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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.
| High-resolution imaging of young M-type stars of the solar neighbourhood: probing for companions down to the mass of Jupiter
Context. High-contrast imaging is a powerful technique when searchingfor gas giant planets and brown dwarfs orbiting at separations greaterthan several AU. Around solar-type stars, giant planets are expected toform by core accretion or by gravitational instability, but since coreaccretion is increasingly difficult as the primary star becomes lighter,gravitational instability would be a probable formation scenario forstill-to-find distant giant planets around a low-mass star. A systematicsurvey for such planets around M dwarfs would therefore provide a directtest of the efficiency of gravitational instability. Aims: Wesearch for gas giant planets orbiting late-type stars and brown dwarfsof the solar neighbourhood. Methods: We obtained deephigh-resolution images of 16 targets with the adaptive optic system ofVLT-NACO in the L' band, using direct imaging and angular differentialimaging. This is currently the largest and deepest survey forJupiter-mass planets around M-dwarfs. We developed and used anintegrated reduction and analysis pipeline to reduce the images andderive our 2D detection limits for each target. The typical contrastachieved is about 9 mag at 0.5? and 11 mag beyond 1?. Foreach target we also determine the probability of detecting a planet of agiven mass at a given separation in our images. Results: Wederived accurate detection probabilities for planetary companions,taking orbital projection effects into account, with in average morethan 50% probability to detect a 3 MJup companion at 10 AUand a 1.5 MJup companion at 20 AU, bringing strongconstraints on the existence of Jupiter-mass planets around this sampleof young M-dwarfs.Based on observations made with the NACO at VLT UT-4 at the ParanalObservatory under programme IDs 084.C-0739, 085.C-0675(A), 087.C-0413(A)and 087.C-0450(B).
| Effect of magnetic activity saturation in chromospheric flux-flux relationships
We present a homogeneous study of chromospheric and coronal flux-fluxrelationships using a sample of 298 late-type dwarf active stars withspectral types F to M. The chromospheric lines were observedsimultaneously in each star to avoid spread as a result of long-termvariability. Unlike other works, we subtract the basal chromosphericcontribution in all the spectral lines studied. For the first time, wequantify the departure of dMe stars from the general relations. We showthat dK and dKe stars also deviate from the general trend. Studying theflux-colour diagrams, we demonstrate that the stars deviating from thegeneral relations are those with saturated X-ray emission and we showthat these stars also present saturation in the H? line. Usingseveral age spectral indicators, we show that these are younger starsthan those following the general relationships. The non-universality offlux-flux relationships found in this work should be taken into accountwhen converting between fluxes in different chromospheric activityindicators.
| 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.
| Magneto-convection and Lithium Age Estimates of the ? Pictoris Moving Group
Although the means of the ages of stars in young groups determined fromLi depletion often agree with mean ages determined fromHertzsprung-Russell (H-R) diagram isochrones, there are oftenstatistically significant differences in the ages of individual starsdetermined by the two methods. We find that inclusion of the effects ofinhibition of convection due to the presence of magnetic fields leads toconsistent ages for the individual stars. We illustrate how ageconsistency arises by applying our results to the ? Pictoris movinggroup (BPMG). We find that, although magnetic inhibition of convectionleads to increased ages from the H-R diagram isochrones for all stars,Li ages are decreased for fully convective M stars and increased forstars with radiative cores. Our consistent age determination for BPMG of40 Myr is larger than previous determinations by a factor of about two.We have also considered models in which the mixing length ratio isadjusted to give consistent ages. We find that our magneto-convectionmodels, which give quantitative estimates of magnetic field strength,provide a viable alternative to models in which the effects of magneticfields (and other processes) are accounted for by reducing the mixinglength ratio.
| 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.
