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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.
| 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.
| 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.
| Chromospheric activity and rotation of FGK stars in the solar vicinity. An estimation of the radial velocity jitter
Context. Chromospheric activity produces both photometric andspectroscopic variations that can be mistaken as planets. Large spotscrossing the stellar disc can produce planet-like periodic variations inthe light curve of a star. These spots clearly affect the spectral lineprofiles, and their perturbations alter the line centroids creating aradial velocity jitter that might “contaminate” thevariations induced by a planet. Precise chromospheric activitymeasurements are needed to estimate the activity-induced noise thatshould be expected for a given star. Aims: We obtain precisechromospheric activity measurements and projected rotational velocitiesfor nearby (d ? 25 pc) cool (spectral types F to K) stars, toestimate their expected activity-related jitter. As a complementaryobjective, we attempt to obtain relationships between fluxes indifferent activity indicator lines, that permit a transformation oftraditional activity indicators, i.e., Ca ii H & K lines, to othersthat hold noteworthy advantages. Methods: We used high resolution(~50 000) echelle optical spectra. Standard data reduction was performedusing the IRAF echelle package. To determine the chromospheric emissionof the stars in the sample, we used the spectral subtraction technique.We measured the equivalent widths of the chromospheric emission lines inthe subtracted spectrum and transformed them into fluxes by applyingempirical equivalent width and flux relationships. Rotational velocitieswere determined using the cross-correlation technique. To inferactivity-related radial velocity (RV) jitter, we used empiricalrelationships between this jitter and the R'_HK index. Results:We measured chromospheric activity, as given by different indicatorsthroughout the optical spectra, and projected rotational velocities for371 nearby cool stars. We have built empirical relationships among themost important chromospheric emission lines. Finally, we used themeasured chromospheric activity to estimate the expected RV jitter forthe active stars in the sample.Based on observations made with the 2.2 m telescope at the CentroAstronómico Hispano Alemán (CAHA) at Calar Alto (Spain)and the Telescopio Nazionale Galileo (TNG) operated on the island of LaPalma by the Istituto Nazionale de Astrofisica Italiano (INAF), in theSpanish Observatorio del Roque de los Muchachos. This research has beensupported by the Programa de Acceso a InfraestructurasCientíficas y Tecnológicas Singulares (ICTS).Tables A1 toA4 are only available in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/520/A79
| ? Pictoris and AB Doradus Moving Groups: Likely New Low-mass Members
We present results from our continuing program to identify new,low-mass, members of the nearby young moving groups (NYMGs) using aproper motion selection algorithm and various observational techniques.We have three goals: (1) to provide high priority targets for exoplanetsearches by direct imaging, (2) to complete the census of the membershipin the NYMGs down to ~0.1 M sun, and thus (3) provide awell-characterized sample of nearby (median distances at least twice asclose as the Taurus and Ophiuchus star-forming regions), young (8-50Myr) stars for detailed study of their physical properties andmultiplicity. Our program proceeds as follows: we apply the selectionalgorithm to a proper motion catalog where initial selection cuts ofcandidate members are based on the mean motion of known NYMG members andthe proper motions and photometric distances of the candidates. NYMGmembership is investigated further using possible signs of youth,including H? emission and X-ray flux, and then verified throughradial velocity measurements. We identify TYC 1766-1431-1 (M3), TYC1208-468-1 and 2 (K3), TYC 7558-655-1 (K5), and PM I04439+3723W and E(M3) as likely members of the ? Pictoris moving group (BPMG) andTYC 1741-2117-1N and S (K7), TYC 1752-63-1 (K7), TYC 523-573-1 (K7), andTYC 4943-192-1 (M0) as likely members of the AB Doradus moving group(ABDMG). We also rule out the membership of several BPMG and ABDMGcandidates. To date our program has identified 16 new NYMG members ofspectral type K3 or later.
| 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.
| A spectroscopic survey of the youngest field stars in the solar neighbourhood. I. The optically bright sample
Aims: We present the first results of an ambitious ground-basedobservation programme conducted on 1-4 m class telescopes. Our sampleconsists of 1097 active and presumably young stars, all of them beingoptical (Tycho Catalogue) counterparts of ROSAT All-Sky Survey X-raysources in the northern hemisphere. In this paper, we concentrate on theoptically brightest (V_T?slant 9.5 m) candidates (704 objects). Weacquired high-resolution optical spectroscopy in the H? and/orlithium spectral regions for 426 of such stars without relevant data inthe literature. We describe the star sample and the observations and westart to discuss the physical properties of the investigated stars. Methods: We used a cross-correlation technique and other toolsdeveloped by us to derive accurate radial and rotational velocities andto perform an automatic spectral classification for both single starsand double-lined systems. The spectral subtraction technique was used toderive chromospheric activity levels and lithium abundances. Weestimated the fraction of young single stars and multiple systems instellar soft X-ray surveys and the contamination by more evolvedsystems, like RS CVn binaries. We classified stars on the basis of theirlithium abundance and give a glimpse of their sky distribution. Results: The sample appears to be a mixture of quite youngPleiades-like and Hyades-like stars plus an older lithium-poorpopulation probably born within the last 1-2 Gyr. Seven stars with alithium abundance compatible with the age of IC 2602 (about 30 Myr) oreven younger were detected as well, although two appear to belithium-rich giants.The discovery of a large number of highly ormoderately lithium-rich giants is another outcome of the present survey. Conclusions: The contamination of soft X-ray surveys by oldsystems in which the activity level is enhanced by tidal synchronisationis not negligible, especially for K-type stars. Five stars with lithiumcontent close to the primordial abundance are probably associated withalready known moving groups in the solar neighbourhood. Some of them aregood post-T Tauri candidates according to their positions in the HRdiagram.Based on observations collected at the Observatoire de Haute Provence(France) and the Catania Astrophysical Observatory (Italy). Data for the426 stars is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/504/829
