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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
| 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.
| 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.
| 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.
| 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%.
| 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.
| Deficit of Wide Binaries in the η Chamaeleontis Young Cluster
We have carried out a sensitive high-resolution imaging survey of starsin the young (6-8 Myr), nearby (97 pc) compact cluster around ηChamaeleontis to search for stellar and substellar companions. Our datawere obtained using the NACO adaptive optics system on the ESO VeryLarge Telescope (VLT). Given its youth and proximity, any substellarcompanions are expected to be luminous, especially in the near-infrared,and thus easier to detect next to their parent stars. Here, we presentVLT NACO adaptive optics imaging with companion detection limits for 17η Cha cluster members, and follow-up VLT ISAAC near-infraredspectroscopy for companion candidates. The widest binary detected is~0.2", corresponding to the projected separation 20 AU, despite oursurvey being sensitive down to substellar companions outside 0.3", andplanetary-mass objects outside 0.5". This implies that the stellarcompanion probability outside 0.3" and the brown dwarf companionprobability outside 0.5" are less than 0.16 with 95% confidence. Wecompare the wide binary frequency of η Cha to that of the similarlyaged TW Hydrae association and estimate the statistical likelihood thatthe wide binary probability is equal in both groups to be less than2×10-4. Even though the η Cha cluster is relativelydense, stellar encounters in its present configuration cannot accountfor the relative deficit of wide binaries. We thus conclude that thedifference in wide binary probability in these two groups providesstrong evidence for multiplicity properties being dependent onenvironment. In two appendices we derive the projected separationprobability distribution for binaries, used to constrain physicalseparations from observed projected separations, and summarizestatistical tools useful for multiplicity studies.
| 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.
| On the age of the TW Hydrae association and 2M1207334-393254
Aims.We have estimated the age of the young moving group TW HydraeAssociation, a cohort of a few dozen stars and brown dwarfs located nearthe Sun which share the same kinematic properties and, presumably, thesame origin and age. Methods: .The chronology has been determinedby analyzing different properties (magnitudes, colors, activity,lithium) of its members and comparing them with several well-known starforming regions and open clusters, as well as theoretical models. Inaddition, by using medium-resolution optical spectra of two M8 membersof the association (2M1139 and 2M1207 - an accreting brown dwarf with aplanetary mass companion), we have derived spectral types and measuredHα and lithium equivalent widths. We have also estimated theireffective temperature and gravity, which were used to produce anindependent age estimation for these two brown dwarfs. We have alsocollected spectra of 2M1315, a candidate member with a L5 spectral typeand measured its Hα equivalent width. Results: .Our ageestimate for the association, 10+10-7 Myr, agreeswith previous values cited in the literature. In the case of the twobrown dwarfs, we have derived an age of 15+15-10Myr, which also agree with our estimate for the whole group.Conclusions: .We compared our results with recent articles published onthe same subject using other techniques, and discuss the limits of theage-dating techniques.
| Dynamical Evolution of the TW Hydrae Association
Using Galactic dynamics we have determined the age of the low-masspost-T Tauri stars in the TW Hya Association (TWA). To do so we appliedthe method of Ortega and coworkers to five stars of the association withHipparcos-measured distances (TWA 1, TWA 4, TWA 9, TWA 11, and TWA 19).The method is based on the calculation of the past three-dimensionalorbits of the stars. Of these stars, only TWA 9 presents a quitedifferent orbit so that it does not appear to be a dynamical member ofthe TWA. The four remaining stars have their first maximum orbitalconfinement at the age of -8.3+/-0.8 Myr, which is considered thedynamical age of the TWA. This confinement fixes the probablethree-dimensional forming region of the TWA within a mean radius of 14.5pc. This region is related to the older subgroups of the Sco-Cen OBassociation, Lower Centaurus Crux and Upper Centaurus Lupus, both with amean age of about 18 Myr. This dynamical age of the TWA and that of theβ Pic Moving Group, 11 Myr, also discussed here, introduce a moreprecise temporal scale for studies of disk evolution and planetaryformation around some stars of these associations. Using the retracedorbit of the runaway star HIP 82868 we examine the possibility that theformation of the TWA was triggered by a supernova explosion. It is shownthat for the four considered TWA stars, the expansion in volume is afactor of 5 from their origin to the present state. This is mainly dueto the currently more distant star TWA 19.
