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Bayesian Analysis to Identify New Star Candidates in Nearby Young Stellar Kinematic Groups
We present a new method based on a Bayesian analysis to identify newmembers of nearby young kinematic groups. The analysis minimally takesinto account the position, proper motion, magnitude, and color of astar, but other observables can be readily added (e.g., radial velocity,distance). We use this method to find new young low-mass stars in theβ Pictoris and AB Doradus moving groups and in the TW Hydrae,Tucana-Horologium, Columba, Carina, and Argus associations. Startingfrom a sample of 758 mid-K to mid-M (K5V-M5V) stars showing youthindicators such as Hα and X-ray emission, our analysis yields 214new highly probable low-mass members of the kinematic groups analyzed.One is in TW Hydrae, 37 in β Pictoris, 17 in Tucana-Horologium, 20in Columba, 6 in Carina, 50 in Argus, 32 in AB Doradus, and theremaining 51 candidates are likely young but have an ambiguousmembership to more than one association. The false alarm rate for newcandidates is estimated to be 5% for β Pictoris and TW Hydrae, 10%for Tucana-Horologium, Columba, Carina, and Argus, and 14% for ABDoradus. Our analysis confirms the membership of 58 stars proposed inthe literature. Firm membership confirmation of our new candidates willrequire measurement of their radial velocity (predicted by ouranalysis), parallax, and lithium 6708 Å equivalent width. We haveinitiated these follow-up observations for a number of candidates, andwe have identified two stars (2MASSJ01112542+1526214,2MASSJ05241914-1601153) as very strong candidate members of the βPictoris moving group and one strong candidate member(2MASSJ05332558-5117131) of the Tucana-Horologium association; thesethree stars have radial velocity measurements confirming theirmembership and lithium detections consistent with young age.Based on observations obtained at the Canada-France-Hawaii Telescope(CFHT) which is operated by the National Research Council of Canada, theInstitut National des Sciences de l'Univers of the Centre National de laRecherche Scientique of France, and the University of Hawaii.
| The Sizes of the Nearest Young Stars
We present moderate resolution (R ~ 3575) optical spectra of 19 known orsuspected members of the AB Doradus and ? Pictoris Moving Groups,obtained with the DeVeny Spectrograph on the 72 inch Perkins telescopeat Lowell Observatory. For four of five recently proposed members,signatures of youth such as Li I 6708 Å absorption and H?emission further strengthen the case for youth and membership. The lackof detected lithium in the proposed ? Pic member TYC 2211-1309-1implies that it is older than all other K-type members and weakens thecase for membership. Effective temperatures are determined via lineratio analyses for the 11 F, G, and early-K stars observed, and viaspectral comparisons for the eight late-K and M stars observed. Weassemble updated candidate membership lists for these moving groups thataccount for known binarity. Currently, the AB Dor Moving Group contains127 proposed members and the ? Pic Moving Group holds 77 proposedmembers. We then use temperature, luminosity, and distance estimates topredict angular diameters for these stars; the motivation is to identifystars that can be spatially resolved with long-baseline optical/infraredinterferometers in order to improve age estimates for these groups andto constrain evolutionary models at young ages. Considering the portionof the sky accessible to northern hemisphere facilities (decl. >- 30), six stars have diameters large enough to be spatiallyresolved (? > 0.4 mas) with the CHARA Array, which currentlyhas the world's longest baseline of 331 m; this subsample includes thelow-mass M2.5 member of AB Dor, GJ 393, which is likely to still bepre-main sequence. For southern hemisphere facilities (decl. < + 30),18 stars have diameters larger than this limiting size, including thelow-mass debris disk star AU Mic (0.72 mas). However, the longestbaselines of southern hemisphere interferometers (160 m) are only ableto resolve the largest of these, the B6 star ? Gru (1.17 mas)proposed long-baseline stations may alleviate the current limitations.
| The Na 8200 Å Doublet as an Age Indicator in Low-mass Stars
We investigate the use of the gravity sensitive neutral sodium (Na I)doublet at 8183 Å and 8195 Å (Na 8200 Å doublet) as anage indicator for M dwarfs. We measured the Na doublet equivalent width(EW) in giants, old dwarfs, young dwarfs, and candidate members of the? Pic moving group using medium-resolution spectra. Our Na 8200Å doublet EW analysis shows that the feature is useful as anapproximate age indicator in M-type dwarfs with (V - K s ) >= 5.0, reliably distinguishing stars older and younger than100 Myr. A simple derivation of the dependence of the Na EW ontemperature and gravity supports the observational results. An analysisof the effects of metallicity shows that this youth indicator is bestused on samples with similar metallicity. The age estimation techniquepresented here becomes useful in a mass regime where traditional youthindicators are increasingly less reliable, is applicable to other alkalilines, and will help identify new low-mass members in other youngclusters and associations.
