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16-20 MJup Radial Velocity Companion Orbiting the Brown Dwarf Candidate Cha Hα 8
We report the discovery of a 16-20 MJup radial velocitycompanion around the very young (~3 Myr) brown dwarf candidate ChaHα 8 (M5.75-M6.5). Based on high-resolution echelle spectra of ChaHα 8 taken between 2000 and 2007 with UVES at the VLT, a companionwas detected through RV variability with a semiamplitude of 1.6 kms-1. A Kepler fit to the data yields an orbital period of thecompanion of 1590 days and an eccentricity of e=0.49. A companionminimum mass M2sini between 16 and 20 MJup isderived when using model-dependent mass estimates for the primary. Themass ratio q≡M2/M1 might be as small as 0.2and, with a probability of 87%, it is less than 0.4. Cha Hα 8harbors most certainly the lowest mass companion detected so far in aclose (~1 AU) orbit around a brown dwarf or very low mass star. From theuncertainty in the orbit solution, it cannot completely be ruled outthat the companion has a mass in the planetary regime. Its discovery isin any case an important step toward RV planet detections around browndwarfs. Further, Cha Hα 8 is the fourth known spectroscopic browndwarf or very low mass binary system with an RV orbital solution and thesecond known very young one.Based on observations obtained at the Very Large Telescope of theEuropean Southern Observatory at Paranal, Chile, in programs75.C-0851(C), 77.C-0831(A+D), and 278.C-5061(A).
| Search for cold debris disks around M-dwarfs
Debris disks are believed to be related to planetesimals left overaround stars after planet formation has ceased. The frequency of debrisdisks around M-dwarfs which account for 70% of the stars in the Galaxyis unknown while constrains have already been found for A- to K-typestars. We have searched for cold debris disks around 32 field M-dwarfsby conducting observations at λ = 850~μm with the SCUBAbolometer array camera at the JCMT and at λ = 1.2 mm with theMAMBO array at the IRAM 30-m telescopes. This is the first survey of alarge sample of M-dwarfs conducted to provide statistical constraints ondebris disks around this type of stars. We have detected a new debrisdisk around the M0.5 dwarf GJ 842.2 at λ = 850~μm, providingevidence for cold dust at large distance from this star (~300 AU). Bycombining the results of our survey with the ones of Liu et al. (2004),we estimate for the first time the detection rate of cold debris disksaround field M-dwarfs with ages between 20 and 200 Myr. This detectionrate is 13+6-8% and is consistent with thedetection rate of cold debris disks (9-23%) around A- to K-type mainsequence stars of the same age. This is an indication that cold disksmay be equally prevalent across stellar spectral types.
| Proto-Brown Dwarf Disks as Products of Protostellar Disk Encounters
The formation of brown dwarfs (BDs) via encounters between protostarshas been confirmed with high-resolution numerical simulations with arestricted treatment of the thermal conditions. The new results indicatethat young BDs formed this way are disklike and often reside in multiplesystems. The newly formed proto-BD disks are up to 18 AU in size andspin rapidly, making small-scale bipolar outflows, fragmentation, andthe possible formation of planetary companions likely, as have recentlybeen observed for BDs. The object masses range from 2 to 73 Jupitermasses, distributed in a manner consistent with the observed substellarinitial mass function. The simulations usually form multiple BDs oneccentric orbits about a star. One such system was hierarchical, a BDbinary in orbit around a star, which may explain recently observedhierarchical systems. One-third of the BDs were unbound after a fewthousand years, and interactions among orbiting BDs may eject more oradd to the number of binaries. Improvements over prior work includeresolution down to a Jupiter mass, self-consistent models of thevertical structure of the initial disks, and careful attention to avoidartificial fragmentation.
| The Gl569 Multiple System
We report the results of high spectral and angular resolution infraredobservations of the multiple system Gl569A and B that were intended tomeasure the dynamical masses of the brown dwarf binary believed tocomprise Gl569B. Our analysis did not yield this result but, instead,revealed two surprises. First, at age ~100 Myr, the system is youngerthan had been reported earlier. Second, our spectroscopic andphotometric results provide support for earlier indications that Gl569Bis actually a hierarchical brown dwarf triple rather than a binary. Ourresults suggest that the three components of Gl569B have roughly equalmass, ~0.04 Msolar.
