Ýçindekiler
Görüntüler
Resim Yükleyin
DSS Images Other Images
Ýlgili Makaleler
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
| The Tucana/Horologium, Columba, AB Doradus, and Argus Associations: New Members and Dusty Debris Disks
We propose 35 star systems within ~70 pc of Earth as newly identifiedmembers of nearby young stellar kinematic groups; these identificationsinclude the first A- and late-B-type members of the AB Doradus movinggroup and field Argus Association. All but one of the 35 systems containa bright solar- or earlier-type star that should make an excellenttarget for the next generation of adaptive optics (AO) imaging systemson large telescopes. AO imaging has revealed four massive planets inorbit around the ? Boo star HR 8799. Initially, the planets wereof uncertain mass due in large part to the uncertain age of the star. Wefind that HR 8799 is a likely member of the ~30 Myr old ColumbaAssociation, implying planet masses ~6 times that of Jupiter. Weconsider Spitzer Space Telescope MIPS photometry of stars in the ~30 Myrold Tucana/Horologium and Columba Associations, the ~40 Myr old fieldArgus Association, and the ~70 Myr old AB Doradus moving group. Thepercentage of stars in these young stellar groups that display excessemission above the stellar photosphere at 24 and 70 ?mwavelengths—indicative of the presence of a dusty debrisdisk—is compared with corresponding percentages for members of 11open clusters and stellar associations with ages between 8 and 750 Myr,thus elucidating the decay of debris disks with time.
| 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.
| 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.
| 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 .
| 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
| 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.
| Keck DEIMOS Spectroscopy of a GALEX UV-Selected Sample from the Medium Imaging Survey
We report results from a pilot program to obtain spectroscopy forobjects detected in the Galaxy Evolution Explorer (GALEX) Medium ImagingSurvey (MIS). Our study examines the properties of galaxies detected byGALEX fainter than the Sloan Digital Sky Survey (SDSS) spectroscopicsurvey. This is the first study to extend the techniques of Salim andcoworkers to estimate stellar masses, star formation rates (SFRs), andthe b (star formation history) parameter for star-forming galaxies outto z~0.7. We obtain redshifts for 50 GALEX MIS sources reaching NUV=23.9(AB mag) having counterparts in the SDSS Data Release 4 (DR4). Of oursample, 43 are star-forming galaxies with z<0.7, 3 have emission-lineratios indicative of active galactic nuclei with z<0.7, and 4 objectswith z>1 are QSOs, 3 of which are not previously cataloged. Wecompare our sample to a much larger sample of ~50,000 matched GALEX/SDSSgalaxies with SDSS spectroscopy; while our survey is shallow, theoptical counterparts to our sources reach ~3 mag fainter in SDSS r thanthe SDSS spectroscopic sample. We use emission-line diagnostics for thegalaxies to determine that the sample contains mostly star-forminggalaxies. The galaxies in the sample populate the blue sequence in theNUV-r versus Mr color-magnitude diagram. The derived stellarmasses of the galaxies range from 108 to 1011Msolar, and derived SFRs are between 10-1 and102 Msolar yr-1. Our sample has SFRs,luminosities, and velocity dispersions that are similar to the samplesof faint compact blue galaxies studied previously in the same redshiftrange by Koo and collaborators, Guzmán and collaborators, andPhillips and collaborators. However, our sample is ~2 mag fainter insurface brightness than the compact blue galaxies. We find that the starformation histories for a majority of the galaxies are consistent with arecent starburst within the last 100 Myr.Some of the data presented herein were obtained at the W. M. KeckObservatory, which is operated as a scientific partnership among theCalifornia Institute of Technology, the University of California, andthe National Aeronautics and Space Administration. The Observatory wasmade possible by the generous financial support of the W. M. KeckFoundation.