| A Moving Cluster Distance to the Exoplanet 2M1207b in the TW Hydrae Association
A candidate extrasolar planet companion to the young brown dwarf 2MASSWJ1207334-393254 (hereafter 2M1207) was recently discovered by Chauvin etal. They find that the temperature and luminosity of 2M1207b areconsistent with a young, ~5MJ planet. The 2M1207 system ispurported to be a member of the TW Hya association (TWA) and situated~70 pc away. Using a revised space motion vector for TWA and improvedproper motion for 2M1207, I use the moving cluster method to estimatethe distance to the 2M1207 system and other TWA members. The deriveddistance for 2M1207 (53+/-6 pc) forces the brown dwarf and planet to behalf as luminous as previously thought. The inferred masses for 2M1207Aand 2M1207b decrease to ~21 and ~3-4MJ, respectively, withthe mass of 2M1207b well below the observed tip of the planetary massfunction and the theoretical deuterium-burning limit. After removingprobable Lower Centaurus Crux (LCC) members from the TWA sample, as wellas the probable nonmember TWA 22, the remaining TWA membership is foundto have distances of 49+/-3 (s.e.m.)+/-12 (1 σ) pc and an internalone-dimensional velocity dispersion of 0.8+0.3-0.2km s-1. There is weak evidence that the TWA is expanding, andthe data are consistent with a lower limit on the expansion age of 10Myr (95% confidence).
| Exploring Terrestrial Planet Formation in the TW Hydrae Association
Spitzer Space Telescope infrared measurements are presented for 24members of the TW Hya association (TWA). High signal-to-noise ratio 24μm photometry is presented for all these stars, including 20 starsthat were not detected by IRAS. Among these 20 stars, only a singleobject, TWA 7, shows excess emission at 24 μm at the level of only40% above the star's photosphere. TWA 7 also exhibits a strong 70 μmexcess that is a factor of 40 brighter than the stellar photosphere atthis wavelength. At 70 μm, an excess of similar magnitude is detectedfor TWA 13, although no 24 μm excess was detected for this binary.For the 18 stars that failed to show measurable IR excesses, thesensitivity of the current 70 μm observations does not rule outsubstantial cool excesses at levels 10-40 times above their stellarcontinua. Measurements of two T Tauri stars, TW Hya and Hen 6-300,confirm that their spectacular IR spectral energy distributions (SEDs)do not turn over even by 160 μm, consistent with the expectation fortheir active accretion disks. In contrast, the Spitzer data for theluminous planetary debris systems in the TWA, HD 98800B and HR 4796A,are consistent with single-temperature blackbody SEDs and agree withprevious IR, submillimeter, and millimeter measurements. The major newresult of this study is the dramatic bimodal distribution found for theassociation in the form of excess emission at a wavelength of 24 μm,indicating negligible amounts of warm (>~100 K) dust and debrisaround 20 of 24 stars in this group of very young stars. This bimodaldistribution is especially striking given that the four stars in theassociation with strong IR excesses are >~100 times brighter at 24μm than their photospheres. Clearly, two terrestrial planetarysystems, HD 98800B and HR 4796A, exist in some form. In addition, thereare at least two active accreting objects, TW Hya and Hen 6-300, thatmay still be forming planetesimals. The remaining stars may possesssignificant amounts of cold dust, as in TWA 7 and 13, that have yet tobe found.
| Expansion of the TW Hydrae association and the encounter with Vega
We investigate the paths of several probable members of the youngassociation around the star TW Hydrae (TWA) with accurate distances,proper motions and radial velocities. We find that three of thepreviously identified members, TWA 1, TWA 4 and TWA 11, together withtwo other young nearby stars, HD 139084 and HD 220476, form a rapidlyexpanding association with an expansion age of 4.7 +/- 0.6 Myr. Initialvelocities of member stars with respect to the common centre of massrange from 4 to 10km s-1. A characteristic size of theassociation in the initial configuration is 21 pc, which may be somewhatbiased upwards due to the uncertainties in the observational data. TheLower Centaurus Crux (LCC) OB association passed near TWA, at a distanceof 36 +/- 6 pc, 11 Myr ago. A plausible scenario, which accounts for thedifference between the isochrone age (~=10 Myr) and expansion age (5Myr), is that star formation was stimulated in the TWA progenitor cloudby the near passage of the LCC, but that the newly formed stars were notreleased from the cloud until a subsequent collision with one of theother molecular clouds in the North Ophiuchus region. Vega was insidethe TWA association, and close to its centre of gravity, at the time ofmaximum compression 4.7 Myr ago. If this alignment is a chanceencounter, the powerful particular disc around Vega could have beenenhanced by the passage through the TWA progenitor cloud at 8 kms-1.
