm. We show that by increasing the optical depth, the same extinction curves allow the spectrum of ULAS J222711−004547 to resemble the spectra of unusually blue L dwarfs and even slightly metal-poor L dwarfs. Grains of similar size also yield very good fits when de-reddening other unusually red L dwarfs in the L5-L7.5 range. These results suggest that the diversity in near-infrared colours and spectra seen in late L dwarfs could be due to differences in the optical thickness of the dust cloud deck.
The discovery of a very cool binary system Burningham, Ben; Leggett, S. K.; Lucas, P. W. ...
Monthly notices of the Royal Astronomical Society,
June 2010, Letnik:
404, Številka:
4
Journal Article
Recenzirano
We report the discovery of a very cool d/sdL7+T7.5p common proper motion binary system, SDSS J1416+13AB, found by cross-matching the United Kingdom Infrared Telescope (UKIRT) Infrared Deep Sky Survey ...(UKIDSS) Large Area Survey Data Release 5 (UKIDSS LAS DR4) against the Sloan Digital Sky Survey Data Release 7. The d/sdL7 is blue in J−H and H−K and has other features suggestive of low metallicity and/or high gravity. The T7.5p displays spectral peculiarity seen before in earlier type dwarfs discovered in UKIDSS LAS DR4, and referred to as CH4-J-early peculiarity, where the CH4-J index, based on the absorption to the red side of the J-band peak, suggests an earlier spectral type than the H2O-J index, based on the blue side of the J-band peak, by ∼2 subtypes. We suggest that CH4-J-early peculiarity arises from low metallicity and/or high gravity, and speculate as to its use for classifying T dwarfs. UKIDSS and follow-up United Kingdom Infrared Telescope/Wide Field CAMera (UKIRT/WFCAM) photometry shows the T dwarf to have the bluest near-infrared colours yet seen for such an object with H−K=−1.31 ± 0.17. Warm Spitzer IRAC photometry shows the T dwarf to have extremely red H−4.5= 4.86 ± 0.04, which is the reddest yet seen for a substellar object. The lack of parallax measurement for the pair limits our ability to estimate parameters for the system. However, applying a conservative distance estimate of 5–15 pc suggests a projected separation in range 45–135 au. By comparing H−K:H−4.5 colours of the T dwarf to spectral models, we estimate that Teff= 500 K and M/H∼− 0.30, with log g∼ 5.0. This suggests a mass of ∼30 MJupiter for the T dwarf and an age of ∼10 Gyr for the system. The primary would then be a 75 MJupiter object with log g∼ 5.5 and a relatively dust-free Teff∼ 1500 K atmosphere. Given the unusual properties of the system we caution that these estimates are uncertain. We eagerly await parallax measurements and high-resolution imaging which will constrain the parameters further.
The properties of the T8.5p dwarf Ross 458C Burningham, Ben; Leggett, S. K.; Homeier, D. ...
Monthly notices of the Royal Astronomical Society,
07/2011, Letnik:
414, Številka:
4
Journal Article
Recenzirano
Odprti dostop
We present near-infrared photometry and spectroscopy, and warm-Spitzer IRAC photometry of the young very cool T dwarf Ross 458C, which we have typed as T8.5p. By applying the fiducial age constraints ...(≤1 Gyr) imposed by the properties of the active M dwarf Ross 458A, we have used these data to determine that Ross 458C has T
eff= 695 ± 60 K, log g= 4.0-4.7 and an inferred mass of 5-20M
J. We have compared fits of the near-infrared spectrum and IRAC photometry to the BT Settl and Saumon & Marley model grids, and have found that both sets provide best fits that are consistent with our derived properties, whilst the former provide a marginally closer match to the data for all scenarios explored here. The main difference between the model grids arises in the 4.5-μm region, where the BT Settl models are able to better predict the flux through the IRAC filter, suggesting that non-equilibrium effects on the CO-CO2 ratio are important for shaping the mid-infrared spectra of very cool T dwarfs. We have also revisited the issue of the dust opacity in the spectra of Ross 458C that was raised by Burgasser et al. We have found that the BT Settl models which also incorporate a condensate cloud model provide a better match to the near-infrared spectrum of this target than the Saumon & Marley model with f
sed= 2 and we briefly discuss the influence of condensate clouds on T dwarf spectra.