| A novel L-band imaging search for giant planets in the Tucana and β Pictoris moving groups
Context: Direct imaging using various techniques for suppressing thestellar halo nowadays can achieve the contrast levels required to detectand characterize the light of substellar companions at orbital distancesgreater than a few astronomical units from their host stars. The methodnicely complements the radial velocity surveys that provide evidencethat giant extrasolar planets in close-in orbits are relatively common. Aims: The paper presents results from a small survey of 22 young, nearbystars that was designed to detect substellar companions and ultimatelygiant extrasolar planets down to Jupiter masses. The targets are membersof the Tucana and β Pictoris moving groups, apart from the somewhatolder star HIP 71395, which has a radial velocity trend suggesting amassive planet in large orbit. Methods: The survey was carried out inthe L-band using adaptive optics-assisted imaging with NAOS-CONICA(NACO) at the VLT. The chosen observation wavelength is well-suited tosearching for close companions around young stars and it deliversunprecedented detection limits. The presented technique reaches some ofthe best sensitivities as of today and is currently the most sensitivemethod for the contrast-limited detection of substellar companions thatare cooler than about 1000 K. Results: The companion to 51 Eri, GJ3305, was found to be a very close binary on an eccentric orbit. Nosubstellar companions were found around the target stars, although themethod permitted companions to be detected down to a few Jupiter massesat orbital distances typically of 5 astronomical units. A planet with amass ≥1 M_Jup at distances ≥5 AU around AU Mic can be excluded atthe time of our observations. The absence of detected planets setsconstraints on the frequency distribution and maximum orbital distanceof giant exoplanets. For example, a radial distribution power law indexof 0.2 in combination with a maximum orbital radius exceeding 30 AU canbe rejected at a 90% confidence level.Based on observations collected at the European Southern Observatory,Chile, through the proposals 073.C-0834(A) and 074.C-0323(A). Figure 6is only available in electronic form at http://www.aanda.org
| SIM PlanetQuest Key Project Precursor Observations to Detect Gas Giant Planets around Young Stars
We present a review of precursor observing programs for the SIMPlanetQuest Key Project devoted to detecting Jupiter-mass planets aroundyoung stars. In order to ensure that the stars in the sample are free ofvarious sources of astrometric noise that might impede the detection ofplanets, we have initiated programs to collect photometry, high-contrastimages, interferometric data, and radial velocities for stars in boththe northern and southern hemispheres. We have completed a high-contrastimaging survey of target stars in Taurus and the Pleiades and found nodefinitive common proper motion companions within 1" (140 AU) of the SIMtargets. Our radial velocity surveys have shown that many of the targetstars in Sco-Cen are fast rotators, and a few stars in Taurus and thePleiades may have substellar companions. Interferometric data of a fewstars in Taurus show no signs of stellar or substellar companions withseparations of 5-50 mas. The photometric survey suggests thatapproximately half of the stars initially selected for this program arevariable to a degree (1 σ > 0.1 mag) that would degrade theastrometric accuracy achievable for that star. While the precursorprograms are still a work in progress, we provide a comprehensive listof all targets and rank them according to their viability as a result ofthe observations taken to date. The observable that removes by far themost targets from the SIM young stellar object (YSO) program isphotometric variability.
| Rotation and Activity of Pre-Main-Sequence Stars
We present a study of rotation (vsini) and chromospheric activity(Hα equivalent width) based on an extensive set of high-resolutionoptical spectra obtained with the MIKE instrument on the 6.5 m MagellanClay telescope. Our targets are 74 F-M dwarfs in four young stellarassociations, spanning ages from 6 to 30 Myr. By comparing Hα EWsin our sample to results in the literature, we see a clear evolutionarysequence: Chromospheric activity declines steadily from the T Tauriphase to the main sequence. Using activity as an age indicator, we finda plausible age range for the Tuc-Hor association of 10-40 Myr. Between5 and 30 Myr, we do not see evidence for rotational braking in the totalsample, and thus angular momentum is conserved, in contrast to youngerstars. This difference indicates a change in the rotational regulationat ~5-10 Myr, possibly because disk braking cannot operate longer thantypical disk lifetimes, allowing the objects to spin up. Therotation-activity relation is flat in our sample; in contrast tomain-sequence stars, there is no linear correlation for slow rotators.We argue that this is because young stars generate their magnetic fieldsin a fundamentally different way from main-sequence stars, and not justthe result of a saturated solar-type dynamo. By comparing our rotationalvelocities with published rotation periods for a subset of stars, wedetermine ages of 13+7-6 and9+8-2 Myr for the η Cha and TWA associations,respectively, consistent with previous estimates. Thus we conclude thatstellar radii from evolutionary models by Baraffe et al. (1998) are inagreement with the observed radii to within +/-15%.
