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Ýlgili Makaleler
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.
| Deep imaging survey of young, nearby austral stars . VLT/NACO near-infrared Lyot-coronographic observations
Context. High contrast and high angular resolution imaging is theoptimal search technique for substellar companions to nearby stars atphysical separations larger than typically 10 AU. Two distinctpopulations of substellar companions, brown dwarfs and planets, can beprobed and characterized. As a result, fossile traces of processes offormation and evolution can be revealed by physical and orbitalproperties, both for individual systems and as an ensemble. Aims:Since November 2002, we have conducted a large, deep imaging, survey ofyoung, nearby associations of the southern hemisphere. Our goal isdetection and characterization of substellar companions with projectedseparations in the range 10-500 AU. We have observed a sample of 88stars, primarily G to M dwarfs, younger than 100 Myr, and within 100 pcof Earth. Methods: The VLT/NACO adaptive optics instrument of theESO Paranal Observatory was used to explore the faint circumstellarenvironment between typically 0.1 and 10''. Diffraction-limitedobservations in H and K_s-band combined with Lyot-coronagraphy enabledus to reach primary star-companion brightness ratios as small as10-6. The existence of planetary mass companions couldtherefore be probed. We used a standardized observing sequence toprecisely measure the position and flux of all detected sources relativeto their visual primary star. Repeated observations at several epochsenabled us to discriminate comoving companions from background objects. Results: We report the discovery of 17 new close (0.1-5.0'')multiple systems. HIP 108195 AB and C (F1 III-M6), HIP 84642 AB (a~14AU, K0-M5) and TWA22 AB (a~1.8 AU; M6-M6) are confirmed comovingsystems. TWA22 AB is likely to be a rare astrometric calibrator that canbe used to test evolutionary model predictions. Among our completesample, a total of 65 targets were observed with deep coronagraphicimaging. About 240 faint companion candidates were detected around 36stars. Follow-up observations with VLT or HST for 83% of these starsenabled us to identify a large fraction of background contaminants. Ourlatest results that pertain to the substellar companions to GSC08047-00232, AB Pic and 2M1207 (confirmed during this survey andpublished earlier), are reviewed. Finally, a statistical analysis of ourcomplete set of coronagraphic detection limits enables us to placeconstraints on the physical and orbital properties of giant planetsbetween typically 20 and 150 AU.Table 8 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/509/A52
| 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
| The Geneva-Copenhagen survey of the solar neighbourhood. III. Improved distances, ages, and kinematics
Context: Ages, chemical compositions, velocity vectors, and Galacticorbits for stars in the solar neighbourhood are fundamental test datafor models of Galactic evolution. The Geneva-Copenhagen Survey of theSolar Neighbourhood (Nordström et al. 2004; GCS), amagnitude-complete, kinematically unbiased sample of 16 682 nearby F andG dwarfs, is the largest available sample with complete data for starswith ages spanning that of the disk. Aims: We aim to improve theaccuracy of the GCS data by implementing the recent revision of theHipparcos parallaxes. Methods: The new parallaxes yield improvedastrometric distances for 12 506 stars in the GCS. We also use theparallaxes to verify the distance calibration for uvby? photometryby Holmberg et al. (2007, A&A, 475, 519; GCS II). We add newselection criteria to exclude evolved cool stars giving unreliableresults and derive distances for 3580 stars with large parallax errorsor not observed by Hipparcos. We also check the GCS II scales of T_effand [Fe/H] and find no need for change. Results: Introducing thenew distances, we recompute MV for 16 086 stars, and U, V, W,and Galactic orbital parameters for the 13 520 stars that also haveradial-velocity measurements. We also recompute stellar ages from thePadova stellar evolution models used in GCS I-II, using the new valuesof M_V, and compare them with ages from the Yale-Yonsei andVictoria-Regina models. Finally, we compare the observed age-velocityrelation in W with three simulated disk heating scenarios to show thepotential of the data. Conclusions: With these revisions, thebasic data for the GCS stars should now be as reliable as is possiblewith existing techniques. Further improvement must await consolidationof the T_eff scale from angular diameters and fluxes, and the Gaiatrigonometric parallaxes. We discuss the conditions for improvingcomputed stellar ages from new input data, and for distinguishingdifferent disk heating scenarios from data sets of the size andprecision of the GCS.Full Table 1 is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/501/941
