目录
图像
上传图像
DSS Images Other Images
相关文章
Distance and Kinematics of the TW Hydrae Association from Parallaxes
From common proper motion and signatures of youth, researchers haveidentified about 30 members of a putative TW Hydrae Association. Onlyfour of these had parallactic distances from Hipparcos. We have measuredparallaxes and proper motions for 14 primary members. We combine thesewith literature values of radial velocities to show that the Galacticspace motions of the stars, with the exception of TWA 9 and 22, areparallel and do not indicate convergence at a common formation pointsometime in the last few million years. The space motions of TWA 9 and22 do not agree with the others and indicate that they are not TWAmembers. The median parallax is 18 mas or 56 pc. We further analyze thestars' absolute magnitudes on pre-main-sequence evolutionary tracks andfind a range of ages with a median of 10.1 Myr and no correlationbetween age and Galactic location. The TWA stars may have formed from anextended and filamentary molecular cloud but are not necessarilyprecisely coeval.
| Bayesian Analysis to Identify New Star Candidates in Nearby Young Stellar Kinematic Groups
We present a new method based on a Bayesian analysis to identify newmembers of nearby young kinematic groups. The analysis minimally takesinto account the position, proper motion, magnitude, and color of astar, but other observables can be readily added (e.g., radial velocity,distance). We use this method to find new young low-mass stars in theβ Pictoris and AB Doradus moving groups and in the TW Hydrae,Tucana-Horologium, Columba, Carina, and Argus associations. Startingfrom a sample of 758 mid-K to mid-M (K5V-M5V) stars showing youthindicators such as Hα and X-ray emission, our analysis yields 214new highly probable low-mass members of the kinematic groups analyzed.One is in TW Hydrae, 37 in β Pictoris, 17 in Tucana-Horologium, 20in Columba, 6 in Carina, 50 in Argus, 32 in AB Doradus, and theremaining 51 candidates are likely young but have an ambiguousmembership to more than one association. The false alarm rate for newcandidates is estimated to be 5% for β Pictoris and TW Hydrae, 10%for Tucana-Horologium, Columba, Carina, and Argus, and 14% for ABDoradus. Our analysis confirms the membership of 58 stars proposed inthe literature. Firm membership confirmation of our new candidates willrequire measurement of their radial velocity (predicted by ouranalysis), parallax, and lithium 6708 Å equivalent width. We haveinitiated these follow-up observations for a number of candidates, andwe have identified two stars (2MASSJ01112542+1526214,2MASSJ05241914-1601153) as very strong candidate members of the βPictoris moving group and one strong candidate member(2MASSJ05332558-5117131) of the Tucana-Horologium association; thesethree stars have radial velocity measurements confirming theirmembership and lithium detections consistent with young age.Based on observations obtained at the Canada-France-Hawaii Telescope(CFHT) which is operated by the National Research Council of Canada, theInstitut National des Sciences de l'Univers of the Centre National de laRecherche Scientique of France, and the University of Hawaii.
| Young stars in ɛ Chamaleontis and their disks: disk evolution in sparse associations
Context. The nearby young stellar association ɛ Cha has anestimated age of 3-5 Myr, making it an ideal laboratory to study thedisk dissipation process and provide empirical constraints on thetimescale of planet formation. Aims: We wish to complementexisting optical and near-infrared data of the ɛ Chaassociation, which provide the stellar properties of its members, withmid-infrared data that probe the presence, geometry, and mineralogicalcomposition of protoplanetary disks around individual stars. Methods: We combine the available literature data with our Spitzer/IRSspectroscopy and VLT/VISIR imaging data. We use proper motions to refinethe membership of ɛ Cha. Masses and ages of individual stars areestimated by fitting model atmospheres to the optical and near-infraredphotometry, followed by placement in the Hertzsprung-Russell diagram.The Spitzer/IRS spectra are analyzed using the two-layer temperaturedistribution spectral decomposition method. Results: Two starspreviously identified as members, CXOU J120152.8 and 2MASS J12074597,have proper motions that are very different from those of the otherstars. But other observations suggest that the two stars are still youngand thus might still be related to ɛ Cha. HD 104237C is thelowest mass member of ɛ Cha with an estimated mass of ~13-15Jupiter masses. The very low mass stars USNO-B120144.7 and 2MASSJ12005517 show globally depleted spectral energy distributions, pointingat strong dust settling. 2MASS J12014343 may have a disk with a veryspecific inclination, where the central star is effectively screened bythe cold outer parts of a flared disk, but the 10 μm radiation of thewarm inner disk can still reach us. We find that the disks in sparsestellar associations are dissipated more slowly than those in denser(cluster) environments. We detect C2H2rovibrational band around 13.7 μm on the IRS spectrum ofUSNO-B120144.7. We find strong signatures of grain growth andcrystallization in all ɛ Cha members with 10 μm featuresdetected in their IRS spectra. We combine the dust properties derived inthe ɛ Cha sample with those found using identical or similarmethods in the MBM 12, Coronet, η Cha associations, and in thecores-to-disks legacy program. We find that disks around low-mass youngstars show a negative radial gradient in the mass-averaged grain sizeand mass fraction of crystalline silicates. A positive correlationexists between the mass-averaged grain sizes of amorphous silicates andthe accretion rates if the latter is above ~10-9M&sun; yr-1, possibly indicating that those disksare sufficiently turbulent to prevent grains of several microns in sizeto sink into the disk interior.Based on observations performed at ESO's La Silla-Paranal observatoryunder programme 076.C-0470.
| TW HYA Association Membership and New WISE-detected Circumstellar Disks
We assess the current membership of the nearby, young TW Hydraeassociation and examine newly proposed members with the Wide-fieldInfrared Survey Explorer (WISE) to search for infrared excess indicativeof circumstellar disks. Newly proposed members TWA 30A, TWA 30B, TWA 31,and TWA 32 all show excess emission at 12 and 22 ?m providing clearevidence for substantial dusty circumstellar disks around theselow-mass, ~8 Myr old stars that were previously shown to likely beaccreting circumstellar material. TWA 30B shows large amounts ofself-extinction, likely due to an edge-on disk geometry. We also confirmpreviously reported circumstellar disks with WISE and determine a 22?m excess fraction of 42+10 - 9% based onour results.
