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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
| 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.
| 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.
| 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.
| Dwarf K and M stars of high proper motion found in a hemispheric survey
A recently completed visual/red spectral region objective-prism surveyof more than half the sky found some 2200 dwarf K and M stars ofnegligible proper motion (Stephenson, 1986). The present paper adds the1800-odd spectroscopically identified dwarfs that did prove to havesignificant proper motions. About half of these had previous spectralclassifications of some sort, especially by Vyssotsky (1952, 1956). Forthe great majority, the present coordinates are more accurate thanprevious data. The paper includes about 50 stars with unpublishedparallaxes, likely to have parallaxes of 0.05 arcsec or more. Combiningthe present data with the first paper suggests that the number oflow-proper-motion stars in that paper was not unreasonable.
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