| Kinematic structure of the corona of the Ursa Major flow found using proper motions and radial velocities of single stars
Aims.We study the kinematic structure of peripheral areas of the UrsaMajoris stream (Sirius supercluster). Methods.We use diagrams ofindividual stellar apexes developed by us and the classical technique ofproper motion diagrams generalized to a star sample distributed over thesky. Results.Out of 128 cluster members we have identified threecorona (sub)structures comprised of 13, 13 and 8 stars. Thesubstructures have a spatial extension comparable to the size of thecorona. Kinematically, these groups are distinguished by their propermotions, radial velocities and by the directions of their spatialmotion. Coordinates of their apexes significantly differ from those ofthe apexes of the stream and its nucleus. Our analysis shows that thesesubstructures do not belong to known kinematic groups, such as Hyades orCastor. We find kinematic inhomogeneity of the corona of the UMa stream.
| Ca II H and K Chromospheric Emission Lines in Late-K and M Dwarfs
We have measured the profiles of the Ca II H and K chromosphericemission lines in 147 main-sequence stars of spectral type M5-K7 (masses0.30-0.55 Msolar) using multiple high-resolution spectraobtained during 6 years with the HIRES spectrometer on the Keck Itelescope. Remarkably, the average FWHM, equivalent widths, and lineluminosities of Ca II H and K increase by a factor of 3 with increasingstellar mass over this small range of stellar masses. We fit the Ca II Hand K lines with a double-Gaussian model to represent both thechromospheric emission and the non-LTE central absorption. Most of thesample stars display a central absorption that is typically redshiftedby ~0.1 km s-1 relative to the emission. This implies thatthe higher level, lower density chromospheric material has a smalleroutward velocity (or higher inward velocity) by 0.1 km s-1than the lower level material in the chromosphere, but the nature ofthis velocity gradient remains unknown. The FWHM of the Ca II H and Kemission lines increase with stellar luminosity, reminiscent of theWilson-Bappu effect in FGK-type stars. Both the equivalent widths andFWHM exhibit modest temporal variability in individual stars. At a givenvalue of MV, stars exhibit a spread in both the equivalentwidth and FWHM of Ca II H and K, due both to a spread in fundamentalstellar parameters, including rotation rate, age, and possiblymetallicity, and to the spread in stellar mass at a given MV.The K line is consistently wider than the H line, as expected, and itscentral absorption is more redshifted, indicating that the H and K linesform at slightly different heights in the chromosphere where thevelocities are slightly different. The equivalent width of Hαcorrelates with Ca II H and K only for stars having Ca II equivalentwidths above ~2 Å, suggesting the existence of a magneticthreshold above which the lower and upper chromospheres become thermallycoupled.Based on observations obtained at the W. M. Keck Observatory, which isoperated jointly by the University of California and the CaliforniaInstitute of Technology. Keck time has been granted by both NASA and theUniversity of California.
| X-ray emission of brown dwarfs: towards constraining the dependence on age, luminosity, and temperature
Aims.We observed brown dwarfs in different evolutionary stages with theChandra X-ray Observatory with the aim to disentangle the influence ofdifferent stellar parameters on the X-ray emission of substellarobjects. The ages of our three targets (HR 7329 B, Gl 569 Bab, and HD130948 BC) are constrained by them being companions to main-sequencestars of known age. With both known age and effective temperature orbolometric luminosity, the mass can be derived from evolutionary models.Methods.Combining the new observations with previous studies presentedin the literature yields a brown dwarf sample that covers the age rangefrom ~1 Myr to ~1 Gyr. Since the atmospheric temperature of brown dwarfsis approximately constant at young ages, a sample with a large agespread is essential for investigating the possible influence ofeffective temperature on X-ray activity. Results.Two out of three browndwarfs are detected with Chandra, with variable lightcurves andcomparatively soft spectra. Combining our results with published dataallows us to consider a subsample of high-mass brown dwarfs (with0.05-0.07 M_ȯ), thus eliminating mass from the list of freeparameters. We find evidence that X-ray luminosity declines withdecreasing bolometric luminosity steeper than expected from thecanonical relation for late-type stars (L_x/L_bol =10-3...-5). Effective temperature is identified as a likelyparameter responsible for the additional decline of X-ray activity inthe more evolved (and therefore cooler) brown dwarfs of the "high-mass"sample. In another subsample of brown dwarfs characterized by similareffective temperature, the X-ray luminosity scales with the bolometricluminosity without indications for a deviation from the canonical rangeof 10-3...-5 observed for late-type stars.Conclusions.Ourfindings support the idea that effective temperature plays a criticalrole for the X-ray activity in brown dwarfs. This underlines an earliersuggestion based on observations of chromospheric Hα emission inultracool dwarfs that the low ionization fraction in the cool browndwarf atmospheres may suppress magnetic activity.