| 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.
| Accretion Disks around Young Stars: Lifetimes, Disk Locking, and Variability
We report the findings of a comprehensive study of disk accretion andrelated phenomena in four of the nearest young stellar associationsspanning 6-30 million years in age, an epoch that may coincide with thelate stages of planet formation. We have obtained ~650 multiepochhigh-resolution optical spectra of 100 low-mass stars that are likelymembers of the η Chamaeleontis (~6 Myr), TW Hydrae (~8 Myr), βPictoris (~12 Myr), and Tucanae-Horologium (~30 Myr) groups. Our datawere collected over 12 nights between 2004 December and 2005 July on theMagellan Clay 6.5 m telescope. Based on Hα line profiles, alongwith a variety of other emission lines, we find clear evidence ofongoing accretion in 3 out of 11 η Cha stars and 2 out of 32 TWHydrae members. None of the 57 β Pic or Tuc-Hor members showsmeasurable signs of accretion. Together, these results imply significantevolution of the disk-accretion process within the first several Myr ofa low-mass star's life. While a few disks can continue to accrete for upto ~10 Myr, our findings suggest that disks accreting for beyond thattimescale are rather rare. This result provides an indirect constrainton the timescale for gas dissipation in inner disks and, in turn, ongas-giant planet formation. All accretors in our sample are slowrotators, whereas nonaccretors cover a large range in rotationalvelocities. This may hint at rotational braking by disks at ages up to~8 Myr. Our multiepoch spectra confirm that emission-line variability iscommon even in somewhat older T Tauri stars, among which accretors tendto show particularly strong variations. Thus, our results indicate thataccretion and wind activity undergo significant and sustained variationsthroughout the lifetime of accretion disks.
| On the Rotation of Post-T Tauri Stars in Associations
Nearby associations or moving groups of post-T Tauri stars with agesbetween ~10 and 30 Myr are excellent objects for the study of theinitial spin-up phase during the pre-main-sequence evolution. Anempirical approach is adopted here for the first time with these starsto infer their rotations, properties, and relations to X-ray emission.Three nearby associations with distances less than 100 pc areconsidered: the TW Hydrae association (TWA) with an age of 8 Myr, theβ Pictoris moving group (BPMG) with an age of 12 Myr, and acombination of Tucana and Horologium associations (Tuc/HorA; 30 Myr).Two low- and high-rotation modes are considered for each association,with stellar masses of0.1Msolar<=M<1.5Msolar and1.5Msolar<=M<=2.6Msolar, respectively.Because no observed rotational periods are known for these stars, we usea mathematical tool to infer representative equatorial rotationvelocities v0(eq) from the observed distribution of projectedrotational velocities (vsini). This is done for each mode and for eachassociation. A spin-up is found for the high-rotation mode, whereas inthe low-rotation mode the v0(eq) do not increasesignificantly. This insufficient increase of v0(eq) isprobably the cause of a decrease of the total mean specific angularmomentum for the low-mass stars between 8 and 30 Myr. However, for thehigh-mass stars, where a sufficient spin-up is present, the specificangular momentum is practically conserved in this same time interval. Atwo-dimensional (mass and vsini) K-S statistical test yields resultscompatible with a spin-up scenario. By supposing that the distributionof the masses of these three associations follows a universal massfunction, we estimate the number of members of these associations thatremain to be detected. The analysis of rotational and stellar massesusing the luminosity X-ray indicators LX andLX/Lb present similar properties, as does thedependence on stellar mass and rotation, at least for the youngerassociations TWA and BPMG, to those obtained for T Tauri stars in theOrion Nebula Cluster (1 Myr). A strong desaturation effect appears at~30 Myr, the age of Tuc/HorA, measured essentially by the early-G andlate-F type stars. This effect seems to be provoked by the minimumconfiguration of the stellar convection layers, attained for the firsttime for the higher mass stars at ~30 Myr. The desaturation appears tobe independent of rotation at this stage.