| Precise Infrared Radial Velocities from Keck/NIRSPEC and the Search for Young Planets
We present a high-precision infrared radial velocity (RV) study oflate-type stars using spectra obtained with NIRSPEC at the W. M. KeckObservatory. RV precisions of 50 m s-1 are achieved for oldfield mid-M dwarfs using telluric features for wavelength calibration.Using this technique, 20 young stars in the β Pic (age ~ 12 Myr)and TW Hya (age ~ 8 Myr) Associations were monitored over several yearsto search for low-mass companions; we also included thechromospherically active field star GJ 873 (EV Lac) in this survey.Based on comparisons with previous optical observations of these youngactive stars, RV measurements at infrared wavelengths mitigate the RVnoise caused by star spots by a factor of ~3. Nevertheless, star spotnoise is still the dominant source of measurement error for young starsat 2.3 μm, and limits the precision to ~77 m s-1 for theslowest rotating stars (v sin i < 6 km s-1), increasing to~168 m s-1 for rapidly rotating stars (v sin i > 12 kms-1). The observations reveal both GJ 3305 and TWA 23 to besingle-lined spectroscopic binaries; in the case of GJ 3305, the motionis likely caused by its 0farcs09 companion, identified after this surveybegan. The large amplitude, short-timescale variations of TWA 13A areindicative of a hot Jupiter-like companion, but the available data areinsufficient to confirm this. We label it as a candidate RV variable.For the remainder of the sample, these observations exclude the presenceof any "hot" (P < 3 days) companions more massive than 8 MJup and any "warm" (P < 30 days) companions more massivethan 17 M Jup, on average. Assuming an edge-on orbit for theedge-on disk system AU Mic, these observations exclude the presence ofany hot Jupiters more massive than 1.8 M Jup or warm Jupitersmore massive than 3.9 M Jup.
| Binaries among Debris Disk Stars
We have gathered a sample of 112 main-sequence stars with known debrisdisks. We collected published information and performed adaptive opticsobservations at Lick Observatory to determine if these debris disks areassociated with binary or multiple stars. We discovered a previouslyunknown M-star companion to HD 1051 at a projected separation of 628 AU.We found that 25% ± 4% of our debris disk systems are binary ortriple star systems, substantially less than the expected ~50%. Theperiod distribution for these suggests a relative lack of systems with1-100 AU separations. Only a few systems have blackbody disk radiicomparable to the binary/triple separation. Together, these twocharacteristics suggest that binaries with intermediate separations of1-100 AU readily clear out their disks. We find that the fractional diskluminosity, as a proxy for disk mass, is generally lower for multiplesystems than for single stars at any given age. Hence, for a binary topossess a disk (or form planets) it must either be a very widelyseparated binary with disk particles orbiting a single star or it mustbe a small separation binary with a circumbinary disk.
| Mapping the Shores of the Brown Dwarf Desert. III. Young Moving Groups
We present the results of an aperture-masking interferometry survey forsubstellar companions around 67 members of the young (~8-200 Myr) nearby(~5-86 pc) AB Doradus, ? Pictoris, Hercules-Lyra, TW Hya, andTucana-Horologium stellar associations. Observations were made atnear-infrared wavelengths between 1.2 and 3.8 ?m using the adaptiveoptics facilities of the Keck II, Very Large Telescope UT4, and PalomarHale Telescopes. Typical contrast ratios of ~100-200 were achieved atangular separations between ~40 and 320 mas, with our survey being 100%complete for companions with masses below ~0.25 M &sun;across this range. We report the discovery of a 0.52 ± 0.09 M&sun; companion to HIP 14807, as well as the detections andorbits of previously known stellar companions to HD 16760, HD 113449,and HD 160934. We show that the companion to HD 16760 is in a face-onorbit, resulting in an upward revision of its mass from M2sin i ~ 14 M J to M 2 = 0.28 ±0.04 M &sun;. No substellar companions were detected aroundany of our sample members, despite our ability to detect companions withmasses below 80 M J for 50 of our targets: of these, oursensitivity extended down to 40 M J around 30 targets, with asubset of 22 subject to the still more stringent limit of 20 MJ. A statistical analysis of our non-detection of substellarcompanions allows us to place constraints on their frequency around~0.2-1.5 M &sun; stars. In particular, consideringcompanion mass distributions that have been proposed in the literature,we obtain an upper limit estimate of ~9%-11% for the frequency of 20-80M J companions between 3 and 30 AU at 95% confidence,assuming that their semimajor axes are distributed according to d {N}/da\propto a^{-1} in this range.