| Measuring physical properties of very young brown dwarfs
I discuss the problem of finding the fundamental parameters (mass,temperature, gravity, radius, and age) of ultra low-mass objects whenthey are very young. This is the part of parameter space wheretheoretical evolutionary models, which typically are used to extract thedifficult parameters of age, mass, and radius from more observationallydirect parameters like luminosity and spectral type, are expected to beleast reliable. Studying binaries is one way to get around this problem;determining fundamental parameters from high resolution spectra isanother (which is applicable to single stars). All these methods havetheir advantages and problems, and it is essential to make comparisonsbetween them to develop trust in any of their answers.Workshop ``Ultralow-mass star formation and evolution'', see AN 326, No.10
| Lithium depletion in the brown dwarf binary GJ 569Bab
GJ 569Bab is the first brown dwarf binary for which the mass of eachcomponent has been derived by solving the astrometric and spectroscopicorbit of the pair, i.e., independently of any theoretical assumption.This allows us to test the predictions of the various evolutionarymodels available in the literature. Particularly interesting are thepredictions of lithium depletion for the mass (0.08-0.05Mȯ) and likely age (300-800 Myr) of the substellarcomponents. High-resolution optical spectra of GJ 569B (the pair is notresolved) obtained with HIRES at the Keck telescope show that there hasbeen significant lithium depletion in both components. We will comparethese results to state-of-the-art theoretical calculations.
| Kelu-1 Is a Binary L Dwarf: First Brown Dwarf Science from Laser Guide Star Adaptive Optics
We present near-infrared (1-2.4 μm) imaging of the L dwarf Kelu-1obtained with the Keck sodium laser guide star adaptive optics (LGS AO)system as part of a high angular resolution survey for substellarbinaries. Kelu-1 was one of the first free-floating L dwarfs identified,and the origin of its overluminosity compared to objects of similarspectral type has been a long-standing question. Our images clearlyresolve Kelu-1 into a 0.29" (5.4 AU) binary with near-infrared fluxratios of ~0.5 mag. A previous nondetection of binarity by the HubbleSpace Telescope demonstrates that the system is a true physical pair andthat its projected orbital motion has been significant over the last 7yr. Binarity explains the properties of Kelu-1 that were previouslynoted to be anomalous compared to other early L dwarfs. We estimatespectral types of L1.5-L3 and L3-L4.5 for the two components, givingmodel-derived masses of 0.05-0.07 and 0.045-0.065 Msolar foran estimated age of 0.3-0.8 Gyr. More distant companions are notdetected to a limit of ~5-9 MJup. The presence of Li 6708Å absorption indicates that both components are substellar, butthe weakness of this feature relative to other L dwarfs can be explainedif only Kelu-1B is Li-bearing. Determining whether both or only one ofthe components possesses lithium could constrain the age of Kelu-1 (andother Li-bearing L binaries) with higher precision than is possible formost ultracool field objects. These results are the first LGS AOobservations of brown dwarfs and demonstrate the potential of this newinstrumental capability for substellar astronomy.
| X-Ray Emission from Young Brown Dwarfs in the Orion Nebula Cluster
We use the sensitive X-ray data from the Chandra Orion Ultradeep Project(COUP) to study the X-ray properties of 34 spectroscopically identifiedbrown dwarfs with near-infrared spectral types between M6 and M9 in thecore of the Orion Nebula Cluster. Nine of the 34 objects are clearlydetected as X-ray sources. The apparently low detection rate is in manycases related to the substantial extinction of these brown dwarfs;considering only the brown dwarfs with AV<=5 mag, nearlyhalf of the objects (7 out of 16) are detected in X-rays. Our 10 daylong X-ray light curves of these objects exhibit strong variability,including numerous flares. While one of the objects was only detectedduring a short flare, a statistical analysis of the light curvesprovides evidence for continuous (``quiescent'') emission in addition toflares for all other objects. Of these, the ~M9 brown dwarf COUP 1255(=HC 212) is one of the coolest known objects with a clear detection ofquiescent X-ray emission. The X-ray properties (spectra, fractionalX-ray luminosities, flare rates) of these young brown dwarfs are similarto those of the low-mass stars in the Orion Nebula Cluster, and thusthere is no evidence for changes in the magnetic activity around thestellar/substellar boundary, which lies at ~M6 for Orion Nebula Clustersources. Since the X-ray properties of the young brown dwarfs are alsosimilar to those of M6-M9 field stars, the key to the magnetic activityin very cool objects seems to be the effective temperature, whichdetermines the degree of ionization in the atmosphere.