| New Members of the Scorpius-Centaurus Complex and Ages of Its Sub-regions
We have spectroscopically identified ~100 G-, K-, and M-type members ofthe Scorpius-Centaurus complex. To deduce the age of these young starswe compare their Li ?6708 absorption line strengths against thoseof stars in the TW Hydrae association and ? Pictoris moving group.These line strengths indicate that Sco-Cen stars are younger than ?Pic stars whose ages of ~12 Myr have previously been derived from akinematic traceback analysis. Our derived age, ~10 Myr, for stars in theLower Centaurus Crux and Upper Centaurus Lupus subgroups of ScoCen isyounger than previously published ages based on the moving clustermethod and upper main-sequence fitting. The discrepant ages are likelydue to an incorrect (or lack of) cross-calibration betweenmodel-dependent and model-independent age-dating methods.
| Precise Infrared Radial Velocities from Keck/NIRSPEC and the Search for Young Planets
We present a high-precision infrared radial velocity (RV) study oflate-type stars using spectra obtained with NIRSPEC at the W. M. KeckObservatory. RV precisions of 50 m s-1 are achieved for oldfield mid-M dwarfs using telluric features for wavelength calibration.Using this technique, 20 young stars in the β Pic (age ~ 12 Myr)and TW Hya (age ~ 8 Myr) Associations were monitored over several yearsto search for low-mass companions; we also included thechromospherically active field star GJ 873 (EV Lac) in this survey.Based on comparisons with previous optical observations of these youngactive stars, RV measurements at infrared wavelengths mitigate the RVnoise caused by star spots by a factor of ~3. Nevertheless, star spotnoise is still the dominant source of measurement error for young starsat 2.3 μm, and limits the precision to ~77 m s-1 for theslowest rotating stars (v sin i < 6 km s-1), increasing to~168 m s-1 for rapidly rotating stars (v sin i > 12 kms-1). The observations reveal both GJ 3305 and TWA 23 to besingle-lined spectroscopic binaries; in the case of GJ 3305, the motionis likely caused by its 0farcs09 companion, identified after this surveybegan. The large amplitude, short-timescale variations of TWA 13A areindicative of a hot Jupiter-like companion, but the available data areinsufficient to confirm this. We label it as a candidate RV variable.For the remainder of the sample, these observations exclude the presenceof any "hot" (P < 3 days) companions more massive than 8 MJup and any "warm" (P < 30 days) companions more massivethan 17 M Jup, on average. Assuming an edge-on orbit for theedge-on disk system AU Mic, these observations exclude the presence ofany hot Jupiters more massive than 1.8 M Jup or warm Jupitersmore massive than 3.9 M Jup.
| Mapping the Shores of the Brown Dwarf Desert. III. Young Moving Groups
We present the results of an aperture-masking interferometry survey forsubstellar companions around 67 members of the young (~8-200 Myr) nearby(~5-86 pc) AB Doradus, ? Pictoris, Hercules-Lyra, TW Hya, andTucana-Horologium stellar associations. Observations were made atnear-infrared wavelengths between 1.2 and 3.8 ?m using the adaptiveoptics facilities of the Keck II, Very Large Telescope UT4, and PalomarHale Telescopes. Typical contrast ratios of ~100-200 were achieved atangular separations between ~40 and 320 mas, with our survey being 100%complete for companions with masses below ~0.25 M &sun;across this range. We report the discovery of a 0.52 ± 0.09 M&sun; companion to HIP 14807, as well as the detections andorbits of previously known stellar companions to HD 16760, HD 113449,and HD 160934. We show that the companion to HD 16760 is in a face-onorbit, resulting in an upward revision of its mass from M2sin i ~ 14 M J to M 2 = 0.28 ±0.04 M &sun;. No substellar companions were detected aroundany of our sample members, despite our ability to detect companions withmasses below 80 M J for 50 of our targets: of these, oursensitivity extended down to 40 M J around 30 targets, with asubset of 22 subject to the still more stringent limit of 20 MJ. A statistical analysis of our non-detection of substellarcompanions allows us to place constraints on their frequency around~0.2-1.5 M &sun; stars. In particular, consideringcompanion mass distributions that have been proposed in the literature,we obtain an upper limit estimate of ~9%-11% for the frequency of 20-80M J companions between 3 and 30 AU at 95% confidence,assuming that their semimajor axes are distributed according to d {N}/da\propto a^{-1} in this range.