| Lithium abundances of nearby young solar-type stars based on optical high-resolution spectroscopy
Context. Lithium has long been recognized as a powerful tool forinvestigating the internal mixing of low-mass stars. In addition,accurate measurements of lithium abundances in young solar-type starsprovide independent and reliable age diagnostics. Aims: To studythe relationship between lithium and activity in normal solar type-typestars and determine the effectiveness of lithium and kinematics asactivity indicators, we perform a lithium survey of nearby late-typestars. We search for nearby young solar-type stars and investigate therelationship between lithium abundances and X-ray activity by measuringlog Lx. Methods: On the basis of high-resolutionspectroscopic observations, we derived the lithium abundances of 33young solar-type stars by comparing the measured Li I λ 670.8 nmequivalent widths with the curve of growth calculations in non-LTE(NLTE) conditions. Results: We obtain the lithium abundances of asample of young solar-type stars. Using the lithium abundance and X-rayluminosity, Lx, we investigate the relationship between Liabundance and X-ray activity. We find a clear correlation betweenlithium abundance (log N(Li)) and X-ray luminosity, log Lx,for our sample stars. Conclusions: As the X-ray luminosity,Lx, becomes stronger, the lithium abundance decreases in oursample (11 pre-main-sequence, 15 zero-age-main-sequence, and 7 youngmain-sequence) stars. The results imply that there is a large number ofsolar-type stars. A correlation appears to exist between Li abundanceand age, thus confirming the presence of very active young stars closeto the Sun.
| 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
| Random forest automated supervised classification of Hipparcos periodic variable stars
We present an evaluation of the performance of an automatedclassification of the Hipparcos periodic variable stars into 26 types.The sub-sample with the most reliable variability types available in theliterature is used to train supervised algorithms to characterize thetype dependencies on a number of attributes. The most useful attributesevaluated with the random forest methodology include, in decreasingorder of importance, the period, the amplitude, the V-I colour index,the absolute magnitude, the residual around the folded light-curvemodel, the magnitude distribution skewness and the amplitude of thesecond harmonic of the Fourier series model relative to that of thefundamental frequency. Random forests and a multi-stage scheme involvingBayesian network and Gaussian mixture methods lead to statisticallyequivalent results. In standard 10-fold cross-validation (CV)experiments, the rate of correct classification is between 90 and 100per cent, depending on the variability type. The main mis-classificationcases, up to a rate of about 10 per cent, arise due to confusion betweenSPB and ACV blue variables and between eclipsing binaries, ellipsoidalvariables and other variability types. Our training set and thepredicted types for the other Hipparcos periodic stars are availableonline.
| Bayesian inference of stellar parameters and interstellar extinction using parallaxes and multiband photometry
Astrometric surveys provide the opportunity to measure the absolutemagnitudes of large numbers of stars, but only if the individualline-of-sight extinctions are known. Unfortunately, extinction is highlydegenerate with stellar effective temperature when estimated frombroad-band optical/infrared photometry. To address this problem, Iintroduce a Bayesian method for estimating the intrinsic parameters of astar and its line-of-sight extinction. It uses both photometry andparallaxes in a self-consistent manner in order to provide anon-parametric posterior probability distribution over the parameters.The method makes explicit use of domain knowledge by employing theHertzsprung-Russell Diagram (HRD) to constrain solutions and to ensurethat they respect stellar physics. I first demonstrate this method byusing it to estimate effective temperature and extinction from BVJHKdata for a set of artificially reddened Hipparcos stars, for whichaccurate effective temperatures have been estimated from high-resolutionspectroscopy. Using just the four colours, we see the expected strongdegeneracy (positive correlation) between the temperature andextinction. Introducing the parallax, apparent magnitude and the HRDreduces this degeneracy and improves both the precision (reduces theerror bars) and the accuracy of the parameter estimates, the latter byabout 35 per cent. The resulting accuracy is about 200 K in temperatureand 0.2 mag in extinction. I then apply the method to estimate theseparameters and absolute magnitudes for some 47 000 F, G, K Hipparcosstars which have been cross-matched with Two-Micron All-Sky Survey(2MASS). The method can easily be extended to incorporate the estimationof other parameters, in particular metallicity and surface gravity,making it particularly suitable for the analysis of the 109stars from Gaia.