| The Physical Natures of Class I and Flat-Spectrum Protostellar Photospheres: A Near-Infrared Spectroscopic Study
We present high-resolution (R~=18,000), high signal-to-noise ratio, 2μm spectra of 52 IR-selected Class I and flat-spectrum young stellarobjects in the Taurus-Auriga, ρ Ophiuchi, Serpens, Perseus, andCorona Australis dark clouds. We detect key absorption lines in 41objects and fit synthetic spectra generated from pre-main-sequencemodels to deduce the effective temperatures, surface gravities, near-IRveilings, rotation velocities, and radial velocities of each of these 41sources. We find these objects to span ranges in effective temperature,surface gravity, and stellar luminosity that appear similar to those oflate spectral type Class II sources and classical T Tauri stars.However, because of significant but uncertain corrections for scatteringand extinction, the derived luminosities for the embedded protostellarobjects must be regarded as being highly uncertain. We determine thatthe mean 2 μm veiling of Class I and flat-spectrum objects issignificantly higher than that of Class II objects in the same regionwhere both types of objects are extensively observed (ρ Oph). Wefind that a significant fraction of our protostellar sample alsoexhibits emission lines. Twenty-three objects show H2emission, which is usually indicative of the presence of energeticoutflows. Thirty-four sources show H I Brγ emission, and a numberof these exhibit profile asymmetries consistent with infall. Eightsources show significant Δv=2 CO emission suggestive of emissionfrom a circumstellar disk. Overall, these observations indicate thatClass I and flat-spectrum objects are self-embedded protostarsundergoing significant mass accretion, although the objects appear tospan a broad range of mass accretion activity.Data presented herein were obtained at the W. M. Keck Observatory fromtelescope time allocated to the National Aeronautics and SpaceAdministration through the agency's scientific partnership with theCalifornia Institute of Technology and the University of California. TheObservatory was made possible by the generous financial support of theW. M. Keck Foundation.
| A Decade of Brown Dwarfs
Not Available
| Statistical Constraints for Astrometric Binaries with Nonlinear Motion
Useful constraints on the orbits and mass ratios of astrometric binariesin the Hipparcos catalog are derived from the measured proper motiondifferences of Hipparcos and Tycho-2 (Δμ), accelerations ofproper motions (μ˙), and second derivatives of proper motions(μ̈). It is shown how, in some cases, statistical bounds can beestimated for the masses of the secondary components. Two catalogs ofastrometric binaries are generated, one of binaries with significantproper motion differences and the other of binaries with significantaccelerations of their proper motions. Mathematical relations betweenthe astrometric observables Δμ, μ˙, and μ̈ andthe orbital elements are derived in the appendices. We find a remarkabledifference between the distribution of spectral types of stars withlarge accelerations but small proper motion differences and that ofstars with large proper motion differences but insignificantaccelerations. The spectral type distribution for the former sample ofbinaries is the same as the general distribution of all stars in theHipparcos catalog, whereas the latter sample is clearly dominated bysolar-type stars, with an obvious dearth of blue stars. We point outthat the latter set includes mostly binaries with long periods (longerthan about 6 yr).
| LP 349-25: A new tight M8V binary
We present the discovery of a tight M8V binary, with a separation ofonly 1.2 astronomical units, obtained with the PUEO and NACO adaptiveoptics systems, respectively at the CFHT and VLT telescopes. Theestimated period of LP 349-25 is approximately 5 years, and this makesit an excellent candidate for a precise mass measurement.
| A Catalog of Northern Stars with Annual Proper Motions Larger than 0.15" (LSPM-NORTH Catalog)
The LSPM catalog is a comprehensive list of 61,977 stars north of theJ2000 celestial equator that have proper motions larger than 0.15"yr-1 (local-background-stars frame). The catalog has beengenerated primarily as a result of our systematic search for high propermotion stars in the Digitized Sky Surveys using our SUPERBLINK software.At brighter magnitudes, the catalog incorporates stars and data from theTycho-2 Catalogue and also, to a lesser extent, from the All-SkyCompiled Catalogue of 2.5 million stars. The LSPM catalog considerablyexpands over the old Luyten (Luyten Half-Second [LHS] and New LuytenTwo-Tenths [NLTT]) catalogs, superseding them for northern declinations.Positions are given with an accuracy of <~100 mas at the 2000.0epoch, and absolute proper motions are given with an accuracy of ~8 masyr-1. Corrections to the local-background-stars propermotions have been calculated, and absolute proper motions in theextragalactic frame are given. Whenever available, we also give opticalBT and VT magnitudes (from Tycho-2, ASCC-2.5),photographic BJ, RF, and IN magnitudes(from USNO-B1 catalog), and infrared J, H, and Ks magnitudes(from 2MASS). We also provide an estimated V magnitude and V-J color fornearly all catalog entries, useful for initial classification of thestars. The catalog is estimated to be over 99% complete at high Galacticlatitudes (|b|>15deg) and over 90% complete at lowGalactic latitudes (|b|>15deg), down to a magnitudeV=19.0, and has a limiting magnitude V=21.0. All the northern starslisted in the LHS and NLTT catalogs have been reidentified, and theirpositions, proper motions, and magnitudes reevaluated. The catalog alsolists a large number of completely new objects, which promise to expandvery significantly the census of red dwarfs, subdwarfs, and white dwarfsin the vicinity of the Sun.Based on data mining of the Digitized Sky Surveys (DSSs), developed andoperated by the Catalogs and Surveys Branch of the Space TelescopeScience Institute (STScI), Baltimore.Developed with support from the National Science Foundation (NSF), aspart of the NASA/NSF NStars program.