| Rotation periods for stars of the TW Hydrae association: the evidence for two spatially and rotationally distinct pre-main-sequence populations
We have conducted a photometric study of late-type members of the TWHydrae association (TWA) and measured the rotation periods for 16 starsin 12 systems. For TWA stars listed by Webb et al. and Sterzik et al.(TWA 1-13 led by TW Hya = TWA 1) we find a median period of 4.7 d.However, for stars that we measured in the TWA 14-19 group identified byZuckerman et al., we find a median period of only 0.7 d. The perioddistributions of the two groups cannot be reconciled at the 3σsignificance level. Using photometric arguments supported by theHipparcos distance to HD 102458 (= TWA 19A), we find that TWA 14-19reside at an average distance of d~ 90 pc, spatially at the nearboundary of the Lower Centaurus-Crux (LCC) subgroup of theOphiuchus-Scorpius-Centaurus OB-star association. Proper motions for HD102458, TWA 14, 18 and 19B link these stars to the LCC subgroup. FromHertzsprung-Russell diagram placement, we derive an age of ~17 Myr forthe HD 102458 system that may be the representative age for the TWA14-19 group. Merging various lines of evidence, we conclude that thesestars form a spatially and rotationally distinct population of olderpre-main-sequence (PMS) stars, rather than being an extension of the TWAbeyond those stars associated with TW Hya that have an age of ~10 Myrand reside at d~ 55 pc. Instead, TWA 14-19 likely represent thepopulation of low-mass stars still physically associated with the LCCsubgroup.
| SSSPM J1102-3431: A probable new young brown dwarf member of the TW Hydrae Association
We have used archival survey data, most importantly from the SuperCOSMOSSky Surveys, to derive accurate proper motions and thus confirm themembership of two previously announced isolated brown dwarfs in thenearby, ˜10 Myr old TW Hydrae Association (TWA), 2MASSWJ1207334-393254 and 2MASSW 1139511-315921 (Gizis \cite{gizis02}), and todiscover a new substellar candidate. This new object, SSSPM J1102-3431,has extremely red optical, optical-to-infrared, and near-infraredcolours (R-I=3.2, I-J=3.4, J-Ks=1.15), while low-resolutionnot only classifies it as a late-type (M8.5) object, but also showsspectral signatures of low gravity as expected for a young brown dwarf.If it is a true TWA member, brown dwarf cooling models predict its massto be ˜ 20 MJup. SSSPM J1102-3431 lies only ˜12arcmin away from the T Tauri star TW Hya and their proper motions agreeto within the errors, and thus it is possible that the two objects forman extremely wide binary system, with a separation of ˜40 000 AUat d=56 pc. We have also obtained improved proper motions for SSS120727.4-324700 and SSS 101726.7-535428, recent additions to the list ofTWA members (Song et al. \cite{song03}). The relatively large propermotion of the latter object is consistent with the suggested proximity(d˜22 pc) of this lowest-mass stellar representative of the TWA.Based on archival data from the SuperCOSMOS Sky Surveys, 2MASS, andDENIS, and on spectroscopic observations with the ESO 3.6-m telescope(ESO 072.C-0630).
| 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.
| A Search for Warm Circumstellar Disks in the TW Hydrae Association
A search for previously undetected optically thin disks around stars inthe nearby, young, TW Hydrae Association (TWA) was conducted around 16stars with sensitive 12 and 18 ?m photometry. The survey could detectzodiacal-like dust, with temperature 200-300 K, at levels ofLIR/L*=7×10-3. Possiblemid-infrared excess emission from TWA 17 was detected at the 2 ?level, but none of the other stars showed evidence for circumstellardust. The rapid disappearance of large amounts of dust around the K- andM-type stars in this sample may mean that any planet formation in theterrestrial planet region was completed very quickly. There appears tobe a bimodal distribution of dust disks in TWA, with stars having eithercopious or negligible warm dust.