| 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.
| Detectability of Exoplanets in the ? Pic Moving Group with the Gemini Planet Imager
We model the detectability of exoplanets around stars in the ? PicMoving Group (BPMG) using the Gemini Planet Imager (GPI), acoronagraphic instrument designed to detect companions by imaging.Members of the BPMG are considered promising targets for exoplanetsearches because of their youth (~12 Myr) and proximity (median distance~35 pc). We wrote a modeling procedure to generate hypotheticalcompanions of given mass, age, eccentricity, and semi-major axis, andplace them around BPMG members that fall within the V-band range of theGPI. We count companions lying within the GPI's field of view and H-bandfluxes that have a host-companion flux ratio placing them within itssensitivity as possible detections. The fraction of companions thatcould be detected depends on their brightness at 12 Myr, and henceformation mechanism, and on their distribution of semi-major axes. Weused brightness models for formation by disk instability andcore-accretion. We considered the two extreme cases of the semi-majoraxis distribution—the log-normal distribution of the nearby F- andG-type stars and a power-law distribution indicated by the exoplanetsdetected by the radial velocity technique. We find that the GPI coulddetect exoplanets of all the F and G spectral type stars in the BPMGsample with a probability that depends on the brightness model andsemi-major axis distribution. At spectral type K-M1, exoplanetdetectability depends on brightness and hence distance of the host star.GPI will be able to detect the companions of M stars later than M1 onlyif they are closer than 10 pc. Of the four A stars in the BPMG sample,only one has a V-band brightness in the range of GPI; the others are toobright.
| Reaching the boundary between stellar kinematic groups and very wide binaries . II. ? Librae + KU Librae: a common proper motion system in Castor separated by 1.0 pc
Aims: I investigate the gravitational binding of a nearby commonproper motion system in the young Castor moving group (? ~ 200 Ma),which is formed by the bright quadruple star ? Lib (Zubenelgenubi)and the young solar analogue KU Lib. The system has an exceptionallywide angular separation of about 2.6 deg, which corresponds to aprojected physical separation of about 1.0 pc. Methods: Icompiled basic information on the system, compared its binding energywith those of other weakly bound systems in the field, and studied thephysical separations of resolved multiple systems in Castor. Results: KU Lib has roughly the same proper motion, parallacticdistance, radial velocity, and metallicity as the young hierarchicalquadruple system ? Lib. It also displays youth features. Theresemblance between these basic parameters and the relatively highestimated binding energy indicate that the five stars aregravitationally bound. KU Lib and ? Lib constitute the widestknown multiple system in all mass domains, and probably represent themost extreme example of young wide binaries on the point of beingdisrupted. Besides this, I make a comprehensive compilation of starcandidates in Castor, including new ones.