| Formation and Evolution of Planetary Systems: Properties of Debris Dust Around Solar-Type Stars
We present Spitzer photometric (IRAC and MIPS) and spectroscopic (IRSlow resolution) observations for 314 stars in the Formation andEvolution of Planetary Systems Legacy program. These data are used toinvestigate the properties and evolution of circumstellar dust aroundsolar-type stars spanning ages from approximately 3 Myr-3 Gyr.We identify 46 sources that exhibit excess infrared emission above thestellar photosphere at 24 μm, and 21 sources with excesses at 70μm. Five sources with an infrared excess have characteristics ofoptically thick primordial disks, while the remaining sources haveproperties akin to debris systems. The fraction of systems exhibiting a24 μm excess greater than 10.2% above the photosphere is 15% for ages< 300 Myr and declines to 2.7% for older ages. The upper envelope tothe 70 μm fractional luminosity appears to decline over a similar agerange. The characteristic temperature of the debris inferred from theIRS spectra range between 60 and 180 K, with evidence for the presenceof cooler dust to account for the strength of the 70 μm excessemission. No strong correlation is found between dust temperature andstellar age. Comparison of the observational data with disk modelscontaining a power-law distribution of silicate grains suggests that thetypical inner-disk radius is gsim 10 AU. Although the interpretation isnot unique, the lack of excess emission shortward of 16 μm and therelatively flat distribution of the 24 μm excess for ages lsim 300Myr is consistent with steady-state collisional models.
| The Formation and Evolution of Planetary Systems: Description of the Spitzer Legacy Science Database
We present the science database produced by the Formation and Evolutionof Planetary Systems (FEPS) Spitzer Legacy program. Data reduction andvalidation procedures for the IRAC, MIPS, and IRS instruments aredescribed in detail. We also derive stellar properties for the FEPSsample from available broadband photometry and spectral types, andpresent an algorithm to normalize Kurucz synthetic spectra to opticaland near-infrared photometry. The final FEPS data products include IRACand MIPS photometry for each star in the FEPS sample and calibrated IRSspectra.
| 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.
| 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.
| The main sequence from F to K stars of the solar neighbourhood in SDSS colours
For an understanding of Galactic stellar populations in the SDSS filtersystem well defined stellar samples are needed. The nearby stars providea complete stellar sample representative for the thin disc population.We compare the filter transformations of different authors applied tothe main sequence stars from F to K dwarfs to SDSS filter system anddiscuss the properties of the main sequence. The location of the meanmain sequence in colour-magnitude diagrams is very sensitive tosystematic differences in the filter transformation. A comparison withfiducial sequences of star clusters observed in g', r', and i' show goodagreement. Theoretical isochrones from Padua and from Dartmouth havestill some problems, especially in the (r-i) colours.
| An Imaging Survey for Extrasolar Planets around 45 Close, Young Stars with the Simultaneous Differential Imager at the Very Large Telescope and MMT
We present the results of a survey of 45 young (<~250 Myr), close(<~50 pc) stars with the Simultaneous Differential Imager (SDI)implemented at the VLT and the MMT for the direct detection ofextrasolar planets. As part of the survey, we observed 54 objects,consisting of 45 close, young stars; two more distant (<150 pc),extremely young (<=10 Myr) stars; three stars with known radialvelocity planets; and four older, very nearby (<=20 pc) solaranalogs. Our SDI devices use a double Wollaston prism and a quad filterto take images simultaneously at three wavelengths surrounding the 1.62μm methane absorption bandhead found in the spectrum of cool browndwarfs and gas giant planets. By differencing adaptive optics-correctedimages in these filters, speckle noise from the primary star issignificantly attenuated, resulting in photon (andflat-field)-noise-limited data. In our VLT data, we achieved H-bandcontrasts>~10 mag (5 σ) at a separation of 0.5" from theprimary star on 45% of our targets and H-band contrasts>~9 mag at aseparation of 0.5" on 80% of our targets. With these contrasts, we canimage (5 σ detection) a 7 MJ planet 15 AU from a 70 MyrK1 star at 15 pc or a 7.8 MJ planet at 2 AU from a 12 Myr Mstar at 10 pc. We detected no candidates with S/N>2 σ whichbehaved consistently like a real object. From our survey null result, wecan rule out (with 93% confidence) a model planet population whereN(a)~constant out to a distance of 45 AU.Steward Observatory, University of Arizona, Tucson, AZ 85721.