| A Uniform Analysis of 118 Stars with High-contrast Imaging: Long-period Extrasolar Giant Planets are Rare Around Sun-like Stars
We expand on the results of Nielsen et al., using the null result forgiant extrasolar planets around the 118 target stars from the Very LargeTelescope (VLT) NACO H- and Ks-band planet search (conducted byMasciadri and collaborators in 2003 and 2004), the VLT and MMTSimultaneous Differential Imager survey, and the Gemini Deep PlanetSurvey to set constraints on the population of giant extrasolar planets.Our analysis is extended to include the planet luminosity models ofFortney et al., as well as the correlation between stellar mass andfrequency of giant planets found by Johnson et al. Doubling the samplesize of FGKM stars strengthens our conclusions: a model for extrasolargiant planets with power laws for mass and semimajor axis as given byCumming et al. cannot, with 95% confidence, have planets beyond 65 AU,compared to the value of 94 AU reported by Nielsen et al., using themodels of Baraffe et al. When the Johnson et al. correction for stellarmass (which gives fewer Jupiter-mass companions to M stars with respectto solar-type stars) is applied, however, this limit moves out to 82 AU.For the relatively new Fortney et al. models, which predict fainterplanets across most of parameter space, these upper limits, with andwithout a correction for stellar mass, are 182 and 234 AU, respectively.
| 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.
| Rotational velocities of nearby young stars
Context. Stellar rotation is a crucial parameter driving stellarmagnetism, activity and mixing of chemical elements. Measuringrotational velocities of young stars can give additional insight in theinitial conditions of the star formation process. Furthermore, theevolution of stellar rotation is coupled to the evolution ofcircumstellar disks. Disk-braking mechanisms are believed to beresponsible for rotational deceleration during the accretion phase, androtational spin-up during the contraction phase after decoupling fromthe disk for fast rotators arriving at the ZAMS. On the ZAMS, stars getrotationally braked by solar-type winds. Aims: We investigate theprojected rotational velocities v sin i of a sample of young stars withrespect to the stellar mass and disk evolutionary state to search forpossible indications of disk-braking mechanisms. Furthermore, we searchfor signs of rotational spin-up of stars that have already decoupledfrom their circumstellar disks. Methods: We analyse the stellarspectra of 220 nearby (mostly <100 pc) young (2-600 Myr) stars fortheir v sin i, stellar age, H? emission, and accretion rates. Thestars have been observed with FEROS at the 2.2 m MPG/ESO telescope andHARPS at the 3.6 m telescope in La Silla, Chile. The spectra have beencross-correlated with appropriate theoretical templates. We build a newcalibration to be able to derive v sin i values from thecross-correlated spectra. Stellar ages are estimated from the Li Iequivalent width at 6708 Å. The equivalent width and width at 10%height of the H? emission are measured to identify accretors andused to estimate accretion rates dot{M}_acc. The v sin i is thenanalysed with respect to the evolutionary state of the circumstellardisks to search for indications of disk-braking mechanisms in accretors. Results: We find that the broad v sin i distribution of ourtargets extends to rotation velocities of up to more than 100 kms-1 and peaks at a value of 7.8 ± 1.2 km s-1, and that 70% of our stars show v sin i < 30 kms-1. Furthermore, we can find indications for disk-braking inaccretors and rotational spin-up of stars which are decoupled from theirdisks. In addition, we show that a number of young stars are suitablefor precise radial-velocity measurements for planet-search surveys.Based upon observations with FEROS at the 2.2 m MPG/ESO telescope andHARPS at the 3.6 m telescope in La Silla, Chile.
| A Test of Pre-Main-Sequence Lithium Depletion Models
Despite the extensive study of lithium depletion duringpre-main-sequence (PMS) contraction, studies of individual stars showdiscrepancies between ages determined from the Hertzsprung-Russell (H-R)diagram and ages determined from lithium depletion, indicating openquestions in the PMS evolutionary models. To further test these models,we present high-resolution spectra for members of the ? PictorisMoving Group (BPMG), which is young and nearby. We measure equivalentwidths of the 6707.8 Å Li I line in these stars and use them todetermine lithium abundances. We combine the lithium abundance with thepredictions of PMS evolutionary models in order to calculate a lithiumdepletion age for each star. We compare this age to the age predicted bythe H-R diagram of the same model. We find that the evolutionary modelsunderpredict the amount of lithium depletion for the BPMG given itsnominal H-R diagram age of ~12 Myr, particularly for the mid-M stars,which have no observable Li I line. This results in systematically olderages calculated from lithium depletion isochrones than from the H-Rdiagram. We suggest that this discrepancy may be related to thediscrepancy between measured M-dwarf radii and the smaller radiipredicted by evolutionary models.