| Search for associations containing young stars (SACY). III. Ages and Li abundances
Context: Our study is a follow-up of the SACY project, an extended highspectral resolution survey of more than two thousand opticalcounterparts to X-ray sources in the southern hemisphere targeted tosearch for young nearby association. Nine associations have either beennewly identified, or have had their member list revised. Groupsbelonging to the Sco-Cen-Oph complex are not considered in the presentstudy. Aims: These nine associations, with ages of between about 6Myr and 70 Myr, form an excellent sample to study the Li depletion inthe pre-main sequence (PMS) evolution. In the present paper, weinvestigate the use of Li abundances as an independent clock toconstrain the PMS evolution. Methods: Using our measurements ofthe equivalent widths of the Li resonance line and assuming fixedmetallicities and microturbulence, we calculated the LTE Li abundancesfor 376 members of various young associations. In addition, weconsidered the effects of their projected stellar rotation.Results: We present the Li depletion as a function of age in the firsthundred million years for the first time for the most extended sample ofLi abundances in young stellar associations. Conclusions: A clearLi depletion can be measured in the temperature range from 5000 K to3500 K for the age span covered by the nine associations studied in thispaper. The age sequence based on the Li-clock agrees well with theisochronal ages, the ?Cha association being the only possibleexception. The lithium depletion patterns for the associations presentedhere resemble those of the young open clusters with similar ages,strengthening the notion that the members proposed for these loose youngassociations have indeed a common physical origin. The observed scatterin the Li abundances hampers the use of Li in determining reliable agesfor individual stars. For velocities above 20 km s-1,rotation seems to play an important role in inhibiting the Li depletion.Based on observations collected at the ESO - La Silla and at theLNA-OPD.Tables [see full textsee full text]-[see full textsee full text] areonly available in electronic form at http://www.aanda.org
| XID II: Statistical Cross-Association of ROSAT Bright Source Catalog X-ray Sources with 2MASS Point Source Catalog Near-Infrared Sources
The 18,806 ROSAT All Sky Survey Bright Source Catalog (RASS/BSC) X-raysources are quantitatively cross-associated with near-infrared (NIR)sources from the Two Micron All Sky Survey Point Source Catalog(2MASS/PSC). An association catalog is presented, listing the mostlikely counterpart for each RASS/BSC source, the probability Pid that the NIR source and X-ray source are uniquelyassociated, and the probability P no-id that none of the2MASS/PSC sources are associated with the X-ray source. The catalogincludes 3853 high quality (P id>0.98) X-ray-NIR matches,2280 medium quality (0.98 >= P id>0.9) matches, and4153 low quality (0.9 >= P id>0.5) matches. Of the highquality matches, 1418 are associations that are not listed in the SIMBADdatabase, and for which no high quality match with a USNO-A2 opticalsource was presented for the RASS/BSC source in previous work. Thepresent work offers a significant number of new associations withRASS/BSC objects that will require optical/NIR spectroscopy forclassification. For example, of the 6133 P id>0.92MASS/PSC counterparts presented in the association catalog, 2411 haveno classification listed in the SIMBAD database. These 2MASS/PSC sourceswill likely include scientifically useful examples of known sourceclasses of X-ray emitters (white dwarfs, coronally active stars, activegalactic nuclei), but may also contain previously unknown sourceclasses. It is determined that all coronally active stars in theRASS/BSC should have a counterpart in the 2MASS/PSC, and that the uniqueassociation of these RASS/BSC sources with their NIR counterparts thusis confusion limited.
| Kinematic analysis and membership status of TWA22 AB
Context: TWA22 was initially regarded as a member of the TW Hydraeassociation (TWA). In addition to being one of the youngest (≈8 Myr)and nearest (≈20 pc) stars to Earth, TWA22 has proven to be veryinteresting after being resolved as a tight, very low-mass binary. Thisbinary can serve as a very useful dynamical calibrator for pre-mainsequence evolutionary models. However, its membership in the TWA hasbeen recently questioned despite due to the lack of accurate kinematicmeasurements. Aims: Based on proper motion, radial velocity, andtrigonometric parallax measurements, we aim here to re-analyze themembership of TWA22 to young, nearby associations. Methods: Usingthe ESO NTT/SUSI2 telescope, we observed TWA22 AB during 5 differentobserving runs over 1.2 years to measure its trigonometric parallax andproper motion. This is a part of a larger project measuringtrigonometric parallaxes and proper motions of most known TWA members ata sub-milliarcsec level. HARPS at the ESO 3.6 m telescope was also usedto measure the system's radial velocity over 2 years. Results: Wereport an absolute trigonometric parallax of TWA22 AB, π =57.0±0.7 mas, corresponding to a distance 17.5±0.2 pc fromEarth. Measured proper motions of TWA 22AB areμαcos(δ) = -175.8±0.8 mas/yr andμδ = -21.3±0.8 mas/yr. Finally, from HARPSmeasurements, we obtain a radial velocity V_rad = 14.8±2.1 kms-1. Conclusions: A kinematic analysis of TWA22 ABspace motion and position implies that a membership of TWA22 AB to knownyoung, nearby associations can be excluded except for the βPictoris and TW Hydrae associations. Membership probabilities based onthe system's Galactic space motion and/or the trace-back techniquesupport a higher chance of being a member to the β Pictorisassociation. Membership of TWA22 in the TWA cannot be fully excludedbecause of large uncertainties in parallax measurements and radialvelocities and to the uncertain internal velocity dispersion of itsmembers.Based on observations performed at the European Southern Observatory,Chile (76.C-0543, 077.C-0112, 078.C-0158, 079.C-0229). Table 4 is onlyavailable in electronic form at http://www.aanda.org