| 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.
| An infrared imaging search for low-mass companions to members of the young nearby β Pic and Tucana/Horologium associations
We present deep high dynamic range infrared images of young nearby starsin the Tucana/Horologium and β Pic associations, all 10 to 35Myrs young and at 10 to 60 pc distance. Such young nearby starsare well-suited for direct imaging searches for brown dwarf and evenplanetary companions, because young sub-stellar objects are stillself-luminous due to contraction and accretion. We performed ourobservations at the ESO 3.5m NTT with the normal infrared imagingdetector SofI and the MPE speckle camera Sharp-I. Three arc sec north ofGSC 8047-0232 in Horologium a promising brown dwarf companion candidateis detected, which needs to be confirmed by proper motion and/orspectroscopy. Several other faint companion candidates are alreadyrejected by second epoch imaging. Among 21 stars observed inTucana/Horologium, there are not more than one to five brown dwarfcompanions outside of 75 AU (1.5'' at 50 pc); most certainly only <=5% of the Tuc/HorA stars have brown dwarf companions (13 to 78 Jupitermasses) outside of 75 AU. For the first time, we can report an upperlimit for the frequency of massive planets ( 10 Mjup) atwide separations ( 100 AU) using a meaningfull and homogeneoussample: Of 11 stars observed sufficiently deep in β Pic (12 Myrs),not more than one has a massive planet outside of 100 AU, i.e.massive planets at large separations are rare (<= 9%).Based on observations obtained on La Silla, Chile, in ESO programs65.L-0144(B), 66.D-0135, 66.C-0310(A), 67.C-0209(B), 67.C-0213(A),68.C-0008(A), and 68.C-0009(A)} }
| Late-type members of young stellar kinematic groups - I. Single stars
This is the first paper of a series aimed at studying the properties oflate-type members of young stellar kinematic groups. We concentrate ourstudy on classical young moving groups such as the Local Association(Pleiades moving group, 20-150Myr), IC 2391 supercluster (35Myr), UrsaMajor group (Sirius supercluster, 300Myr), and Hyades supercluster(600Myr), as well as on recently identified groups such as the Castormoving group (200Myr). In this paper we compile a preliminary list ofsingle late-type possible members of some of these young stellarkinematic groups. Stars are selected from previously established membersof stellar kinematic groups based on photometric and kinematicproperties as well as from candidates based on other criteria such astheir level of chromospheric activity, rotation rate and lithiumabundance. Precise measurements of proper motions and parallaxes takenfrom the Hipparcos Catalogue, as well as from the Tycho-2 Catalogue, andpublished radial velocity measurements are used to calculate theGalactic space motions (U, V, W) and to apply Eggen's kinematic criteriain order to determine the membership of the selected stars to thedifferent groups. Additional criteria using age-dating methods forlate-type stars will be applied in forthcoming papers of this series. Afurther study of the list of stars compiled here could lead to a betterunderstanding of the chromospheric activity and their age evolution, aswell as of the star formation history in the solar neighbourhood. Inaddition, these stars are also potential search targets for directimaging detection of substellar companions.
| Tucana Association
Among star clusters, only the sparse Ursa Major nucleus is closer toEarth than the recently identified Tucana association. Based on newphotometric VRI magnitudes, we construct a color-magnitude diagramcomposed of likely and possible Tucana members. The implied age of theTucana association stars, <~40 Myr, is consistent with the agedetermined from a previous analysis by Stelzer & Neuhäuser ofthe X-ray luminosities of plausible association members. Based primarilyon space motions and X-ray fluxes, we identify potential new members ofthe Tucana association not considered in earlier studies. Torres andcoworkers recently identified a group of post-T Tauri stars, theHorologium association, which is located near the Tucana association inright ascension and declination. Because the Horologium stars have thesame space motions, age, distance from Earth, volume density, and rangeof spectral types as Tucana stars, we suggest that, rather than beingcharacterized as a separate group, it would be appropriate andeconomical to subsume the Horologium association stars into the Tucanastream.