| New Members of the TW Hydrae Association, β Pictoris Moving Group, and Tucana/Horologium Association
We have identified five new members of the TW Hydrae association (TWA),11 new members of the β Pic moving group, and 11 newTucana/Horologium association members. These are the three youngest(<~30 Myr) known kinematic stellar groups near the Earth. Newlyidentified β Pic group members are located mostly in the northernhemisphere, and they have a slightly different U-component of Galacticvelocity compared to that of previously known members. Tracing themotion of β Pic members backward in time for 12 Myr indicates thatthey might have formed in a small region with an initial velocitydispersion of ~8 km s-1. A couple of mid-M spectral typeβ Pic members show emission features [He Iλ5876+λ6678) and Na D λ5890+λ5896)] seenamong earlier spectral type stars in the TWA and β Pic groups. Toderive the distances of the non-Hipparcos members of these groups, wehave constructed a V-K versus MK color-magnitude diagram thatis very useful in separating young K/M stars from older main-sequencecounterparts and constraining theoretical pre-main-sequence evolutionarytracks. All newly identified K- and M-type members of the three groupsshow saturated X-ray activity(LX/Lbol~10-3). One newly identifiedTWA member, SSS 101727-5354, is estimated to be only 22 pc away fromEarth. Its extreme youth, late spectral type (~M5), and proximity toEarth make SSS 101727-5354 perhaps the best target for direct imagingdetection of cooling planets.
| Proper motion and X-ray selected search for new members of the young TW Hya association
We have searched for new members of the TW Hya association (TWA) amongunidentified ROSAT X-ray sources by identifying them in proper motioncatalogues and selecting those that would be consistent with kinematicalmembership to the TWA. Spectroscopic follow-up observations of 19 membercandidates revealed the detection of moderate lithium absorption linesfor the following three stars: GSC 7206 845, TYC 7216-55, and TYC7247-12. The isochronal ages of the latter TYC stars are estimated to be 20 Myr while the other one has 100 Myr age based on akinematic distance estimate that assumes TWA membership. However, themoderately Li-rich stars are not likely to be new pre-main sequencemembers of TWA partly because of the discrepant radial velocities.Infrared follow-up imaging in the H-band for the 3 stars shows companioncandidates near two of them. While one system (TYC 7216-55) is probablya near-equal-magnitude stellar binary, our follow-up H-band spectrum ofthe faint companion candidate near GSC 7206 845 shows that it is insteada background K-type star rather than a companion.Based on observations obtained in ESO programs 66.C-0540, 66.C-0138B,and 68.C-0009 on La Silla, Chile, as well as with ROSAT and theUniversity of Hawaii Institute for Astronomy 2.2m telescope on MaunaKea.
| Flare stars in the TW Hydrae association: the HIP 57269 syste
We discuss a new member candidate of the TW Hydrae association (TWA)among the stars of the Gershberg et al. ( te{gershberg}) flare starcatalog. TWA is one of the closest known associations of young stars atabout 60 pc. Three supposedly young flare stars are located in the sameregion of the sky as TWA. One of them (HIP 57269)shows strong lithium absorption with spectral type K1/K2V and a highlevel of chromospheric and coronal activity. It is located at a distanceof 48.7±6.3 pc in common with the five TWA members observed withHipparcos (46.7 to 103.9 pc). HIP 57268 A has a wide companion C whichalso shows lithium absorption at 6707 Å and which has commonproper motion with HIP 57269, as well as a closecompanion resolved visually by Tycho. HIP 57269 A&C lie above themain sequence and are clearly pre-main-sequence stars. The UVW-spacevelocity is more consistent with the star system being a Pleiades supercluster member. The two other flare stars in the TWA sky region do notshow lithium at all and are, hence, unrelated.Data obtained at ESO La Silla and ESO Paranal with programs:66.C-0138(B), 67.C-0073(A), 68.C-0009(A), 68.C-0018(A)
| Activity and kinematics of members of the TW Hydrae association
We present high-resolution echelle spectroscopy of 20 stars in 16systems catalogued as members of the TW Hydrae association, and 16 starsidentified as possible new members. We have calibrated the range ofcoronal and chromospheric activity expected for such young stars as afunction of spectral type by combining our observations with literaturedata for field and open cluster stars. We also compute space motions forTWA members and candidate members with proper motion measurements, usingtwo techniques to estimate distances to stars lacking directtrigonometric parallax measurements. The mean space motion of the fourTWA members with known parallaxes is (U, V, W: -10.0, -17.8, -4.6) kms-1. 14 of the candidates have properties inconsistent withcluster membership; the remaining two are potential new members,although further observations are required to confirm this possibility.