We report the discovery of 76 new T dwarfs from the UKIRT Infrared Deep Sky Survey (UKIDSS) Large Area Survey (LAS). Near-infrared broad- and narrow-band photometry and spectroscopy are presented for ...the new objects, along with Wide-field Infrared Survey Explorer (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 T8p+M4 pair and HD 118865AB, 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. First, it appears that the H − 4.5 and J − W2 colours are more sensitive to metallicity than has previously been recognized, 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 benchmarks. This is supported by the apparently similar distribution of late-T dwarfs and earlier type T dwarfs on reduced proper motion diagrams that we present. Finally, we present updated space densities for the late-T dwarfs, and compare our values to simulation predictions and those from WISE.
We have searched the Wide-field Infrared Survey Explorer first data release for widely separated (≤10 000 au) late T dwarf companions to Hipparcos and Gliese stars. We have discovered a new binary ...system containing a K-band suppressed T8p dwarf WISEP J142320.86+011638.1 and the mildly metal poor (Fe/H =−0.38 ± 0.06) primary BD +01° 2920 (HIP 70319), a G1 dwarf at a distance of 17.2 pc. This new benchmark has T
eff= 680 ± 55 K and a mass of 20-50M
Jup. Its spectral properties are well modelled except for known discrepancies in the Y and K bands. Based on the well-determined metallicity of its companion, the properties of BD +01° 2920B imply that the currently known T dwarfs are dominated by young low-mass objects. We also present an accurate proper motion for the T8.5 dwarf WISEP J075003.84+272544.8.
We report the discovery of a brown dwarf on an eccentric orbit and with a semimajor axis that places it in the brown dwarf desert region around the star HD 191760. The star has a spectral type of ...G3IV/V and a metallicity (Fe/H) of 0.29 dex. HD 191760 adds to the small number of metal-rich stars with brown dwarf companions. The brown dwarf (HD 191760b) is found to have an orbital period of 505.57 ± 0.40 d and semimajor axis of 1.35 ± 0.01 au, placing it firmly in the brown dwarf desert. The eccentricity of HD 191760b is found to be 0.63 ± 0.01, meaning it reaches as close as 0.5 au from the host star. Dynamical simulations indicate that no inner planets could reside at separations beyond ∼0.17 au due to the disastrous gravity imposed by HD 191760b. In addition to these first results, we also refine the orbits found for the exoplanets around the stars HD 48265, HD 143361 and HD 154672. All one-planet solutions are in agreement with those previously published by the Magellan Planet Search.
Using a simulated disc brown dwarf (BD) population, we find that new large area infrared surveys are expected to identify enough BDs covering wide enough mass–age ranges to potentially measure the ...present day mass function down to ∼0.03 M⊙, and the BD formation history out to 10 Gyr, at a level that will be capable of establishing if BD formation follows star formation. We suggest these capabilities are best realized by spectroscopic calibration of BD properties (Teff, g and M/H) which when combined with a measured luminosity and an evolutionary model can give BD mass and age relatively independent of BD atmosphere models. Such calibration requires an empirical understanding of how BD spectra are affected by variations in these properties, and thus the identification and study of ‘benchmark BDs’ whose age and composition can be established independently. We identify the best sources of benchmark BDs as young open cluster members, moving group members, and wide (>1000 au) BD companions to both subgiant stars and high-mass white dwarfs (WDs). To accurately asses the likely number of wide companion BDs available, we have constrained the wide L dwarf companion fraction using the 2-Micron All Sky Survey (2MASS), and find a companion fraction of 2.7+0.7−0.5 per cent for separations of ∼1000–5000 au. This equates to a BD companion fraction of 34+9−6 per cent if one assumes an α∼ 1 companion mass function. Using this BD companion fraction, we simulate populations of wide BD binaries, and estimate that 80+21−14 subgiant–BD binaries, and 50+13−10 benchmark WD–BD binaries could be identified using current and new facilities. The WD–BD binaries should all be identifiable using the Large Area Survey component of the United Kingdom Infrared Telescope (UKIRT) Infrared Deep Sky Survey, combined with the Sloan Digital Sky Survey. Discovery of the subgiant–BD binaries will require a near-infrared imaging campaign around a large (∼900) sample of Hipparcos subgiants. If identified, spectral studies of these benchmark BD populations could reveal the spectral sensitivities across the Teff, g and M/H space probed by new surveys.
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