| RACE-OC project: rotation and variability in the ? Chamaeleontis, Octans, and Argus stellar associations
Context. Rotational properties of late-type low-mass members ofassociations of known age provide a fundamental source of information onstellar internal structure and its evolution. Aims: We aim atdetermining the rotational and magnetic-related activity properties ofstars at different stages of evolution. We focus our attention primarilyon members of young stellar associations of known ages. Specifically, weextend our previous analysis in Paper I (Messina et al. 2010, A&A520, A15) to 3 additional young stellar associations beyond 100 pc andwith ages in the range 6-40 Myr: ? Chamaeleontis (~6 Myr),Octans (~20 Myr), and Argus (~40 Myr). Additional rotational data of? Chamaeleontis and IC 2391 clusters are also considered. Methods: Rotational periods were determined by applying the Lomb-Scargleperiodogram technique to photometric time-series data obtained by theAll Sky Automated Survey (ASAS) and the Wide Angle Search for Planets(SuperWASP) archives. The magnetic activity level was derived from theamplitude of the V light curves. Results: We detected therotational modulation and measured the rotation periods of 56 stars forthe first time, confirmed 11 and revised 3 rotation periods alreadyknown from the literature. Adding the periods of 10 additional starsretrieved from the literature we determined a sample of 80 periodicstars at ages of ~6, ~20, and ~40 Myr. Using the SuperWASP data we alsorevisited some of the targets studied in Paper I. Conclusions:With the present study we have completed the analysis of the rotationalproperties of the late-type members of all known young looseassociations in the solar neighbourhood. Considering also the results ofPaper I, we have derived the rotation periods of 241 targets: 171confirmed, 44 likely, 26 uncertain. The period of the remaining 50 starsknown to be part of loose associations still remains unknown. Therotation period distributions we provided in the 0.8-1.2M&sun; mass range span nine different ages from 1 to ~100Myr. This rotation period catalogue, and specifically the newinformation presented in this paper at ~6, 20, and 40 Myr, contributessignificantly to a better observational description of the angularmomentum evolution of young stars. The results of the angular momentumevolution model based on this period database will be presented inforthcoming papers.Based on the All Sky Automated Survey (ASAS) and Wide Angle Search forPlanets (SuperWASP) photometric data.Appendices A and B are available inelectronic form at http://www.aanda.org
| Near-infrared Spectroscopy of TW Hya: A Revised Spectral Type and Comparison with Magnetospheric Accretion Models
We present high signal-to-noise ratio (S/N), moderate spectralresolution (R ~ 2000-2500) near-infrared (0.8-5.0 ?m) spectroscopy ofthe nearby T Tauri star TW Hya. By comparing the spectrum and theequivalent widths of several atomic and molecular features with thosefor stars in the IRTF near-infrared library, we revise the spectral typeto M2.5V, which is later than what is usually adopted (K7V). Thisimplies a substantially cooler stellar temperature than previouslyassumed. Comparison with various pre-main-sequence models suggests thatTW Hya is only ~3 Myr old, much younger than the usually adopted 8-10Myr. Analysis of the relative strengths of the H lines seen in thespectrum yields estimates for the temperature and density of theemitting region of Te >= 7500 K and ne ~1012-1013 cm-3. The thickness of theemitting region is 102-104 km and the coveringfraction is f * ~ 0.04. Our derived physical parameter valuesagree with the predictions of the magnetospheric accretion scenario. Thehighest S/N H lines have profiles that indicate multiple emissioncomponents. We derive an excess spectrum (above that of the M2.5Vtemplate) that peaks in the H band. Although our derived veiling values(~0.1) agree with previous estimates, the excess spectrum does not matchthat of current models in which this flux is generated by an inneroptically thin disk. We suggest that the excess flux spectrum insteadreflects the differences in atmospheric opacity, gravity, and agebetween TW Hya and older, higher gravity, field M2.5 dwarfs.
| An X-shooter survey of star forming regions: Low-mass stars and sub-stellar objects
We present preliminary results of our X-shooter survey in star formingregions. In this contribution we focus on sub-samples of young stellarand sub-stellar objects (YSOs) in the Lupus star forming region and inthe TW Hya association. We show that the X-shooter spectra are suitablefor conducting several parallel studies such as YSO + disk fundamentalparameters, accretion and outflow activity in the very low-mass (VLM)and sub-stellar regimes, as well as magnetic activity in young VLM YSOs,and Li abundance determinations. The capabilities of X-shooter in termsof wide spectral coverage, resolution and limiting magnitudes, allow usto assess simultaneously the accretion/outflow, magnetic activity, anddisk diagnostics, from the UV and optical to the near-IR, avoidingambiguities due to possible YSO variability.Based on observations collected at the European Southern Observatory,Chile, under Programmes 084.C-0269 and 085.C-0238.
| Bayesian inference of stellar parameters and interstellar extinction using parallaxes and multiband photometry
Astrometric surveys provide the opportunity to measure the absolutemagnitudes of large numbers of stars, but only if the individualline-of-sight extinctions are known. Unfortunately, extinction is highlydegenerate with stellar effective temperature when estimated frombroad-band optical/infrared photometry. To address this problem, Iintroduce a Bayesian method for estimating the intrinsic parameters of astar and its line-of-sight extinction. It uses both photometry andparallaxes in a self-consistent manner in order to provide anon-parametric posterior probability distribution over the parameters.The method makes explicit use of domain knowledge by employing theHertzsprung-Russell Diagram (HRD) to constrain solutions and to ensurethat they respect stellar physics. I first demonstrate this method byusing it to estimate effective temperature and extinction from BVJHKdata for a set of artificially reddened Hipparcos stars, for whichaccurate effective temperatures have been estimated from high-resolutionspectroscopy. Using just the four colours, we see the expected strongdegeneracy (positive correlation) between the temperature andextinction. Introducing the parallax, apparent magnitude and the HRDreduces this degeneracy and improves both the precision (reduces theerror bars) and the accuracy of the parameter estimates, the latter byabout 35 per cent. The resulting accuracy is about 200 K in temperatureand 0.2 mag in extinction. I then apply the method to estimate theseparameters and absolute magnitudes for some 47 000 F, G, K Hipparcosstars which have been cross-matched with Two-Micron All-Sky Survey(2MASS). The method can easily be extended to incorporate the estimationof other parameters, in particular metallicity and surface gravity,making it particularly suitable for the analysis of the 109stars from Gaia.
| A catalogue of young runaway Hipparcos stars within 3 kpc from the Sun
Traditionally, runaway stars are O- and B-type stars with large peculiarvelocities. We would like to extend this definition to young stars (upto ?50 Myr) of any spectral type and to identify those present in theHipparcos catalogue by applying different selection criteria, such aspeculiar space velocities or peculiar one-dimensional velocities.Runaway stars are important for studying the evolution of multiple starsystems or star clusters, as well as for identifying the origins ofneutron stars. We compile the distances, proper motions, spectral types,luminosity classes, V magnitudes and B-V colours, and we utilizeevolutionary models from different authors to obtain star ages. We studya sample of 7663 young Hipparcos stars within 3 kpc from the Sun. Theradial velocities are obtained from the literature. We investigate thedistributions of the peculiar spatial velocity and the peculiar radialvelocity as well as the peculiar tangential velocity and itsone-dimensional components and we obtain runaway star probabilities foreach star in the sample. In addition, we look for stars that aresituated outside any OB association or OB cluster and the Galactic planeas well as stars for which the velocity vector points away from themedian velocity vector of neighbouring stars or the surrounding local OBassociation/cluster (although the absolute velocity might be small). Wefind a total of 2547 runaway star candidates (with a contamination ofnormal Population I stars of 20 per cent at most). Thus, aftersubtracting these 20 per cent, the runaway frequency among young starsis about 27 per cent. We compile a catalogue of runaway stars, which isavailable via VizieR.