| X-ray emission probing the limiting cases of stellar dynamos
Magnetic activity, driven by a dynamo, is a frequently observedphenomenon on solar-like stars and the Sun itself. Its manifestationsinclude chromospheric and coronal emission. Solar-like stars regenerateand accumulate their magnetic fields in the transition zone between theradiative core and the convective envelope. Consequently, stars in whichthis `overshoot' region is absent are not expected to display activity.In particular, the critical cases are (A) the fully radiativeintermediate-mass stars (M > 2 M_ȯ), and (B) the fullyconvective lowest mass stars (M < 0.3 M_ȯ) and brown dwarfs.Nevertheless, high-energy emission is observed from both classes ofobjects. I discuss recent efforts to constrain the emission mechanism inthese limiting regimes of the stellar dynamo by means of X-rayobservations with the Chandra satellite.
| Mesure de masses de naines brunes.
Not Available
| The structure of our stellar system.
Not Available
| Quiescent X-Ray Emission from an Evolved Brown Dwarf?
I report on the X-ray detection of Gl 569Bab. During a 25 ks Chandraobservation, the binary brown dwarf is for the first time spatiallyseparated in X-rays from the flare star primary Gl 569A. Companionshipto Gl 569A constrains the age of the brown dwarf pair to ~300-800 Myr.The observation presented here is only the second X-ray detection of anevolved brown dwarf. About half of the observing time is dominated by alarge flare on Gl 569Bab; the remainder is characterized by weak andnonvariable emission just above the detection limit. This emission-ifnot related to the afterglow of the flare-represents the first detectionof a quiescent corona on a brown dwarf, representing an important piecein the puzzle of dynamos in the substellar regime.
| Dynamical Masses of the Binary Brown Dwarf GJ 569 Bab
We have obtained new images and high-resolution (R~22,400) near-infrared(1.2400-1.2575 μm) spectra of each component of the brown dwarfbinary GJ 569 Bab using the adaptive optics facility of the Keck IItelescope and the NIRSPEC spectrometer. These data have allowed us toimprove the determination of the astrometric orbit and to measure radialvelocities of the components. We have used the astrometric andspectroscopic measurements to derive the dynamical mass of each browndwarf and the systemic velocity of the pair by means of aχ2 fitting technique. From various considerations, themass of each component is likely in the range 0.034-0.070Msolar (GJ 569 Bb) and 0.055-0.087 Msolar (GJ 569Ba). This implies that the mass ratio q of the binary is greater than0.4, the most likely value being q=0.75-0.85. Adopting 0.072Msolar as the most conservative location of the substellarlimit for solar metallicity, our analysis confirms GJ 569 Bb as thefirst genuine brown dwarf known without any theoretical assumptions. Wehave compared the dynamical masses of GJ 569 Ba and Bb, and theireffective temperatures and luminosities, to the predictions ofstate-of-the-art theoretical evolutionary isochrones, finding thatmodels exhibit good performance in the regime of high substellar massesif the binary is about a few hundred million years old. However, thesurface gravities of GJ 569 Ba (M8.5 V) and Bb (M9 V) derived from ourspectral analysis (the observed data have been compared to the latestsynthetic spectra) appear to be smaller than the values provided by theevolutionary models.
| Chromospheric Ca II Emission in Nearby F, G, K, and M Stars
We present chromospheric Ca II H and K activity measurements, rotationperiods, and ages for ~1200 F, G, K, and M type main-sequence stars from~18,000 archival spectra taken at Keck and Lick Observatories as a partof the California and Carnegie Planet Search Project. We have calibratedour chromospheric S-values against the Mount Wilson chromosphericactivity data. From these measurements we have calculated medianactivity levels and derived R'HK, stellar ages,and rotation periods from general parameterizations for 1228 stars,~1000 of which have no previously published S-values. We also presentprecise time series of activity measurements for these stars.Based on observations obtained at Lick Observatory, which is operated bythe University of California, and on observations obtained at the W. M.Keck Observatory, which is operated jointly by the University ofCalifornia and the California Institute of Technology. The KeckObservatory was made possible by the generous financial support of theW. M. Keck Foundation.
| Nearby stars of the Galactic disk and halo. III.