| The NaI D resonance lines in main-sequence late-type stars
We study the sodium D lines (D1: 5895.92Å D2: 5889.95Å) inlate-type dwarf stars. The stars have spectral types between F6 and M5.5(B - V between 0.457 and 1.807) and metallicity between [Fe/H] = -0.82and 0.6. We obtained medium-resolution echelle spectra using the 2.15-mtelescope at the Argentinian observatory Complejo Astronómico ElLeoncito (CASLEO). The observations have been performed periodicallysince 1999. The spectra were calibrated in wavelength and in flux. Adefinition of the pseudo-continuum level is found for all ourobservations. We also define a continuum level for calibration purposes.The equivalent width of the D lines is computed in detail for all ourspectra and related to the colour index (B - V) of the stars. Whenpossible, we perform a careful comparison with previous studies.Finally, we construct a spectral index (R'D) as the ratiobetween the flux in the D lines and the bolometric flux. We find that,once corrected for the photospheric contribution, this index can be usedas a chromospheric activity indicator in stars with a high level ofactivity. Additionally, we find that combining some of our results, weobtain a method to calibrate in flux stars of unknown colour.
| Hα and the Ca II H and K lines as activity proxies for late-type stars
Context: The main chromospheric activity indicator is the S index, whichis the ratio of the flux in the core of the Ca II H and K lines to thecontinuum nearby, and is well studied for stars from F to K. Anotherchromospheric proxy is the Hα line, which is believed to betightly correlated with the Ca II index. Aims: In this work wecharacterize both chromospheric activity indicators, the one associatedwith the H and K Ca II lines and the other with Hα, for the wholerange of late type stars, from F to M. Methods: We present periodicmedium-resolution echelle observations covering the complete visualrange, taken at the CASLEO Argentinean Observatory over 7 years. We usea total of 917 flux-calibrated spectra for 109 stars that range from F6to M5. We statistically study these two indicators for stars ofdifferent activity levels and spectral types. Results: We directlyderive the conversion factor that translates the known S index to fluxin the Ca II cores, and extend its calibration to a wider spectralrange. We investigate the relation between the activity measurements inthe calcium and hydrogen lines, and found that the usual correlationobserved is the product of the dependence of each flux on stellarcolour, and not the product of similar activity phenomena.Tables 1 and 2 and full Figs. 1 and 6 are only available in electronicform at http://www.aanda.org
| On the common origin of the AB Doradus moving group and the Pleiades cluster
AB Doradus (AB Dor) is the nearest identified moving group. As withother such groups, the age is important for understanding of several keyquestions. It is important, for example, in establishing the origin ofthe group and also in comparative studies of the properties of planetarysystems, eventually surrounding some of the AB Dor group members, withthose existing in other groups. For AB Dor two rather differentestimates for its age have been proposed: the first one, of the order of50 Myr, by Zuckerman and coworkers from a comparison with theTucana/Horologium moving group and a second one of about 100-125 Myr byLuhman and coworkers from colour-magnitude diagrams. Using this lastvalue and the closeness in velocity space of AB Dor and the Pleiadesgalactic cluster, Luhman and coworkers suggested coevality for thesesystems. Because strictly speaking such a closeness does not stillguarantee coevality, here we address this problem by computing andcomparing the full 3D orbits of AB Dor, Pleiades, α Persei and IC2602. The latter two open clusters have estimated ages of about 85-90and 50 Myr. The resulting age 119 +/- 20 Myr is consistent with AB Dorand Pleiades being coeval. Our solution and the scenario of open clusterformation proposed by Kroupa and collaborators suggest that the AB Dormoving group may be identified with the expanding subpopulation (GroupI) present in this scenario. We also discuss other related aspects asiron and lithium abundances, eventual stellar mass segregation duringthe formation of the systems and possible fraction of debris discs inthe AB Dor group.