| Imaging Young Giant Planets From Ground and Space
High-contrast imaging can find and characterize gas giant planets aroundnearby young stars and the closest M stars, complementing radialvelocity and astrometric searches by exploring orbital separationsinaccessible to indirect methods. Ground-based coronagraphs are alreadyprobing within 25 AU of nearby young stars to find objects as small as .This paper contrasts near-term and future ground-based capabilities withhigh-contrast imaging modes of the James Webb Space Telescope (JWST).Monte Carlo modeling reveals that JWST can detect planets with masses assmall as across a broad range of orbital separations. We present newcalculations for planet brightness as a function of mass and age forspecific JWST filters and extending to .
| Search for associations containing young stars (SACY). III. Ages and Li abundances
Context: Our study is a follow-up of the SACY project, an extended highspectral resolution survey of more than two thousand opticalcounterparts to X-ray sources in the southern hemisphere targeted tosearch for young nearby association. Nine associations have either beennewly identified, or have had their member list revised. Groupsbelonging to the Sco-Cen-Oph complex are not considered in the presentstudy. Aims: These nine associations, with ages of between about 6Myr and 70 Myr, form an excellent sample to study the Li depletion inthe pre-main sequence (PMS) evolution. In the present paper, weinvestigate the use of Li abundances as an independent clock toconstrain the PMS evolution. Methods: Using our measurements ofthe equivalent widths of the Li resonance line and assuming fixedmetallicities and microturbulence, we calculated the LTE Li abundancesfor 376 members of various young associations. In addition, weconsidered the effects of their projected stellar rotation.Results: We present the Li depletion as a function of age in the firsthundred million years for the first time for the most extended sample ofLi abundances in young stellar associations. Conclusions: A clearLi depletion can be measured in the temperature range from 5000 K to3500 K for the age span covered by the nine associations studied in thispaper. The age sequence based on the Li-clock agrees well with theisochronal ages, the ?Cha association being the only possibleexception. The lithium depletion patterns for the associations presentedhere resemble those of the young open clusters with similar ages,strengthening the notion that the members proposed for these loose youngassociations have indeed a common physical origin. The observed scatterin the Li abundances hampers the use of Li in determining reliable agesfor individual stars. For velocities above 20 km s-1,rotation seems to play an important role in inhibiting the Li depletion.Based on observations collected at the ESO - La Silla and at theLNA-OPD.Tables [see full textsee full text]-[see full textsee full text] areonly available in electronic form at http://www.aanda.org
| 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
| 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.
| Constraints on Extrasolar Planet Populations from VLT NACO/SDI and MMT SDI and Direct Adaptive Optics Imaging Surveys: Giant Planets are Rare at Large Separations
We examine the implications for the distribution of extrasolar planetsbased on the null results from two of the largest direct imaging surveyspublished to date. Combining the measured contrast curves from 22 of thestars observed with the VLT NACO adaptive optics system by Masciadri andcoworkers and 48 of the stars observed with the VLT NACO SDI and MMT SDIdevices by Biller and coworkers (for a total of 60 unique stars), weconsider what distributions of planet masses and semimajor axes can beruled out by these data, based on Monte Carlo simulations of planetpopulations. We can set the following upper limit with 95% confidence:the fraction of stars with planets with semimajor axis between 20 and100 AU, and mass above 4 MJup, is 20% or less. Also, with adistribution of planet mass of dN/dM~M-1.16 in the range of0.5-13 MJup, we can rule out a power-law distribution forsemimajor axis (dN/da~aα) with index 0 and upper cutoffof 18 AU, and index -0.5 with an upper cutoff of 48 AU. For thedistribution suggested by Cumming et al., a power-law of index -0.61, wecan place an upper limit of 75 AU on the semimajor axis distribution. Ingeneral, we find that even null results from direct imaging surveys arevery powerful in constraining the distributions of giant planets (0.5-13MJup) at large separations, but more work needs to be done toclose the gap between planets that can be detected by direct imaging,and those to which the radial velocity method is sensitive.