| The Age, Stellar Content, and Star Formation Timescale of the B59 Dense Core
We have investigated the stellar content of Barnard 59 (B59), the mostactive star-forming core in the Pipe Nebula. Using the SpeX spectrographon the NASA Infrared Telescope Facility, we obtained moderateresolution, near-infrared (NIR) spectra for 20 candidate young stellarobjects (YSOs) in B59 and a representative sample of NIR and mid-IRbright sources distributed throughout the Pipe. Measuring luminosity andtemperature sensitive features in these spectra, we identified likelybackground giant stars and measured each star's spectral type,extinction, and NIR continuum excess. To measure B59's age, we place itscandidate YSOs in the Hertzsprung-Russell diagram and compare theirlocation to YSOs in several well-studied star-forming regions, as wellas predictions of pre-main-sequence (PMS) evolutionary models. We findthat B59 is composed of late-type (K4-M6) low-mass (0.9-0.1 Msun) YSOs whose median stellar age is comparable to, if notslightly older than, that of YSOs within the ? Oph, Taurus, andChameleon star-forming regions. Deriving absolute age estimates from PMSmodels computed by D'Antona et al., and accounting only for statisticaluncertainties, we measure B59's median stellar age to be 2.6 ±0.8 Myr. Including potential systematic effects increases the errorbudget for B59's median (DM98) stellar age to 2.6+4.1-2.6 Myr. We also find that the relative age orderingsimplied by PMS evolutionary tracks depend on the range of stellar massessampled, as model isochrones possess significantly different massdependences. The maximum likelihood median stellar age we measure forB59, and the region's observed gas properties, suggests that the B59dense core has been stable against global collapse for roughly sixdynamical timescales and is actively forming stars with a star formationefficiency per dynamical time of ~6%. While the ~150% uncertaintiesassociated with our age measurement propagate directly into thesederived star formation timescales, the maximum likelihood valuesnonetheless agree well with recent star formation simulations thatincorporate various forms of support against collapse, such assubcritical magnetic fields, outflows, and radiative feedback fromprotostellar heating.
| RACE-OC project: Rotation and variability of young stellar associations within 100 pc
Context. Examining the angular momentum of stars and its interplay withtheir magnetic fields represent a promising way to probe the stellarinternal structure and evolution of low-mass stars. Aims: Weattempt to determine the rotational and magnetic-related activityproperties of stars at different stages of evolution.We focused ourattention primarily on members of clusters and young stellarassociations of known ages. In this study, our targets are 6 young loosestellar associations within 100 pc and with ages in the range 8-70 Myr:TW Hydrae (~8 Myr), ? Pictoris (~10 Myr), Tucana/Horologium,Columba, Carina (~30 Myr), and AB Doradus (~70 Myr). Additionalrotational data for ? Persei and the Pleiades from the literatureare also considered. Methods: Rotational periods of starsexhibiting rotational modulation due to photospheric magnetic activity(i.e., starspots) were determined by applying the Lomb-Scargleperiodogram technique to photometric time-series data obtained by theAll Sky Automated Survey (ASAS). The magnetic activity level was derivedfrom the amplitude of the V lightcurves. The statistical significance ofthe rotational evolution at different ages was inferred by applying atwo-sided Kolmogorov-Smirnov test to subsequent age-bins. Results: We detected the rotational modulation and measured the rotationperiods of 93 stars for the first time, and confirmed the periods of 41stars already known from the literature. For an additional 10 stars, werevised the period determinations by other authors. The sample wasaugmented with periods of 21 additional stars retrieved from theliterature. In this way, for the first time we were able to determinethe largest set of rotation periods at ages of ~8, ~10 and ~30 Myr, aswell as increase by 150% the number of known periodic members of AB Dor. Conclusions: The analysis of the rotation periods in youngstellar associations, supplemented by Orion Nebula Cluster (ONC) and NGC2264 data from the literature, has allowed us to find that in the0.6-1.2 M? range the most significant variations in therotation period distribution are the spin-up between 9 and 30 Myr andthe spin-down between 70 and 110 Myr. Variations of between 30 and 70Myr are rather doubtful, despite the median period indicating asignificant spin-up. The photospheric activity level is found to becorrelated with rotation at ages greater than ~70 Myr and to show someadditional age dependence besides that related to rotation and mass.Tables 1.1-1.7 and Figs. 1.1-1.22 are only available in electronic format http://www.aanda.orgBased on theAll Sky Automated Survey photometric data.
| A Uniform Analysis of 118 Stars with High-contrast Imaging: Long-period Extrasolar Giant Planets are Rare Around Sun-like Stars
We expand on the results of Nielsen et al., using the null result forgiant extrasolar planets around the 118 target stars from the Very LargeTelescope (VLT) NACO H- and Ks-band planet search (conducted byMasciadri and collaborators in 2003 and 2004), the VLT and MMTSimultaneous Differential Imager survey, and the Gemini Deep PlanetSurvey to set constraints on the population of giant extrasolar planets.Our analysis is extended to include the planet luminosity models ofFortney et al., as well as the correlation between stellar mass andfrequency of giant planets found by Johnson et al. Doubling the samplesize of FGKM stars strengthens our conclusions: a model for extrasolargiant planets with power laws for mass and semimajor axis as given byCumming et al. cannot, with 95% confidence, have planets beyond 65 AU,compared to the value of 94 AU reported by Nielsen et al., using themodels of Baraffe et al. When the Johnson et al. correction for stellarmass (which gives fewer Jupiter-mass companions to M stars with respectto solar-type stars) is applied, however, this limit moves out to 82 AU.For the relatively new Fortney et al. models, which predict fainterplanets across most of parameter space, these upper limits, with andwithout a correction for stellar mass, are 182 and 234 AU, respectively.
