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We report the parallax and proper motion of five L dwarfs obtained with observations from the robotic Liverpool Telescope. Our derived proper motions are consistent with published values and have considerably smaller errors. Based on our spectral type versus absolute magnitude diagram, we do not find any evidence for binaries among our sample, or, at least no comparable mass binaries. Their space velocities locate them within the thin disk and based on the model comparisons they have solar-like abundances. For all five objects, we derived effective temperature, luminosity, radius, gravity and mass from a evolutionary model(CBA00) and our measured parallax; moreover, we derived their effective temperature by integrating observed optical and near-infrared spectra and model spectra (BSH06 or BT-Dusty respectively) at longer wavelengths to obtain bolometric {bf flux using} the classical Stefan-Boltzmann law: generally the three temperatures for one object derived using two different methods with three models are consistent, while at lower temperature(e.g. for L4) the differences among the three temperatures are slightly larger than that at higher temperature(e.g. for L1).
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We report new parallax measurements for ten L and early T type dwarfs, five of which have no previous published values, using observations over 3 years at the robotic Liverpool Telescope. The resulting parallaxes and proper motions have median errors of 2,mas and 1.5,mas/year respectively. Their space motions indicate they are all Galactic disk members. We combined this sample with other objects with astrometry from the Liverpool Telescope and with published literature astrometry to construct a sample of 260 L and early T type dwarfs with measured parallaxes, designated the Astrometry Sample. We study the kinematics of the Astrometry Sample, and derived a solar motion of $(U,V,W)_{bigodot} = (7.9pm1.7,13.2pm1.2,7.2pm1.0)$,kms~ with respect to the local standard of rest, in agreement with recent literature. We derive a kinematic age of 1.5-1.7,Gyr for the Astrometry Sample assuming the age increases monotonically with the total velocity for a given disk sample. This kinematic age is less than half literature values for other low mass dwarf samples. We believe this difference arises for two reasons (1) the sample is mainly composed of mid to late L dwarfs which are expected to be relatively young and (2) the requirement that objects have a measured parallax biases the sample to the brighter examples which tend to be younger.
We present new results from the Parallaxes of Southern Extremely Cool dwarfs program to measure parallaxes, proper motions and multi-epoch photometry of L and early T dwarfs. The observations were made on 108 nights over the course of 8 years using the Wide Field Imager on the ESO 2.2m telescope. We present 118 new parallaxes of L & T dwarfs of which 52 have no published values and 24 of the 66 published values are preliminary estimates from this program. The parallax precision varies from 1.0 to 15.5 mas with a median of 3.8 mas. We find evidence for 2 objects with long term photometric variation and 24 new moving group candidates. We cross-match our sample to published photometric catalogues and find standard magnitudes in up to 16 pass-bands from which we build spectral energy distributions and H-R diagrams. This allows us to confirm the theoretically anticipated minimum in radius between stars and brown dwarfs across the hydrogen burning minimum mass. We find the minimum occurs between L2 and L6 and verify the predicted steep dependence of radius in the hydrogen burning regime and the gentle rise into the degenerate brown dwarf regime. We find a relatively young age of 2 Gyr from the kinematics of our sample.
We present trigonometric parallax and proper motion measurements for two T-type brown dwarfs. We derive our measurements from infrared laser guide star adaptive optics observations spanning five years from the ShaneAO/SHARCS and NIRC2/medium-cam instruments on the Shane and Keck telescopes, respectively. To improve our astrometric precision, we measure and apply a distortion correction to our fields for both instruments. We also transform the Keck and ShaneAO astrometric reference frames onto the ICRS using five-parameter parallax and proper motion solutions for background reference stars from Gaia DR2. Fitting for parallax and proper motion, we measure parallaxes of $73.5pm9.2$ mas and $70.1pm6.7$ mas for WISEJ19010703+47181688 (WISE1901) and WISEJ21543294+59421370 (WISE2154), respectively. We utilize Monte Carlo methods to estimate the error in our sparse field methods, taking into account overfitting and differential atmospheric refraction. Comparing to previous measurements in the literature, all of our parallax and proper motion values fall within $2sigma$ of the published measurements, and 4 of 6 measurements are within $1sigma$. These data are among the first parallax measurements of these T dwarfs and serve as precise measurements for calibrating stellar formation models. These two objects are the first results of an ongoing survey of T dwarfs with Keck/NIRC2 and the Shane Adaptive Optics system at Lick Observatory.
We have followed up the three Y0 dwarfs WISEPA J041022.71+150248.5, WISEPA J173835.53+273258.9 and WISEPC J205628.90+145953.3 using the UKIRT/WFCAM telescope/instruments. We find parallaxes that are more consistent and accurate than previously published values. We estimate absolute magnitudes in photometric pass-bands from $Y$ to $W3$ and find them to be consistent between the three Y0 dwarfs indicating the inherent cosmic absolute magnitude spread of these objects is small. We examine the MKO $J$ magnitudes over the four year time line and find small but significant monotonic variations. Finally we estimate physical parameters from a comparison of spectra and parallax to equilibrium and non-equilibrium models finding values consistent with solar metallicity, an effective temperature of 450-475,K and log~g of 4.0-4.5.
We present absolute parallaxes and proper motions for seven members of the Hyades open cluster, pre-selected to lie in the core of the cluster. Our data come from archival astrometric data from FGS 3, and newer data for 3 Hyads from FGS 1R, both white-light interferometers on the Hubble Space Telescope (HST). We obtain member parallaxes from six individual Fine Guidance Sensor (FGS) fields and use the field containing van Altena 622 and van Altena 627 (= HIP 21138) as an example. Proper motions, spectral classifications and VJHK photometry of the stars comprising the astrometric refer- ence frames provide spectrophotometric estimates of reference star absolute parallaxes. Introducing these into our model as observations with error, we determine absolute parallaxes for each Hyad. The parallax of vA 627 is significantly improved by including a perturbation orbit for this previously known spectroscopic binary, now an astrometric binary. Compared to our original (1997) determina- tions, a combination of new data, updated calibration, and improved analysis lowered the individual parallax errors by an average factor of 4.5. Comparing parallaxes of the four stars contained in the Hipparcos catalog, we obtain an average factor of 11 times improvement with the HST . With these new results, we also have better agreement with Hipparcos for the four stars in common. These new parallaxes provide an average distance for these seven members, < D > = 47.5 pc, for the core a pm 1 - {sigma} dispersion depth of 3.6 pc, and a minimum depth from individual components of 16.0 pm 0.9 pc. Absolute magnitudes for each member are compared to established main sequences, with excellent agreement. We obtain a weighted average distance modulus for the core of the Hyades of m-M=3.376 pm 0.01, a value close to the previous Hipparcos values, m-M=3.33pm 0.02.
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