| Unraveling the Origins of Nearby Young Stars
A systematic search for close conjunctions and clusterings in the pastof nearby stars younger than the Pleiades is undertaken, which mayreveal the time, location, and mechanism of formation of these oftenisolated, disconnected from clusters and star-forming regions, objects.The sample under investigation includes 101 T Tauri, post-TT, andmain-sequence stars and stellar systems with signs of youth, culled fromthe literature. Their Galactic orbits are traced back in time and nearapproaches are evaluated in time, distance, and relative velocity.Numerous clustering events are detected, providing clues to the originof very young, isolated stars. Each star's orbit is also matched withthose of nearby young open clusters, OB and TT associations andstar-forming molecular clouds, including the Ophiuchus, Lupus, CoronaAustralis, and Chamaeleon regions. Ejection of young stars from openclusters is ruled out for nearly all investigated objects, but thenearest OB associations in Scorpius-Centaurus, and especially, the denseclouds in Ophiuchus and Corona Australis have likely played a major rolein the generation of the local streams (TWA, Beta Pic, andTucana-Horologium) that happen to be close to the Sun today. The core ofthe Tucana-Horologium association probably originated from the vicinityof the Upper Scorpius association 28 Myr ago. A few proposed members ofthe AB Dor moving group were in conjunction with the coeval Cepheus OB6association 38 Myr ago.
| The Formation and Evolution of Planetary Systems: Placing Our Solar System in Context with Spitzer
We provide an overview of the Spitzer Legacy Program, Formation andEvolution of Planetary Systems, that was proposed in 2000, begun in2001, and executed aboard the Spitzer Space Telescope between 2003 and2006. This program exploits the sensitivity of Spitzer to carry outmid-infrared spectrophotometric observations of solar-type stars. With asample of ~328 stars ranging in age from ~3 Myr to ~3 Gyr, we trace theevolution of circumstellar gas and dust from primordial planet-buildingstages in young circumstellar disks through to older collisionallygenerated debris disks. When completed, our program will help define thetimescales over which terrestrial and gas giant planets are built,constrain the frequency of planetesimal collisions as a function oftime, and establish the diversity of mature planetary architectures. Inaddition to the observational program, we have coordinated a concomitanttheoretical effort aimed at understanding the dynamics of circumstellardust with and without the effects of embedded planets, dust spectralenergy distributions, and atomic and molecular gas line emission.Together with the observations, these efforts will provide anastronomical context for understanding whether our solar system-and itshabitable planet-is a common or a rare circumstance. Additionalinformation about the FEPS project can be found on the team Web site.
| Search for associations containing young stars (SACY). I. Sample and searching method
We report results from a high-resolution optical spectroscopic surveyaimed to search for nearby young associations and young stars amongoptical counterparts of ROSAT All-Sky Survey X-ray sources in theSouthern Hemisphere. We selected 1953 late-type (B-V~≥~0.6),potentially young, optical counterparts out of a total of 9574 1RXSsources for follow-up observations. At least one high-resolutionspectrum was obtained for each of 1511 targets. This paper is the firstin a series presenting the results of the SACY survey. Here we describeour sample and our observations. We describe a convergence method in the(UVW) velocity space to find associations. As an example, we discuss thevalidity of this method in the framework of the β Pic Association.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| Contributions to the Nearby Stars (NStars) Project: Spectroscopy of Stars Earlier than M0 within 40 pc-The Southern Sample
We are obtaining spectra, spectral types, and basic physical parametersfor the nearly 3600 dwarf and giant stars earlier than M0 in theHipparcos catalog within 40 pc of the Sun. Here we report on resultsfor 1676 stars in the southern hemisphere observed at Cerro TololoInter-American Observatory and Steward Observatory. These resultsinclude new, precise, homogeneous spectral types, basic physicalparameters (including the effective temperature, surface gravity, andmetallicity [M/H]), and measures of the chromospheric activity of ourprogram stars. We include notes on astrophysically interesting stars inthis sample, the metallicity distribution of the solar neighborhood, anda table of solar analogs. We also demonstrate that the bimodal nature ofthe distribution of the chromospheric activity parameterlogR'HK depends strongly on the metallicity, andwe explore the nature of the ``low-metallicity'' chromosphericallyactive K-type dwarfs.