| Absolute proper motions of open clusters. I. Observational data
Mean proper motions and parallaxes of 205 open clusters were determinedfrom their member stars found in the Hipparcos Catalogue. 360 clusterswere searched for possible members, excluding nearby clusters withdistances D < 200 pc. Members were selected using ground basedinformation (photometry, radial velocity, proper motion, distance fromthe cluster centre) and information provided by Hipparcos (propermotion, parallax). Altogether 630 certain and 100 possible members werefound. A comparison of the Hipparcos parallaxes with photometricdistances of open clusters shows good agreement. The Hipparcos dataconfirm or reject the membership of several Cepheids in the studiedclusters. Tables 1 and 2 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| X-ray emission from young stars in the Tucanae association
We report on X-ray emission from members of the recently discoveredTucanae association, a group of stars with youth signatures and similarspace motion. The Tucanae association is the nearest known region ofrecent star formation ( ~ 45 pc) far from molecular clouds(\cite{Zuckerman00.1}). We have made use of the ROSAT Data Archive andsearched for X-rays from Tucanae stars in both ROSAT All-Sky Survey(RASS) and pointed observations. While the RASS provides completecoverage of the sky, only three potential Tucanae members have beenobserved during PSPC pointings. All three stars have been detected. Forthe RASS the percentage of detections is 59%. The comparison of theX-ray luminosity function of Tucanae to that of other star formingregions may provide clues to the uncertain age of the association. Wefind that the distribution of X-ray luminosities is very similar to theones derived for the TW Hya association, the Taurus-Auriga T TauriStars, and the IC 2602 cluster, but significantly brighter than theluminosity distribution of the Pleiades. We conclude that the stars inTucanae are most likely young, on the order of 10-30 Myr. Strongvariability of most stars emerges from the X-ray lightcurves whereseveral flares and irregular variations are observed.
| Identification of a Nearby Stellar Association in theHipparcos Catalog: Implications for Recent, Local Star Formation
The TW Hydrae Association (~55 pc from Earth) is the nearest knownregion of recent star formation. Based primarily on the Hipparcoscatalog, we have identified a group of nine or 10 comoving star systemsat a common distance (~45 pc) from Earth that appear to compriseanother, somewhat older association (``the Tucanae Association'').Together with ages and motions recently determined for some nearby fieldstars, the existence of the Tucanae and TW Hydrae Associations suggeststhat the Sun is now close to a region that was the site of substantialstar formation only 10-50 Myr ago. The TW Hydrae Association representsa final chapter in the local star formation history.
| The 74th Special Name-list of Variable Stars
We present the Name-list introducing GCVS names for 3153 variable starsdiscovered by the Hipparcos mission.
| Photometry of Southern Hemisphere red dwarf stars
Results are presented for a photometric investigation of aspectroscopically selected sample of red dwarf stars in the SouthernHemisphere. Absolute magnitudes and distances for the stars areestimated from broadband red colors. Three stars which may besubluminous are identified, as are several stars which may be within 25pc. The tangential velocity and velocity dispersion of the sample aresimilar to values found in other studies of nearby late-type stars.
|
Yeni bir Makale Öner
Ýlgili Baðlantýlar
Yeni Bir Baðlantý Öner
sonraki gruplarýn üyesi:
|
Gözlemler ve gökölçümü verileri
| Takýmyýldýz: | Tukan |
| Sað Açýklýk: | 00h25m14.56s |
| Yükselim: | -61°30'47.8" |
| Görünürdeki Parlaklýk: | 11.376 |
| Uzaklýk: | 37.467 parsek |
| özdevim Sað Açýklýk: | 0 |
| özdevim Yükselim: | 0 |
| B-T magnitude: | 13.704 |
| V-T magnitude: | 11.569 |
Kataloglar ve belirtme:
|
INHABITED SKY