| Formation scenarios for the young stellar associations between galactic longitudes l = 280degr - 360degr
We investigate the spatial distribution, the space velocities and agedistribution of the pre-main sequence (PMS) stars belonging toOphiuchus, Lupus and Chamaeleon star-forming regions (SFRs), and of theyoung early-type star members of the Scorpius-Centaurus OB association.These young stellar associations extend over the galactic longituderange from 280degr to 360degr , and are at a distance interval ofaround 100 and 200 pc. This study is based on a compilation ofdistances, proper motions and radial velocities from the literature forthe kinematic properties, and of basic stellar data for the constructionof Hertzsprung-Russel diagrams. Although there was no well-known OBassociation in Chamaeleon, the distances and the proper motions of agroup of 21 B- and A-type stars, taken from the Hipparcos Catalogue,lead us to propose that they form a young association. We show that theyoung early-type stars of the OB associations and the PMS stars of theSFRs follow a similar spatial distribution, i.e., there is no separationbetween the low and the high-mass young stars. We find no difference inthe kinematics nor in the ages of these two populations studied.Considering not only the stars selected by kinematic criteria but thewhole sample of young early-type stars, the scattering of their propermotions is similar to that of the PMS stars and all the young starsexhibit a common direction of motion. The space velocities of theHipparcos PMS stars of each SFR are compatible with the mean values ofthe OB associations. The PMS stars in each SFR span a wide range of ages(from 1 to 20 Myr). The ages of the OB subgroups are 8-10 Myr for UpperScorpius (US), and 16-20 Myr for Upper Centaurus Lupus (UCL) and forLower Centaurus Crux (LCC). Thus, our results do not confirm that UCL isolder than the LCC association. Based on these results and theuncertainties associated with the age determination, we cannot say thatthere is indeed a difference in the age of the two populations. Weanalyze the different scenarios for the triggering of large-scalestar-formation that have been proposed up to now, and argue that mostprobably we are observing a spiral arm that passes close to the Sun. Thealignment of young stars and molecular clouds and the average velocityof the stars in the opposite direction to the Galactic rotation agreewith the expected behavior of star formation in nearby spiral arms.Tables 1 to 4 are 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/404/913
| Radial Velocity Survey of Members and Candidate Members of the TW Hydrae Association
We report spectroscopic observations of stars belonging to the youngnearby group known as the TW Hydrae association, as well as of a numberof potential members of the association identified in kinematic andX-ray surveys. Multiple radial velocity measurements were obtained foreach object, several of which turn out to be multiple systems. Orbitalsolutions are presented for three double-lined binaries, onesingle-lined binary, and a double-lined triple system, all with shortperiods. Effective temperatures and projected rotational velocities arepresented for each visible object. None of the candidate members of theassociation in our sample are confirmed as a true member. The largefraction of close binaries among the candidate members has to do withtheir selection based on X-ray emission from ROSAT, which tends to favorthe inclusion of tidally locked systems that are active but notnecessarily young.Some of the observations reported here were obtained with the MultipleMirror Telescope, a joint facility of the Smithsonian Institution andthe University of Arizona.