| Rotational velocities of nearby young stars
Context. Stellar rotation is a crucial parameter driving stellarmagnetism, activity and mixing of chemical elements. Measuringrotational velocities of young stars can give additional insight in theinitial conditions of the star formation process. Furthermore, theevolution of stellar rotation is coupled to the evolution ofcircumstellar disks. Disk-braking mechanisms are believed to beresponsible for rotational deceleration during the accretion phase, androtational spin-up during the contraction phase after decoupling fromthe disk for fast rotators arriving at the ZAMS. On the ZAMS, stars getrotationally braked by solar-type winds. Aims: We investigate theprojected rotational velocities v sin i of a sample of young stars withrespect to the stellar mass and disk evolutionary state to search forpossible indications of disk-braking mechanisms. Furthermore, we searchfor signs of rotational spin-up of stars that have already decoupledfrom their circumstellar disks. Methods: We analyse the stellarspectra of 220 nearby (mostly <100 pc) young (2-600 Myr) stars fortheir v sin i, stellar age, H? emission, and accretion rates. Thestars have been observed with FEROS at the 2.2 m MPG/ESO telescope andHARPS at the 3.6 m telescope in La Silla, Chile. The spectra have beencross-correlated with appropriate theoretical templates. We build a newcalibration to be able to derive v sin i values from thecross-correlated spectra. Stellar ages are estimated from the Li Iequivalent width at 6708 Å. The equivalent width and width at 10%height of the H? emission are measured to identify accretors andused to estimate accretion rates dot{M}_acc. The v sin i is thenanalysed with respect to the evolutionary state of the circumstellardisks to search for indications of disk-braking mechanisms in accretors. Results: We find that the broad v sin i distribution of ourtargets extends to rotation velocities of up to more than 100 kms-1 and peaks at a value of 7.8 ± 1.2 km s-1, and that 70% of our stars show v sin i < 30 kms-1. Furthermore, we can find indications for disk-braking inaccretors and rotational spin-up of stars which are decoupled from theirdisks. In addition, we show that a number of young stars are suitablefor precise radial-velocity measurements for planet-search surveys.Based upon observations with FEROS at the 2.2 m MPG/ESO telescope andHARPS at the 3.6 m telescope in La Silla, Chile.
| Orbital motion of the young brown dwarf companion TWA 5 B
Context. It is difficult to determine masses and test formation modelsfor brown dwarfs, because they are always above the main sequence, sothat there is a degeneracy between mass and age. However, for browndwarf companions to normal stars, such determinations may be possible,because one can know the distance and age of the primary star. As aresult, brown dwarf companions are well-suited to testing formationtheories and atmosphere models. Aims: With more adaptive opticsimages available, we aim at detecting orbital motion for the first timein the system TWA 5 A+B. Methods: We measured separation andposition angle between TWA 5 A and B in each high-resolution imageavailable and followed their change in time, because B should orbitaround A. The astrometric measurement precision is about one milliarcsec. Results: With ten year difference in epoch, we canclearly detect orbital motion of B around A, a decrease in separation by~0.0054” per year and a decrease in position angle by ~0.26°per year. Conclusions: TWA 5 B is a brown dwarf with ~25 Jupitermasses (Neuhäuser et al. 2000), but having large error bars (4 to145 Jupiter masses, Neuhäuser et al. 2009). Given its largeprojected separation from the primary star, ~86 AU, and its young age(~10 Myr), it has probably formed star-like, and would then be a browndwarf companion. Given the relatively large changes in separation andposition angle between TWA 5 A and B, we can conclude that they orbitaround each other on an eccentric orbit. Some evidence is found for acurvature in the orbital motion of B around A - most consistent with anelliptic (e = 0.45) orbit. Residuals around the best-fit ellipse aredetected and show a small-amplitude (~18 mas) periodic sinusoid with~5.7 yr period, i.e., fully consistent with the orbit of the inner closepair TWA 5 Aa+b. Measuring these residuals caused by the photocenterwobble - even in unresolved images - can yield the total mass of theinner pair, so can test theoretical pre-main sequence models.Based on observations collected at the European Southern Observatory,Chile, in runs 79.C-0103(A) and 81.C-0393(A) as well as on data obtainedfrom the public ESO science archive.
| Imaging Young Giant Planets From Ground and Space
High-contrast imaging can find and characterize gas giant planets aroundnearby young stars and the closest M stars, complementing radialvelocity and astrometric searches by exploring orbital separationsinaccessible to indirect methods. Ground-based coronagraphs are alreadyprobing within 25 AU of nearby young stars to find objects as small as .This paper contrasts near-term and future ground-based capabilities withhigh-contrast imaging modes of the James Webb Space Telescope (JWST).Monte Carlo modeling reveals that JWST can detect planets with masses assmall as across a broad range of orbital separations. We present newcalculations for planet brightness as a function of mass and age forspecific JWST filters and extending to .