| The Nearest Young Moving Groups
The latest results in the research of forming planetary systems have ledseveral authors to compile a sample of candidates for searching forplanets in the vicinity of the Sun. Young stellar associations areindeed excellent laboratories for this study, but some of them are notclose enough to allow the detection of planets through adaptive opticstechniques. However, the existence of very close young moving groups cansolve this problem. Here we have compiled the members of the nearestyoung moving groups, as well as a list of new candidates from ourcatalog of late-type stars that are possible members of young stellarkinematic groups, studying their membership through spectroscopic andphotometric criteria.
| How Dry is the Brown Dwarf Desert? Quantifying the Relative Number of Planets, Brown Dwarfs, and Stellar Companions around Nearby Sun-like Stars
Sun-like stars have stellar, brown dwarf, and planetary companions. Tohelp constrain their formation and migration scenarios, we analyze theclose companions (orbital period <5 yr) of nearby Sun-like stars. Byusing the same sample to extract the relative numbers of stellar, browndwarf, and planetary companions, we verify the existence of a very drybrown dwarf desert and describe it quantitatively. With decreasing mass,the companion mass function drops by almost 2 orders of magnitude from 1Msolar stellar companions to the brown dwarf desert and thenrises by more than an order of magnitude from brown dwarfs toJupiter-mass planets. The slopes of the planetary and stellar companionmass functions are of opposite sign and are incompatible at the 3σ level, thus yielding a brown dwarf desert. The minimum number ofcompanions per unit interval in log mass (the driest part of the desert)is at M=31+25-18MJ. Approximately 16%of Sun-like stars have close (P<5 yr) companions more massive thanJupiter: 11%+/-3% are stellar, <1% are brown dwarf, and 5%+/-2% aregiant planets. The steep decline in the number of companions in thebrown dwarf regime, compared to the initial mass function of individualstars and free-floating brown dwarfs, suggests either a differentspectrum of gravitational fragmentation in the formation environment orpost-formation migratory processes disinclined to leave brown dwarfs inclose orbits.
| The Age of AB Doradus
We have derived a new age estimate for the nearby young star AB Dor andhave investigated the resulting implications for testing theoreticalevolutionary models with the data reported by Close and coworkers forthe low-mass companion AB Dor C. Using color-magnitude diagrams, we findthat the AB Dor moving group is roughly coeval with the Pleiades(τ=100-125 Myr) and is clearly older than IC 2391 (τ=35-50 Myr).In fact, based on a comparison of the kinematics of AB Dor and thePleiades, we suggest that the stars identified by Zuckerman andcoworkers as members of a moving group with AB Dor are remnants of thelarge-scale star-formation event that formed the Pleiades. Using the ageof τ=50+50-20 Myr adopted by Close, theluminosity predicted by the models of Chabrier and Baraffe for AB Dor Cis larger than the value reported by Close but is still within thequoted uncertainties. Meanwhile, the agreement is good when our ageestimate for AB Dor C is adopted. Thus, we find no evidence in the datapresented by Close for AB Dor C to suggest that previous studies usingthe models of Chabrier and Baraffe and bolometric luminosity as the massindicator have significantly underestimated the masses of young low-massstars and brown dwarfs.This publication makes use of data products from the Two Micron All SkySurvey, which is a joint project of the University of Massachusetts andthe Infrared Processing and Analysis Center/California Institute ofTechnology, funded by the National Aeronautics and Space Administrationand the National Science Foundation.
| Young Stars Near the Sun
Until the late 1990s the rich Hyades and the sparse UMa clusters werethe only coeval, comoving concentrations of stars known within 60 pc ofEarth. Both are hundreds of millions of years old. Then beginning in thelate 1990s the TW Hydrae Association, the Tucana/Horologium Association,the Pictoris Moving Group, and the AB Doradus Moving Group wereidentified within 60 pc of Earth, and the Chamaeleontis cluster wasfound at 97 pc. These young groups (ages 8 50 Myr), along with othernearby, young stars, will enable imaging and spectroscopic studies ofthe origin and early evolution of planetary systems.
| The AB Doradus Moving Group
From radio to X-ray wavelengths, AB Doradus has been an intensivelystudied star. We have identified ~30 nearby star systems, each with oneor more characteristics of youth, that are moving through space togetherwith AB Dor. This diverse set of ~50 million year old star systems isthe comoving, youthful group closest to Earth. The group's nucleus is aclustering of a dozen stars ~20 pc from Earth that includes AB Doritself. The AB Dor moving group joins the previously known and somewhatyounger and more distant Tucana/Horologium and TW Hydrae associationsand the β Pictoris moving group as excellent laboratories forinvestigations of forming planetary systems.