| Identifying the Young Low-mass Stars within 25 pc. I. Spectroscopic Observations
We have completed a high-resolution (R ≈ 60,000) opticalspectroscopic survey of 185 nearby M dwarfs identified using ROSAT datato select active, young objects with fractional X-ray luminositiescomparable to or greater than Pleiades members. Our targets are drawnfrom the NStars 20 pc census and the Moving-M sample with distancesdetermined from parallaxes or spectrophotometric relations. We limitedour sample to 25 pc from the Sun, prior to correcting forpre-main-sequence overluminosity or binarity. Nearly half of theresulting M dwarfs are not present in the Gliese catalog and have nopreviously published spectral types. We identified 30 spectroscopicbinaries (SBs) from the sample, which have strong X-ray emission due totidal spin-up rather than youth. This is equivalent to a 16% SBfraction, with at most a handful of undiscovered SBs. We estimate upperlimits on the age of the remaining M dwarfs using spectroscopic youthindicators such as surface gravity-sensitive indices (CaH and K I). Wefind that for a sample of field stars with no metallicity measurements,a single CaH gravity index may not be sufficient, as highermetallicities mimic lower gravity. This is demonstrated in a subsampleof metal-rich radial velocity (RV) standards, which appear to have lowsurface gravity as measured by the CaH index, yet show no other evidenceof youth. We also use additional youth diagnostics such as lithiumabsorption and strong Hα emission to set more stringent agelimits. Eleven M dwarfs with no Hα emission or absorption arelikely old (>400 Myr) and were caught during an X-ray flare. Weestimate that our final sample of the 144 youngest and nearest low-massobjects in the field is less than 300 Myr old, with 30% of them beingyounger than 150 Myr and four very young (lap10 Myr), representing agenerally untapped and well-characterized resource of M dwarfs forintensive planet and disk searches.Based on observations collected at the W. M. Keck Observatory and theCanada-France-Hawaii Telescope. The Keck Observatory is operated as ascientific partnership between the California Institute of Technology,the University of California, and NASA, and was made possible by thegenerous financial support of the W. M. Keck Foundation. The CFHT isoperated by the National Research Council of Canada, the Centre Nationalde la Recherche Scientifique of France, and the University of Hawaii.
| A Formation Scenario of Young Stellar Groups in the Region of the Scorpio Centaurus OB Association
The main objective of this work is to investigate the role played byLower Centaurus Crux (LCC) and Upper Centaurus Lupus (UCL), bothsubcomponents of the Scorpio Centaurus OB association (Sco-Cen), in theformation of the groups β Pictoris, TW Hydrae, and the ηChamaeleontis cluster. The dynamical evolution of all the stellar groupsinvolved and of the bubbles and shells blown by LCC and UCL arecalculated, and followed from the past to the present. This leads to aformation scenario in which (1) the groups β Pictoris, TW Hydraewere formed in the wake of the shells created by LCC and UCL, (2) theyoung cluster η Chamaeleontis was born as a consequence of thecollision of the shells of LCC and UCL, and (3) the formation of UpperScorpius (US), the other main subcomponent of the Sco-Cen association,may have been started by the same process that created ηChamaeleontis.
| Nearby Young Stars Selected by Proper Motion. I. Four New Members of the β Pictoris Moving Group From The Tycho-2 Catalog
We describe a procedure to identify stars from nearby moving groups andassociations out of catalogs of stars with large proper motions. We showthat from the mean motion vector of a known or suspected moving group,one can identify additional members of the group based on proper motiondata and photometry in the optical and infrared, with minimalcontamination from background field stars. We demonstrate this techniqueby conducting a search for low-mass members of the β Pictorismoving group in the Tycho-2 catalog. All known members of the movinggroup are easily recovered, and a list of 51 possible candidates isgenerated. Moving group membership is evaluated for 33 candidates basedon X-ray flux from ROSAT, Hα line emission, and radial velocitymeasurement from high-resolution infrared spectra obtained at InfraredTelescope Facility. We confirm three of the candidates to be new membersof the group: TYC 1186-706-1, TYC 7443-1102-1, and TYC 2211-1309-1 whichare late-K and early-M dwarfs 45-60 pc from the Sun. We also identify acommon proper motion companion to the known β Pictoris Moving Groupmember TYC 7443-1102-1, at a 26farcs3 separation; the new companion isassociated with the X-ray source 1RXS J195602.8 – 320720. We arguethat the present technique could be applied to other large proper motioncatalogs to identify most of the elusive, low-mass members of knownnearby moving groups and associations.Based on data obtained in part with the 2.4 m Hiltner telescope of theMDM observatory. Based on data obtained in part with the CTIO 1.5 mtelescope, operated by SMARTS, the Small and Medium Aperture TelescopeSystem consortium, under contract with the Associated Universities forResearch in Astronomy (AURA).