| Post-T Tauri Stars in the Nearest OB Association
We present results of a spectroscopic survey of X-ray- andproper-motion-selected samples of late-type stars in the LowerCentaurus-Crux (LCC) and Upper Centaurus-Lupus (UCL) subgroups of thenearest OB association: Scorpius-Centaurus. The primary goals of thesurvey are to determine the star formation history of the OB subgroupsand to assess the frequency of accreting stars in a sample dominated by``post-T Tauri'' pre-main-sequence (PMS) stars. We investigate twosamples: (1) proper-motion candidates from the ACT Catalog and TychoReference Catalog (TRC) with X-ray counterparts in the ROSAT All-SkySurvey (RASS) Bright Source Catalog and (2) G- and K-type stars in theHipparcos catalog found to be candidate members by de Zeeuw et al. Weobtained optical spectra of 130 candidates with the Siding Spring 2.3 mdual-beam spectrograph. PMS stars were identified by (1) strong Liλ6707 absorption, (2) subgiant surface gravities, (3) propermotions consistent with Sco-Cen membership, and (4) H-R diagrampositions consistent with being PMS. We find 93% of the RASS-ACT/TRCstars to be probable PMS members, compared with 73% of the Hipparcoscandidates. We demonstrate that measuring the gravity-sensitive bandratio of Sr II λ4077 to Fe I λ4071 is a valuable means ofdiscriminating PMS and zero-age main-sequence (ZAMS) stars. Usingsecular parallaxes and Hipparcos, Tycho-2, and Two Micron All Sky Surveyphotometry, we construct an H-R diagram. Depending on the choice ofpublished evolutionary tracks, we find the mean ages of the PMSpopulations to range between 17 and 23 Myr for LCC and 15 and 22 Myr forUCL. Taking into account observational errors, it appears that 95% ofthe low-mass star formation in each subgroup must have occurred in lessthan 8 Myr (LCC) and 12 Myr (UCL). Using the Bertelli et al. tracks, wefind main-sequence turnoff ages for Hipparcos B-type members to be16+/-1 Myr for LCC and 17+/-1 Myr for UCL. Contrary to previousfindings, it appears that LCC is coeval with, or slightly older than,UCL. The secular parallaxes of the Sco-Cen PMS stars yield distances of85-215 pc, with 12 of the LCC members lying within 100 pc of the Sun.Only one out of 110 (0.9+2.1-0.8%; 1 σ) PMSsolar-type stars in the sample with ages of 13+/-1 (s.e.)+/-6 (1σ) Myr and masses of 1.3+/-0.2 (1 σ) Msolar showsboth enhanced Hα emission and a K-band excess indicative ofaccretion from a truncated circumstellar disk: the nearby (d~=86 pc)classical T Tauri star PDS 66.
| Additional TWA members?. Spectroscopic verification of kinematically selected TWA candidates
We present spectroscopic measurements of the 23 new candidate members ofthe TW Hydrae Association from \citet{MF}. Based on Hα and Li 6708Å strengths together with location on a color-magnitude diagramfor Hipparcos TWA candidates, we found only three possible new members(TYC 7760-0835-1, TYC 8238-1462-1, and TYC 8234-2856-1) in addition tothe already known member, TWA 19. This eliminated most of the candidatesmore distant than 100 pc. Three Tycho stars, almost certainly members ofthe Lower Centaurus Crux association, are the most distant members ofthe TWA. A claim of isotropic expansion of TWA has to be re-evaluatedbased on our new results. Generally, one cannot identify new members ofa diffuse nearby stellar group based solely on kinematic data. Toeliminate interlopers with similar kinematics, spectroscopicverification is essential.
| Astrometric radial velocities. III. Hipparcos measurements of nearby star clusters and associations
Radial motions of stars in nearby moving clusters are determined fromaccurate proper motions and trigonometric parallaxes, without any use ofspectroscopy. Assuming that cluster members share the same velocityvector (apart from a random dispersion), we apply a maximum-likelihoodmethod on astrometric data from Hipparcos to compute radial and spacevelocities (and their dispersions) in the Ursa Major, Hyades, ComaBerenices, Pleiades, and Praesepe clusters, and for theScorpius-Centaurus, alpha Persei, and ``HIP 98321'' associations. Theradial motion of the Hyades cluster is determined to within 0.4 kms-1 (standard error), and that of its individual stars towithin 0.6 km s-1. For other clusters, Hipparcos data yieldastrometric radial velocities with typical accuracies of a few kms-1. A comparison of these astrometric values withspectroscopic radial velocities in the literature shows a good generalagreement and, in the case of the best-determined Hyades cluster, alsopermits searches for subtle astrophysical differences, such as evidencefor enhanced convective blueshifts of F-dwarf spectra, and decreasedgravitational redshifts in giants. Similar comparisons for the ScorpiusOB2 complex indicate some expansion of its associations, albeit slowerthan expected from their ages. As a by-product from the radial-velocitysolutions, kinematically improved parallaxes for individual stars areobtained, enabling Hertzsprung-Russell diagrams with unprecedentedaccuracy in luminosity. For the Hyades (parallax accuracy 0.3 mas), itsmain sequence resembles a thin line, possibly with wiggles in it.Although this main sequence has underpopulated regions at certaincolours (previously suggested to be ``Böhm-Vitense gaps''), suchare not visible for other clusters, and are probably spurious. Futurespace astrometry missions carry a great potential for absoluteradial-velocity determinations, insensitive to the complexities ofstellar spectra. Based on observations by the ESA Hipparcos satellite.Extended versions of Tables \ref{tab1} and \ref{tab2} are available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.125.8) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/381/446
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