| The Disk Population of the Taurus Star-Forming Region
We have analyzed nearly all images of the Taurus star-forming region at3.6, 4.5, 5.8, 8.0, and 24 ?m that were obtained during the cryogenicmission of the Spitzer Space Telescope (46 deg2) and havemeasured photometry for all known members of the region that are withinthese data, corresponding to 348 sources, or 99% of the known stellarpopulation. By combining these measurements with previous observationswith the Spitzer Infrared Spectrograph and other facilities, we haveclassified the members of Taurus according to whether they show evidenceof circumstellar disks and envelopes (classes I, II, and III). Throughthese classifications, we find that the disk fraction in Taurus,N(II)/N(II+III), is ~75% for solar-mass stars and declines to ~45% forlow-mass stars and brown dwarfs (0.01-0.3 M sun). Thisdependence on stellar mass is similar to that measured for Chamaeleon I,although the disk fraction in Taurus is slightly higher overall,probably because of its younger age (1 Myr versus 2-3 Myr). Incomparison, the disk fraction for solar-mass stars is much lower (~20%)in IC 348 and ? Ori, which are denser than Taurus and Chamaeleon Iand are roughly coeval with the latter. These data indicate that disklifetimes for solar-mass stars are longer in star-forming regions thathave lower stellar densities. Through an analysis of multiple epochs ofSpitzer photometry that are available for ~200 Taurus members, we findthat stars with disks exhibit significantly greater mid-infrared(mid-IR) variability than diskless stars, which agrees with the resultsof similar variability measurements for a smaller sample of stars inChamaeleon I. The variability fraction for stars with disks is higher inTaurus than in Chamaeleon I, indicating that the IR variability of disksdecreases with age. Finally, we have used our data in Taurus to refinethe observational criteria for primordial, evolved, and transitionaldisks. The ratio of the number of evolved and transitional disks to thenumber of primordial disks in Taurus is 15/98 for spectral types ofK5-M5, indicating a timescale of 0.15 × ?primordial~ 0.45 Myr for the clearing of the inner regions of optically thickdisks. After applying the same criteria to older clusters andassociations (2-10 Myr) that have been observed with Spitzer, we findthat the proportions of evolved and transitional disks in thosepopulations are consistent with the measurements in Taurus when theirstar formation histories are properly taken into account.Based on observations performed with the Spitzer Space Telescope.
| Search for associations containing young stars (SACY). III. Ages and Li abundances
Context: Our study is a follow-up of the SACY project, an extended highspectral resolution survey of more than two thousand opticalcounterparts to X-ray sources in the southern hemisphere targeted tosearch for young nearby association. Nine associations have either beennewly identified, or have had their member list revised. Groupsbelonging to the Sco-Cen-Oph complex are not considered in the presentstudy. Aims: These nine associations, with ages of between about 6Myr and 70 Myr, form an excellent sample to study the Li depletion inthe pre-main sequence (PMS) evolution. In the present paper, weinvestigate the use of Li abundances as an independent clock toconstrain the PMS evolution. Methods: Using our measurements ofthe equivalent widths of the Li resonance line and assuming fixedmetallicities and microturbulence, we calculated the LTE Li abundancesfor 376 members of various young associations. In addition, weconsidered the effects of their projected stellar rotation.Results: We present the Li depletion as a function of age in the firsthundred million years for the first time for the most extended sample ofLi abundances in young stellar associations. Conclusions: A clearLi depletion can be measured in the temperature range from 5000 K to3500 K for the age span covered by the nine associations studied in thispaper. The age sequence based on the Li-clock agrees well with theisochronal ages, the ?Cha association being the only possibleexception. The lithium depletion patterns for the associations presentedhere resemble those of the young open clusters with similar ages,strengthening the notion that the members proposed for these loose youngassociations have indeed a common physical origin. The observed scatterin the Li abundances hampers the use of Li in determining reliable agesfor individual stars. For velocities above 20 km s-1,rotation seems to play an important role in inhibiting the Li depletion.Based on observations collected at the ESO - La Silla and at theLNA-OPD.Tables [see full textsee full text]-[see full textsee full text] areonly available in electronic form at http://www.aanda.org
| XID II: Statistical Cross-Association of ROSAT Bright Source Catalog X-ray Sources with 2MASS Point Source Catalog Near-Infrared Sources
The 18,806 ROSAT All Sky Survey Bright Source Catalog (RASS/BSC) X-raysources are quantitatively cross-associated with near-infrared (NIR)sources from the Two Micron All Sky Survey Point Source Catalog(2MASS/PSC). An association catalog is presented, listing the mostlikely counterpart for each RASS/BSC source, the probability Pid that the NIR source and X-ray source are uniquelyassociated, and the probability P no-id that none of the2MASS/PSC sources are associated with the X-ray source. The catalogincludes 3853 high quality (P id>0.98) X-ray-NIR matches,2280 medium quality (0.98 >= P id>0.9) matches, and4153 low quality (0.9 >= P id>0.5) matches. Of the highquality matches, 1418 are associations that are not listed in the SIMBADdatabase, and for which no high quality match with a USNO-A2 opticalsource was presented for the RASS/BSC source in previous work. Thepresent work offers a significant number of new associations withRASS/BSC objects that will require optical/NIR spectroscopy forclassification. For example, of the 6133 P id>0.92MASS/PSC counterparts presented in the association catalog, 2411 haveno classification listed in the SIMBAD database. These 2MASS/PSC sourceswill likely include scientifically useful examples of known sourceclasses of X-ray emitters (white dwarfs, coronally active stars, activegalactic nuclei), but may also contain previously unknown sourceclasses. It is determined that all coronally active stars in theRASS/BSC should have a counterpart in the 2MASS/PSC, and that the uniqueassociation of these RASS/BSC sources with their NIR counterparts thusis confusion limited.