| On the kinematic evolution of young local associations and the Scorpius-Centaurus complex
Context: Over the last decade, several groups of young (mainly low-mass)stars have been discovered in the solar neighbourhood (closer than ~100pc), thanks to cross-correlation between X-ray, optical spectroscopy andkinematic data. These young local associations - including an importantfraction whose members are Hipparcos stars - offer insights into thestar formation process in low-density environments, shed light on thesubstellar domain, and could have played an important role in the recenthistory of the local interstellar medium. Aims: To study the kinematicevolution of young local associations and their relation to other youngstellar groups and structures in the local interstellar medium, thuscasting new light on recent star formation processes in the solarneighbourhood. Methods: We compiled the data published in theliterature for young local associations. Using a realistic Galacticpotential we integrated the orbits for these associations and theSco-Cen complex back in time. Results: Combining these data with thespatial structure of the Local Bubble and the spiral structure of theGalaxy, we propose a recent history of star formation in the solarneighbourhood. We suggest that both the Sco-Cen complex and young localassociations originated as a result of the impact of the inner spiralarm shock wave against a giant molecular cloud. The core of the giantmolecular cloud formed the Sco-Cen complex, and some small cloudlets ina halo around the giant molecular cloud formed young local associationsseveral million years later. We also propose a supernova in young localassociations a few million years ago as the most likely candidate tohave reheated the Local Bubble to its present temperature.
| The N2K Consortium. VII. Atmospheric Parameters of 1907 Metal-rich Stars: Finding Planet-Search Targets
We report high-precision atmospheric parameters for 1907 stars in theN2K low-resolution spectroscopic survey, designed to identify metal-richFGK dwarfs likely to harbor detectable planets. Of these stars, 284 arein the ideal temperature range for planet searches,Teff<=6000 K, and have a 10% or greater probability ofhosting planets based on their metallicities. The stars in thelow-resolution spectroscopic survey should eventually yield >60 newplanets, including 8-9 hot Jupiters. Short-period planets have alreadybeen discovered orbiting the survey targets HIP 14810 and HD 149143.
| Unraveling the Origins of Nearby Young Stars
A systematic search for close conjunctions and clusterings in the pastof nearby stars younger than the Pleiades is undertaken, which mayreveal the time, location, and mechanism of formation of these oftenisolated, disconnected from clusters and star-forming regions, objects.The sample under investigation includes 101 T Tauri, post-TT, andmain-sequence stars and stellar systems with signs of youth, culled fromthe literature. Their Galactic orbits are traced back in time and nearapproaches are evaluated in time, distance, and relative velocity.Numerous clustering events are detected, providing clues to the originof very young, isolated stars. Each star's orbit is also matched withthose of nearby young open clusters, OB and TT associations andstar-forming molecular clouds, including the Ophiuchus, Lupus, CoronaAustralis, and Chamaeleon regions. Ejection of young stars from openclusters is ruled out for nearly all investigated objects, but thenearest OB associations in Scorpius-Centaurus, and especially, the denseclouds in Ophiuchus and Corona Australis have likely played a major rolein the generation of the local streams (TWA, Beta Pic, andTucana-Horologium) that happen to be close to the Sun today. The core ofthe Tucana-Horologium association probably originated from the vicinityof the Upper Scorpius association 28 Myr ago. A few proposed members ofthe AB Dor moving group were in conjunction with the coeval Cepheus OB6association 38 Myr ago.
| The Nearest Young Moving Groups
The latest results in the research of forming planetary systems have ledseveral authors to compile a sample of candidates for searching forplanets in the vicinity of the Sun. Young stellar associations areindeed excellent laboratories for this study, but some of them are notclose enough to allow the detection of planets through adaptive opticstechniques. However, the existence of very close young moving groups cansolve this problem. Here we have compiled the members of the nearestyoung moving groups, as well as a list of new candidates from ourcatalog of late-type stars that are possible members of young stellarkinematic groups, studying their membership through spectroscopic andphotometric criteria.
| Young Stars Near the Sun
Until the late 1990s the rich Hyades and the sparse UMa clusters werethe only coeval, comoving concentrations of stars known within 60 pc ofEarth. Both are hundreds of millions of years old. Then beginning in thelate 1990s the TW Hydrae Association, the Tucana/Horologium Association,the Pictoris Moving Group, and the AB Doradus Moving Group wereidentified within 60 pc of Earth, and the Chamaeleontis cluster wasfound at 97 pc. These young groups (ages 8 50 Myr), along with othernearby, young stars, will enable imaging and spectroscopic studies ofthe origin and early evolution of planetary systems.
| The AB Doradus Moving Group
From radio to X-ray wavelengths, AB Doradus has been an intensivelystudied star. We have identified ~30 nearby star systems, each with oneor more characteristics of youth, that are moving through space togetherwith AB Dor. This diverse set of ~50 million year old star systems isthe comoving, youthful group closest to Earth. The group's nucleus is aclustering of a dozen stars ~20 pc from Earth that includes AB Doritself. The AB Dor moving group joins the previously known and somewhatyounger and more distant Tucana/Horologium and TW Hydrae associationsand the β Pictoris moving group as excellent laboratories forinvestigations of forming planetary systems.
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