| 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.
| Spitzer MIPS Observations of Stars in the β Pictoris Moving Group
We present Multiband Imaging Photometer for Spitzer (MIPS) observationsat 24 and 70 μm for 30 stars, and at 160 μm for a subset of 12stars, in the nearby (~30 pc), young (~12 Myr) β Pictoris movinggroup (BPMG). In several cases, the new MIPS measurements resolve sourceconfusion and background contamination issues in the IRAS data for thissample. We find that 7 members have 24 μm excesses, implying a debrisdisk fraction of 23%, and that at least 11 have 70 μm excesses (diskfraction of >=37%). Five disks are detected at 160 μm (out of abiased sample of 12 stars observed), with a range of 160/70 flux ratios.The disk fraction at 24 and 70 μm, and the size of the excessesmeasured at each wavelength, are both consistent with an ``inside-out''infrared excess decrease with time, wherein the shorter wavelengthexcesses disappear before longer wavelength excesses, and consistentwith the overall decrease of infrared excess frequency with stellar age,as seen in Spitzer studies of other young stellar groups. Assuming thatthe infrared excesses are entirely due to circumstellar disks, wecharacterize the disk properties using simple models and fractionalinfrared luminosities. Optically thick disks, seen in the younger TW Hyaand η Cha associations, are entirely absent in the BPMG. Additionalflux density measurements at 24 and 70 μm are reported for nineTucana-Horologium association member stars. Since this is <20% of theassociation membership, limited analysis on the complete disk fractionof this association is possible.
| 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.
| Near-Infrared Interferometric, Spectroscopic, and Photometric Monitoring of T Tauri Inner Disks
We present high angular resolution observations with the KeckInterferometer, high-dispersion spectroscopic observations withKeck/NIRSPEC, and near-IR photometric observations from PAIRITEL of asample of 11 solar-type T Tauri stars in nine systems. We use theseobservations to probe the circumstellar material within 1 AU of theseyoung stars, measuring the circumstellar-to-stellar flux ratios andangular size scales of the 2.2 μm emission. Our sample spans a rangeof stellar luminosities and mass accretion rates, allowing investigationof potential correlations between inner disk properties and stellar oraccretion properties. We suggest that the mechanism by which the dustyinner disk is truncated may depend on the accretion rate of the source;in objects with low accretion rates, the stellar magnetospheres maytruncate the disks, while sublimation may truncate dusty disks aroundsources with higher accretion rates. We have also included in our sampleobjects that are known to be highly variable (based on previousphotometric and spectroscopic observations), and for several sources, weobtained multiple epochs of spectroscopic and interferometric data,supplemented by near-IR photometric monitoring, to search for inner diskvariability. While time-variable veilings and accretion rates areobserved in some sources, no strong evidence for inner disk pulsation isfound.
| 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.
| New periodic variable stars coincident with ROSAT sources discovered using SuperWASP
We present optical lightcurves of 428 periodic variable stars coincidentwith ROSAT X-ray sources, detected using the first run of the SuperWASPphotometric survey. Only 68 of these were previously recognised asperiodic variables. A further 30 of these objects are previously knownpre-main sequence stars, for which we detect a modulation period for thefirst time. Amongst the newly identified periodic variables, many appearto be close eclipsing binaries, their X-ray emission is presumably theresult of RS CVn type behaviour. Others are probably BY Dra stars,pre-main sequence stars and other rapid rotators displaying enhancedcoronal activity. A number of previously catalogued pulsating variables(RR Lyr stars and Cepheids) coincident with X-ray sources are also seen,but we show that these are likely to be misclassifications. We identifyfour objects which are probable low mass eclipsing binary stars, basedon their very red colour and light curve morphology.Tables 1 and 2 are also 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/467/785
| 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.
| 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 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.
| 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.
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