| The Geneva-Copenhagen survey of the Solar neighbourhood. Ages, metallicities, and kinematic properties of 14 000 F and G dwarfs
We present and discuss new determinations of metallicity, rotation, age,kinematics, and Galactic orbits for a complete, magnitude-limited, andkinematically unbiased sample of 16 682 nearby F and G dwarf stars. Our63 000 new, accurate radial-velocity observations for nearly 13 500stars allow identification of most of the binary stars in the sampleand, together with published uvbyβ photometry, Hipparcosparallaxes, Tycho-2 proper motions, and a few earlier radial velocities,complete the kinematic information for 14 139 stars. These high-qualityvelocity data are supplemented by effective temperatures andmetallicities newly derived from recent and/or revised calibrations. Theremaining stars either lack Hipparcos data or have fast rotation. Amajor effort has been devoted to the determination of new isochrone agesfor all stars for which this is possible. Particular attention has beengiven to a realistic treatment of statistical biases and errorestimates, as standard techniques tend to underestimate these effectsand introduce spurious features in the age distributions. Our ages agreewell with those by Edvardsson et al. (\cite{edv93}), despite severalastrophysical and computational improvements since then. We demonstrate,however, how strong observational and theoretical biases cause thedistribution of the observed ages to be very different from that of thetrue age distribution of the sample. Among the many basic relations ofthe Galactic disk that can be reinvestigated from the data presentedhere, we revisit the metallicity distribution of the G dwarfs and theage-metallicity, age-velocity, and metallicity-velocity relations of theSolar neighbourhood. Our first results confirm the lack of metal-poor Gdwarfs relative to closed-box model predictions (the ``G dwarfproblem''), the existence of radial metallicity gradients in the disk,the small change in mean metallicity of the thin disk since itsformation and the substantial scatter in metallicity at all ages, andthe continuing kinematic heating of the thin disk with an efficiencyconsistent with that expected for a combination of spiral arms and giantmolecular clouds. Distinct features in the distribution of the Vcomponent of the space motion are extended in age and metallicity,corresponding to the effects of stochastic spiral waves rather thanclassical moving groups, and may complicate the identification ofthick-disk stars from kinematic criteria. More advanced analyses of thisrich material will require careful simulations of the selection criteriafor the sample and the distribution of observational errors.Based on observations made with the Danish 1.5-m telescope at ESO, LaSilla, Chile, and with the Swiss 1-m telescope at Observatoire deHaute-Provence, France.Complete Tables 1 and 2 are only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/418/989
| Library of flux-calibrated echelle spectra of southern late-type dwarfs with different activity levels
We present Echelle spectra of 91 late-type dwarfs, of spectral typesfrom F to M and of different levels of chromospheric activity, obtainedwith the 2.15 m telescope of the CASLEO Observatory located in theArgentinean Andes. Our observations range from 3890 to 6690 Å, ata spectral resolution from 0.141 to 0.249 Å per pixel(R=λ/δ λ ≈ 26 400). The observations were fluxcalibrated with the aid of long slit spectra. A version of thecalibrated spectra is available via the World Wide Web.Table 2 is also available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/414/699The spectra are available as FITS and ascii-files at the URL:http://www.iafe.uba.ar/cincunegui/spectra/Table2.html. They are alsoavailable 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/414/699. When convertingthe fits to ascii, the spectra were oversampled to a constant δλ ≈ 0.15 Å.Table 2 is also available in electronic form at the CDS via anonymous
| Some anomalies in the occurrence of debris discs around main-sequence A and G stars
Debris discs consist of large dust grains that are generated bycollisions of comets or asteroids around main-sequence stars, and thequantity and distribution of debris may be used to detect the presenceof perturbing planets akin to Neptune. We use stellar and disc surveysto compare the material seen around A- and G-type main-sequence stars.Debris is detected much more commonly towards A stars, even when acomparison is made only with G stars of comparable age. Detection ratesare consistent with disc durations of ~0.5 Gyr, which may occur at anytime during the main sequence. The higher detection rate for A stars canresult from this duration being a larger fraction of the main-sequencelifetime, possibly boosted by a globally slightly larger disc mass thanfor the G-type counterparts. The disc mass range at any given age is afactor of at least ~100 and any systematic decline with time is slow,with a power law estimated to not be steeper than t-1/2.Comparison with models shows that dust can be expected as late as a fewGyr when perturbing planetesimals form slowly at large orbital radii.Currently, the Solar system has little dust because the radius of theKuiper Belt is small and hence the time-scale to produce planetesimalswas less than 1 Gyr. However, the apparently constant duration of ~0.5Gyr when dust is visible is not predicted by the models.