High-resolution spectroscopic observations of about 150 nearby stars orstar systems are presented and discussed. The study of these and another100 objects of the previous papers of this series implies that theGalaxy became reality 13 or 14 Gyr ago with the implementation of amassive, rotationally-supported population of thick-disk stars. The veryhigh star formation rate in that phase gave rise to a rapid metalenrichment and an expulsion of gas in supernovae-driven Galactic winds,but was followed by a star formation gap for no less than three billionyears at the Sun's galactocentric distance. In a second phase, then, thethin disk - our ``familiar Milky Way'' - came on stage. Nowadays ittraces the bright side of the Galaxy, but it is also embedded in a hugecoffin of dead thick-disk stars that account for a large amount ofbaryonic dark matter. As opposed to this, cold-dark-matter-dominatedcosmologies that suggest a more gradual hierarchical buildup throughmergers of minor structures, though popular, are a poor description forthe Milky Way Galaxy - and by inference many other spirals as well - if,as the sample implies, the fossil records of its long-lived stars do notstick to this paradigm. Apart from this general picture that emergeswith reference to the entire sample stars, a good deal of the presentwork is however also concerned with detailed discussions of manyindividual objects. Among the most interesting we mention the bluestraggler or merger candidates HD 165401 and HD 137763/HD 137778, thelikely accretion of a giant planet or brown dwarf on 59 Vir in itsrecent history, and HD 63433 that proves to be a young solar analog at\tau200 Myr. Likewise, the secondary to HR 4867, formerly suspectednon-single from the Hipparcos astrometry, is directly detectable in thehigh-resolution spectroscopic tracings, whereas the visual binary \chiCet is instead at least triple, and presumably even quadruple. Withrespect to the nearby young stars a complete account of the Ursa MajorAssociation is presented, and we provide as well plain evidence foranother, the ``Hercules-Lyra Association'', the likely existence ofwhich was only realized in recent years. On account of its rotation,chemistry, and age we do confirm that the Sun is very typical among itsG-type neighbors; as to its kinematics, it appears however not unlikelythat the Sun's known low peculiar space velocity could indeed be thecause for the weak paleontological record of mass extinctions and majorimpact events on our parent planet during the most recent Galactic planepassage of the solar system. Although the significance of thiscorrelation certainly remains a matter of debate for years to come, wepoint in this context to the principal importance of the thick disk fora complete census with respect to the local surface and volumedensities. Other important effects that can be ascribed to this darkstellar population comprise (i) the observed plateau in the shape of theluminosity function of the local FGK stars, (ii) a small thoughsystematic effect on the basic solar motion, (iii) a reassessment of theterm ``asymmetrical drift velocity'' for the remainder (i.e. the thindisk) of the stellar objects, (iv) its ability to account for the bulkof the recently discovered high-velocity blue white dwarfs, (v) itsmajor contribution to the Sun's 220 km s-1 rotationalvelocity around the Galactic center, and (vi) the significant flatteningthat it imposes on the Milky Way's rotation curve. Finally we note ahigh multiplicity fraction in the small but volume-complete local sampleof stars of this ancient population. This in turn is highly suggestivefor a star formation scenario wherein the few existing single stellarobjects might only arise from either late mergers or the dynamicalejection of former triple or higher level star systems.
| Meeting the Cool Neighbors. V. A 2MASS-Selected Sample of Ultracool Dwarfs
We present the initial results of our effort to create a statisticallyrobust, volume-limited sample of ultracool dwarfs from the Two MicronAll Sky Survey Second Incremental Data Release. We are engaged in amultifaceted search for nearby late-type objects, and this is the firstinstallment of our search using purely photometric selection. The goalof this work is a determination of the low-mass star and brown dwarfluminosity function in the infrared. Here we outline the construction ofthe sample, dubbed 2MU2, and present our first results, including thediscovery of 186 M7-L6 dwarfs-47 of which are likely to be within 20 pcof the Sun. These results represent 66% of the ultracool candidates inour sample yet constitute a 127% increase in the number of ultracooldwarfs known within the volume searched (covering 40% of the sky out to20 pc). In addition, we have identified 10 M4-M6.5 objects that arelikely to be within 20 pc (or within 1 σ). Finally, based on theseinitial data, we present a preliminary luminosity function and discussseveral interesting features of the partial sample presented here. Onceour sample is complete, we will use our measured luminosity function toconstrain the mass function of low-mass stars and brown dwarfs.