| Kinematic analysis and membership status of TWA22 AB
Context: TWA22 was initially regarded as a member of the TW Hydraeassociation (TWA). In addition to being one of the youngest (≈8 Myr)and nearest (≈20 pc) stars to Earth, TWA22 has proven to be veryinteresting after being resolved as a tight, very low-mass binary. Thisbinary can serve as a very useful dynamical calibrator for pre-mainsequence evolutionary models. However, its membership in the TWA hasbeen recently questioned despite due to the lack of accurate kinematicmeasurements. Aims: Based on proper motion, radial velocity, andtrigonometric parallax measurements, we aim here to re-analyze themembership of TWA22 to young, nearby associations. Methods: Usingthe ESO NTT/SUSI2 telescope, we observed TWA22 AB during 5 differentobserving runs over 1.2 years to measure its trigonometric parallax andproper motion. This is a part of a larger project measuringtrigonometric parallaxes and proper motions of most known TWA members ata sub-milliarcsec level. HARPS at the ESO 3.6 m telescope was also usedto measure the system's radial velocity over 2 years. Results: Wereport an absolute trigonometric parallax of TWA22 AB, π =57.0±0.7 mas, corresponding to a distance 17.5±0.2 pc fromEarth. Measured proper motions of TWA 22AB areμαcos(δ) = -175.8±0.8 mas/yr andμδ = -21.3±0.8 mas/yr. Finally, from HARPSmeasurements, we obtain a radial velocity V_rad = 14.8±2.1 kms-1. Conclusions: A kinematic analysis of TWA22 ABspace motion and position implies that a membership of TWA22 AB to knownyoung, nearby associations can be excluded except for the βPictoris and TW Hydrae associations. Membership probabilities based onthe system's Galactic space motion and/or the trace-back techniquesupport a higher chance of being a member to the β Pictorisassociation. Membership of TWA22 in the TWA cannot be fully excludedbecause of large uncertainties in parallax measurements and radialvelocities and to the uncertain internal velocity dispersion of itsmembers.Based on observations performed at the European Southern Observatory,Chile (76.C-0543, 077.C-0112, 078.C-0158, 079.C-0229). Table 4 is onlyavailable in electronic form at http://www.aanda.org
| Search for Remnant Clouds Associated with the TW Hya Association
We report on a search for the parental molecular clouds of the TW Hyaassociation (TWA), using CO emission and NaI absorption lines. TWA isthe nearest young (˜50pc; ˜10Myr) stellar association; yet inspite of its youth there has been no detection of any associated natalmolecular gas, as is the case with other typical young clusters. Usinginfrared maps as a guide, we conducted a CO cloud survey toward a regionwith a dust extinction of E(B ? V) > 0.2mag, or AV> 0.6mag. CO emission was detected in the direction of three IR dustclouds, and we rejected one cloud out of the TWA, because nointerstellar Na absorption was detected in the nearby Hipparcos stars,implying that it is too distant to relate to the TWA. The two otherclouds exhibit only faint and small-scale CO emission. Interstellar NaIabsorptions of Hipparcos targets (HIP 57809, HIP 64837, and HIP 64925,at distances of 133, 81, and 101pc, respectively) by these clouds werealso detected. We conclude that only a small fraction of theinterstellar matter (ISM) toward the infrared (IR) dust cloud is locatedat a distance less than 100pc, which may be all that is left out of theremnant clouds of TWA; the remaining remnant cloud dissipated within thelast ˜1Myr. Such a short-dissipation timescale may be due to anexternal perturbation or kinematic segregation that has a large stellarproper motion relative to the natal cloud.
| Lithium Depletion of Nearby Young Stellar Associations
We estimate cluster ages from lithium depletion in fivepre-main-sequence groups found within 100 pc of the Sun: the TW Hydraeassociation, η Chamaeleontis cluster, β Pictoris moving group,Tucanae-Horologium association, and AB Doradus moving group. Wedetermine surface gravities, effective temperatures, and lithiumabundances for over 900 spectra through least-squares fitting tomodel-atmosphere spectra. For each group, we compare the dependence oflithium abundance on temperature with isochrones from pre-main-sequenceevolutionary tracks to obtain model-dependent ages. We find that theη Cha cluster and the TW Hydrae association are the youngest, withages of 12+/-6 Myr and 12+/-8 Myr, respectively, followed by the βPic moving group at 21+/-9 Myr, the Tucanae-Horologium association at27+/-11 Myr, and the AB Dor moving group at an age of at least 45 Myr(whereby we can only set a lower limit, since the models-unlike realstars-do not show much lithium depletion beyond this age). Here theordering is robust, but the precise ages depend on our choice of bothatmospheric and evolutionary models. As a result, while our ages areconsistent with estimates based on Hertzsprung-Russell isochrone fittingand dynamical expansion, they are not yet more precise. Our observationsdo show that with improved models, much stronger constraints should befeasible, as the intrinsic uncertainties, as measured from the scatterbetween measurements from different spectra of the same star, are verylow: around 10 K in effective temperature, 0.05 dex in surface gravity,and 0.03 dex in lithium abundance.
| Magnetic Properties of Young Stars in the TW Hydrae Association
We present an analysis of infrared echelle spectra of five stars in theTW Hydrae Association (TWA). We model the Zeeman broadening in fourmagnetic-sensitive Ti I lines near 2.2 μm and measure the value ofthe photospheric magnetic field averaged over the surface of each star.To ensure that other broadening mechanisms are properly taken intoaccount, we also inspect several magnetically insensitive CO lines near2.3 μm and find no excess broadening above that produced by stellarrotation and instrumental broadening, providing confidence in themagnetic interpretation of the width of the Ti I lines. We then utilizeour results to test the relationship between stellar magnetic flux andX-ray properties and compare the measured fields with equipartitionfield values. Finally, we use our results and recent results on a largesample of stars in Taurus to discuss the potential evolution of magneticfield properties between the age of Taurus (~2 Myr) and the age of TWA(~10 Myr). We find that the average stellar field strength increaseswith age; however, the total unsigned magnetic flux decreases as thestars contract onto the main sequence.
| On the kinematic evolution of young local associations and the Scorpius-Centaurus complex
Context: Over the last decade, several groups of young (mainly low-mass)stars have been discovered in the solar neighbourhood (closer than ~100pc), thanks to cross-correlation between X-ray, optical spectroscopy andkinematic data. These young local associations - including an importantfraction whose members are Hipparcos stars - offer insights into thestar formation process in low-density environments, shed light on thesubstellar domain, and could have played an important role in the recenthistory of the local interstellar medium. Aims: To study the kinematicevolution of young local associations and their relation to other youngstellar groups and structures in the local interstellar medium, thuscasting new light on recent star formation processes in the solarneighbourhood. Methods: We compiled the data published in theliterature for young local associations. Using a realistic Galacticpotential we integrated the orbits for these associations and theSco-Cen complex back in time. Results: Combining these data with thespatial structure of the Local Bubble and the spiral structure of theGalaxy, we propose a recent history of star formation in the solarneighbourhood. We suggest that both the Sco-Cen complex and young localassociations originated as a result of the impact of the inner spiralarm shock wave against a giant molecular cloud. The core of the giantmolecular cloud formed the Sco-Cen complex, and some small cloudlets ina halo around the giant molecular cloud formed young local associationsseveral million years later. We also propose a supernova in young localassociations a few million years ago as the most likely candidate tohave reheated the Local Bubble to its present temperature.