| Nearby young stars
We present the results of an extensive all-sky survey of nearby stars ofspectral type F8 or later in a systematic search of young (zero-age mainsequence) objects. Our sample has been derived by cross-correlating theROSAT All-Sky Survey and the TYCHO catalogue, yielding a total of 754candidates distributed more or less randomly over the sky. Follow-upspectroscopy of these candidate objects has been performed on 748 ofthem. We have discovered a tight kinematic group of ten stars withextremely high lithium equivalent widths that are presumably youngerthan the Pleiades, but again distributed rather uniformly over the sky.Furthermore, about 43 per cent of our candidates have detectable levelsof lithium, thus indicating that these are relatively young objects withages not significantly above the Pleiades age.Based on observations collected at the European Southern Observatory,Chile (ESO No. 62.I-0650, 66.D-0159(A), 67.D-0236(A)).
| Fast-rotating nearby solar-type stars sin i and X-ray luminosities relationships. II. Li abundances, v sin i and X-ray luminosities relationships
We present an analysis of our high-resolution spectroscopic andhigh-precision UBV(RI)_c photometric observations of a sample of 110nearby late-F and G-type stars selected for their large rotationalvelocity. The relationships between Li abundance, X-ray luminosity, andvsin i are investigated. We find that, as expected, the stars in oursample show statistically higher Li abundance and activity level thanfield star samples with similar characteristics, but slower rotation.Surprisingly, however, we also find four rapidly-rotating singlemain-sequence stars with very low Li abundance. For both single andbinary stars we find a large spread of Li abundance for stars withrotation lower than about 18 km s-1. The well-establishedcorrelation between X-ray luminosity and rotation rate is clearlyobserved. All single unevolved solar type stars with vsin i larger than18 km s-1 are strong X-ray emitters and have high Liabundance. Finally, we find also five evolved stars with very low Liabundance that are still rather fast rotators. The results from oursample confirm the presence of young very active stars close to the Sun,in agreement with recent findings from EUV and X-ray surveys, althoughour sample does not show such extreme characteristics as those selectedfrom EUV and X-ray surveys at the current flux limits.Based on data collected at the European Southern Observatory, La Silla,Chile.Tables, Figures and the complete data set are available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/397/987
| Fast-rotating nearby solar-type stars, Li abundances and X-ray luminosities. I. Spectral classification, v sin i, Li abundances and X-ray luminosities
We present the results of high-resolution spectroscopic andhigh-precision photometric observations on a sample of 129 late-F andG-type nearby stars selected on the basis of their large rotationalvelocity. Using also data from the Hipparcos satellite, CORAVEL and fromthe ROSAT satellite database, we infer spectral types, compute radialvelocities, v sin i, Li abundances and X-ray luminosities andinvestigate the single or binary nature of the sample stars. Such acareful analysis of our sample shows a large fraction of binaries ( =~62%) and of young single disk stars. In particular, at least 9 stars canbe considered bona-fide PMS or ZAMS objects, and 30 stars are identifiedas SBs for the first time. Information on the presence of Ca II Kemission and on optical variability is given for some of the stars ofthe sample. Based on data collected at the European SouthernObservatory, La Silla, Chile. Tables 1, 3, 4 and 5 and the complete dataset are only available in electronic form at the CDS via anonymous ftpto cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/384/491
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| Takýmyýldýz: | Ressam |
| Sað Açýklýk: | 06h22m30.94s |
| Yükselim: | -60°13'07.1" |
| Görünürdeki Parlaklýk: | 6.526 |
| Uzaklýk: | 23.496 parsek |
| özdevim Sað Açýklýk: | -11 |
| özdevim Yükselim: | 65.4 |
| B-T magnitude: | 7.248 |
| V-T magnitude: | 6.586 |
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