| Multiplicity of Nearby Free-Floating Ultracool Dwarfs: A Hubble Space Telescope WFPC2 Search for Companions
We present Hubble Space Telescope (HST) Wide Field Planetary Camera 2(WFPC2) observations of a sample of 134 ultracool objects (spectraltypes later than M7) coming from the Deep Near Infrared Survey (DENIS),Two Micron All Sky Survey (2MASS), and Sloan Digital Sky Survey (SDSS),with distances estimated to range from 7 to 105 pc. Fifteen newultracool binary candidates are reported here. Eleven known binaries areconfirmed, and orbital motion is detected in some of them. We estimatethat the closest binary systems in this sample have periods between 5and 20 yr, and thus dynamical masses will be derived in the near future.For the calculation of binary frequency, we restrict ourselves tosystems with distances less than 20 pc. After correction of the binariesbias, we find a ratio of visual binaries (at the HST limit of detection)of around 10%, and that ~15% of the 26 objects within 20 pc are binarysystems with separations between 1 and 8 AU. The observed frequency ofultracool binaries is similar to that of binaries with G-type primariesin the separation range from 2.1 to 140 AU. There is also a cleardeficit of ultracool binaries with separations greater than 15 AU, and apossible tendency for the binaries to have mass ratios near unity. Mostsystems have indeed visual and near-infrared brightness ratios between 1and 0.3. We discuss our results in the framework of current scenariosfor the formation and evolution of free-floating brown dwarfs.
| Gravity Indicators in the Near-Infrared Spectra of Brown Dwarfs
We investigate the sensitivity to temperature and gravity of the strongabsorption features in the J- and K-band spectra of substellar objects.We compare the spectra of giants and young M dwarfs (of low gravity) tofield M and L dwarfs (of high gravity) and to model spectra from theLyon group. We find that low-resolution spectra of M4-M9 stars and youngbrown dwarfs at R~350 and signal-to-noise ratios greater than 70 candetermine the spectral type to a precision of +/-1 subtype using theH2O and CO bands and can measure the surface gravity to+/-0.5 dex using the atomic lines of K I and Na I. This result pointstoward the development of photometric spectral indices to separatelow-mass members from foreground and background objects in youngclusters and associations. We also emphasize the complexity of theinterpretation of the empirical quantities (e.g., spectral types) interms of the physical variables (e.g., temperature, opacities) in thecool atmospheres of young brown dwarfs.Based on observations obtained with the Multiple Mirror TelescopeObservatory, a joint facility of the University of Arizona and theSmithsonian Institution.
| The Hamburg/RASS Catalogue of optical identifications. Northern high-galactic latitude ROSAT Bright Source Catalogue X-ray sources
We present the Hamburg/RASS Catalogue (HRC) of optical identificationsof X-ray sources at high-galactic latitude. The HRC includes all X-raysources from the ROSAT Bright Source Catalogue (RASS-BSC) with galacticlatitude |b| >=30degr and declination delta >=0degr . In thispart of the sky covering ~ 10 000 deg2 the RASS-BSC contains5341 X-ray sources. For the optical identification we used blue Schmidtprism and direct plates taken for the northern hemisphere Hamburg QuasarSurvey (HQS) which are now available in digitized form. The limitingmagnitudes are 18.5 and 20, respectively. For 82% of the selectedRASS-BSC an identification could be given. For the rest either nocounterpart was visible in the error circle or a plausibleidentification was not possible. With ~ 42% AGN represent the largestgroup of X-ray emitters, ~ 31% have a stellar counterpart, whereasgalaxies and cluster of galaxies comprise only ~ 4% and ~ 5%,respectively. In ~ 3% of the RASS-BSC sources no object was visible onour blue direct plates within 40\arcsec around the X-ray sourceposition. The catalogue is used as a source for the selection of(nearly) complete samples of the various classes of X-ray emitters.