| Constraints on Extrasolar Planet Populations from VLT NACO/SDI and MMT SDI and Direct Adaptive Optics Imaging Surveys: Giant Planets are Rare at Large Separations
We examine the implications for the distribution of extrasolar planetsbased on the null results from two of the largest direct imaging surveyspublished to date. Combining the measured contrast curves from 22 of thestars observed with the VLT NACO adaptive optics system by Masciadri andcoworkers and 48 of the stars observed with the VLT NACO SDI and MMT SDIdevices by Biller and coworkers (for a total of 60 unique stars), weconsider what distributions of planet masses and semimajor axes can beruled out by these data, based on Monte Carlo simulations of planetpopulations. We can set the following upper limit with 95% confidence:the fraction of stars with planets with semimajor axis between 20 and100 AU, and mass above 4 MJup, is 20% or less. Also, with adistribution of planet mass of dN/dM~M-1.16 in the range of0.5-13 MJup, we can rule out a power-law distribution forsemimajor axis (dN/da~aα) with index 0 and upper cutoffof 18 AU, and index -0.5 with an upper cutoff of 48 AU. For thedistribution suggested by Cumming et al., a power-law of index -0.61, wecan place an upper limit of 75 AU on the semimajor axis distribution. Ingeneral, we find that even null results from direct imaging surveys arevery powerful in constraining the distributions of giant planets (0.5-13MJup) at large separations, but more work needs to be done toclose the gap between planets that can be detected by direct imaging,and those to which the radial velocity method is sensitive.
| An accurate distance to 2M1207Ab
Context: In April 2004, the first image was obtained of a planetary masscompanion (now known as 2M1207 b) in orbit around a self-luminous objectdifferent from our own Sun (the young brown dwarf 2MASSWJ1207334-393254, hereafter 2M1207 A). That 2M1207 b probably formed viafragmentation and gravitational collapse offered proof that such amechanism can form bodies in the planetary mass regime. However, thepredicted mass, luminosity, and radius of 2M1207 b depend on its age,distance, and other observables, such as effective temperature. Aims: To refine our knowledge of the physical properties of 2M1207 band its nature, we accurately determined the distance to the 2M1207 Aand b system by measuring of its trigonometric parallax at themilliarcsec level. Methods: With the ESO NTT/SUSI2 telescope, webegan a campaign of photometric and astrometric observations in 2006 tomeasure the trigonometric parallax of 2M1207 A. Results: Anaccurate distance (52.4± 1.1 pc) to 2M1207A was measured. Fromdistance and proper motions we derived spatial velocities that are fullycompatible with TWA membership. Conclusions: With this newdistance estimate, we discuss three scenarios regarding the nature of2M1207 b: (1) a cool (1150±150 K) companion of mass 4±1M{Jup}, (2) a warmer (1600±100 K) and heavier(8±2 M{Jup}) companion occulted by an edge-oncircumsecondary disk, or (3) a hot protoplanet collision afterglow.Based on observations collected at the European SouthernObservatory, Chile (76.C-0543, 077.C-0112, 078.C-0158, 079.C-0229) andat Valinhos meridian circle.
| An Improbable Solution to the Underluminosity of 2M1207B: A Hot Protoplanet Collision Afterglow
We introduce an alternative hypothesis to explain the very lowluminosity of the cool (L-type) companion to the ~25 MJup, ~8Myr old brown dwarf 2M1207A. Recently, Mohanty et al. found thateffective temperature estimates for 2M1207B (1600+/-100 K) are grosslyinconsistent with its lying on the same isochrone as the primary, beinga factor of ~10 underluminous at all bands between I (0.8 μm) andL' (3.6 μm). Mohanty et al. explain this discrepancy bysuggesting that 2M1207B is an 8 MJup object surrounded by anedge-on disk comprised of large dust grains producing 2.5 mag ofachromatic extinction. We offer an alternative explanation: the apparentflux reflects the actual source luminosity. Given the temperature, weinfer a small radius (~49,000 km), and for a range of plausibledensities, we estimate a mass < MJup. We suggest that2M1207B is a hot protoplanet collision afterglow and show that theradiative timescale for such an object is >~1% the age of the system.If our hypothesis is correct, the surface gravity of 2M1207B should bean order of magnitude lower than that predicted by Mohanty et al.
| Kinematics of the Scorpius-Centaurus OB association
A fine structure related to the kinematic peculiarities of threecomponents of the Scorpius-Centaurus association (LCC, UCL, and US) hasbeen revealed in the UV-velocity distribution of Gould Belt stars. Wehave been able to identify the most likely members of these groups byapplying the method of analyzing the two-dimensional probability densityfunction of stellar UV velocities that we developed. A kinematicanalysis of the identified structural components has shown that, ingeneral, the center-of-mass motion of the LCC, UCL, and US groupsfollows the motion characteristic of the Gould Belt, notably itsexpansion. The entire Scorpius-Centaurus complex is shown to possess aproper expansion with an angular velocity parameter of 46 ± 8 kms‑1 kpc‑1 for the kinematic centerwith l 0 = ‑40° and R 0 = 110 pc found.Based on this velocity, we have estimated the characteristic expansiontime of the complex to be 21 ± 4 Myr. The proper rotationvelocity of the Scorpius-Centaurus complex is lower in magnitude, isdetermined less reliably, and depends markedly on the data quality.
|
提交文章
相关链接
提交链接
下列团体成员
|
观测天体数据
目录:
|
INHABITED SKY