| Stellar Kinematic Groups. II. A Reexamination of the Membership, Activity, and Age of the Ursa Major Group
Utilizing Hipparcos parallaxes, original radial velocities and recentliterature values, new Ca II H and K emission measurements,literature-based abundance estimates, and updated photometry (includingrecent resolved measurements of close doubles), we revisit the UrsaMajor moving group membership status of some 220 stars to produce afinal clean list of nearly 60 assured members, based on kinematic andphotometric criteria. Scatter in the velocity dispersions and H-Rdiagram is correlated with trial activity-based membership assignments,indicating the usefulness of criteria based on photometric andchromospheric emission to examine membership. Closer inspection,however, shows that activity is considerably more robust at excludingmembership, failing to do so only for <=15% of objects, perhapsconsiderably less. Our UMa members demonstrate nonzero vertex deviationin the Bottlinger diagram, behavior seen in older and recent studies ofnearby young disk stars and perhaps related to Galactic spiralstructure. Comparison of isochrones and our final UMa group membersindicates an age of 500+/-100 Myr, some 200 Myr older than thecanonically quoted UMa age. Our UMa kinematic/photometric members' meanchromospheric emission levels, rotational velocities, and scattertherein are indistinguishable from values in the Hyades and smaller thanthose evinced by members of the younger Pleiades and M34 clusters,suggesting these characteristics decline rapidly with age over 200-500Myr. None of our UMa members demonstrate inordinately low absolutevalues of chromospheric emission, but several may show residual fluxes afactor of >=2 below a Hyades-defined lower envelope. If one defines aMaunder-like minimum in a relative sense, then the UMa results maysuggest that solar-type stars spend 10% of their entire main-sequencelives in periods of precipitously low activity, which is consistent withestimates from older field stars. As related asides, we note six evolvedstars (among our UMa nonmembers) with distinctive kinematics that liealong a 2 Gyr isochrone and appear to be late-type counterparts to diskF stars defining intermediate-age star streams in previous studies,identify a small number of potentially very young but isolated fieldstars, note that active stars (whether UMa members or not) in our samplelie very close to the solar composition zero-age main sequence, unlikeHipparcos-based positions in the H-R diagram of Pleiades dwarfs, andargue that some extant transformations of activity indices are notadequate for cool dwarfs, for which Ca II infrared triplet emissionseems to be a better proxy than Hα-based values for Ca II H and Kindices.
| Improved Astrometry and Photometry for the Luyten Catalog. II. Faint Stars and the Revised Catalog
We complete construction of a catalog containing improved astrometry andnew optical/infrared photometry for the vast majority of NLTT starslying in the overlap of regions covered by POSS I and by the secondincremental Two Micron All Sky Survey (2MASS) release, approximately 44%of the sky. The epoch 2000 positions are typically accurate to 130 mas,the proper motions to 5.5 mas yr-1, and the V-J colors to0.25 mag. Relative proper motions of binary components are measured to 3mas yr-1. The false-identification rate is ~1% for11<~V<~18 and substantially less at brighter magnitudes. Theseimprovements permit the construction of a reduced proper-motion diagramthat, for the first time, allows one to classify NLTT stars intomain-sequence (MS) stars, subdwarfs (SDs), and white dwarfs (WDs). We inturn use this diagram to analyze the properties of both our catalog andthe NLTT catalog on which it is based. In sharp contrast to popularbelief, we find that NLTT incompleteness in the plane is almostcompletely concentrated in MS stars, and that SDs and WDs are detectedalmost uniformly over the sky δ>-33deg. Our catalogwill therefore provide a powerful tool to probe these populationsstatistically, as well as to reliably identify individual SDs and WDs.
| Hipparcos red stars in the HpV_T2 and V I_C systems
For Hipparcos M, S, and C spectral type stars, we provide calibratedinstantaneous (epoch) Cousins V - I color indices using newly derivedHpV_T2 photometry. Three new sets of ground-based Cousins V I data havebeen obtained for more than 170 carbon and red M giants. These datasetsin combination with the published sources of V I photometry served toobtain the calibration curves linking Hipparcos/Tycho Hp-V_T2 with theCousins V - I index. In total, 321 carbon stars and 4464 M- and S-typestars have new V - I indices. The standard error of the mean V - I isabout 0.1 mag or better down to Hp~9 although it deteriorates rapidly atfainter magnitudes. These V - I indices can be used to verify thepublished Hipparcos V - I color indices. Thus, we have identified ahandful of new cases where, instead of the real target, a random fieldstar has been observed. A considerable fraction of the DMSA/C and DMSA/Vsolutions for red stars appear not to be warranted. Most likely suchspurious solutions may originate from usage of a heavily biased color inthe astrometric processing.Based on observations from the Hipparcos astrometric satellite operatedby the European Space Agency (ESA 1997).}\fnmsep\thanks{Table 7 is onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/397/997
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