| An Imaging Survey for Extrasolar Planets around 45 Close, Young Stars with the Simultaneous Differential Imager at the Very Large Telescope and MMT
We present the results of a survey of 45 young (<~250 Myr), close(<~50 pc) stars with the Simultaneous Differential Imager (SDI)implemented at the VLT and the MMT for the direct detection ofextrasolar planets. As part of the survey, we observed 54 objects,consisting of 45 close, young stars; two more distant (<150 pc),extremely young (<=10 Myr) stars; three stars with known radialvelocity planets; and four older, very nearby (<=20 pc) solaranalogs. Our SDI devices use a double Wollaston prism and a quad filterto take images simultaneously at three wavelengths surrounding the 1.62μm methane absorption bandhead found in the spectrum of cool browndwarfs and gas giant planets. By differencing adaptive optics-correctedimages in these filters, speckle noise from the primary star issignificantly attenuated, resulting in photon (andflat-field)-noise-limited data. In our VLT data, we achieved H-bandcontrasts>~10 mag (5 σ) at a separation of 0.5" from theprimary star on 45% of our targets and H-band contrasts>~9 mag at aseparation of 0.5" on 80% of our targets. With these contrasts, we canimage (5 σ detection) a 7 MJ planet 15 AU from a 70 MyrK1 star at 15 pc or a 7.8 MJ planet at 2 AU from a 12 Myr Mstar at 10 pc. We detected no candidates with S/N>2 σ whichbehaved consistently like a real object. From our survey null result, wecan rule out (with 93% confidence) a model planet population whereN(a)~constant out to a distance of 45 AU.Steward Observatory, University of Arizona, Tucson, AZ 85721.
| 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.
| A novel L-band imaging search for giant planets in the Tucana and β Pictoris moving groups
Context: Direct imaging using various techniques for suppressing thestellar halo nowadays can achieve the contrast levels required to detectand characterize the light of substellar companions at orbital distancesgreater than a few astronomical units from their host stars. The methodnicely complements the radial velocity surveys that provide evidencethat giant extrasolar planets in close-in orbits are relatively common. Aims: The paper presents results from a small survey of 22 young, nearbystars that was designed to detect substellar companions and ultimatelygiant extrasolar planets down to Jupiter masses. The targets are membersof the Tucana and β Pictoris moving groups, apart from the somewhatolder star HIP 71395, which has a radial velocity trend suggesting amassive planet in large orbit. Methods: The survey was carried out inthe L-band using adaptive optics-assisted imaging with NAOS-CONICA(NACO) at the VLT. The chosen observation wavelength is well-suited tosearching for close companions around young stars and it deliversunprecedented detection limits. The presented technique reaches some ofthe best sensitivities as of today and is currently the most sensitivemethod for the contrast-limited detection of substellar companions thatare cooler than about 1000 K. Results: The companion to 51 Eri, GJ3305, was found to be a very close binary on an eccentric orbit. Nosubstellar companions were found around the target stars, although themethod permitted companions to be detected down to a few Jupiter massesat orbital distances typically of 5 astronomical units. A planet with amass ≥1 M_Jup at distances ≥5 AU around AU Mic can be excluded atthe time of our observations. The absence of detected planets setsconstraints on the frequency distribution and maximum orbital distanceof giant exoplanets. For example, a radial distribution power law indexof 0.2 in combination with a maximum orbital radius exceeding 30 AU canbe rejected at a 90% confidence level.Based on observations collected at the European Southern Observatory,Chile, through the proposals 073.C-0834(A) and 074.C-0323(A). Figure 6is only available in electronic form at http://www.aanda.org
| SIM PlanetQuest Key Project Precursor Observations to Detect Gas Giant Planets around Young Stars
We present a review of precursor observing programs for the SIMPlanetQuest Key Project devoted to detecting Jupiter-mass planets aroundyoung stars. In order to ensure that the stars in the sample are free ofvarious sources of astrometric noise that might impede the detection ofplanets, we have initiated programs to collect photometry, high-contrastimages, interferometric data, and radial velocities for stars in boththe northern and southern hemispheres. We have completed a high-contrastimaging survey of target stars in Taurus and the Pleiades and found nodefinitive common proper motion companions within 1" (140 AU) of the SIMtargets. Our radial velocity surveys have shown that many of the targetstars in Sco-Cen are fast rotators, and a few stars in Taurus and thePleiades may have substellar companions. Interferometric data of a fewstars in Taurus show no signs of stellar or substellar companions withseparations of 5-50 mas. The photometric survey suggests thatapproximately half of the stars initially selected for this program arevariable to a degree (1 σ > 0.1 mag) that would degrade theastrometric accuracy achievable for that star. While the precursorprograms are still a work in progress, we provide a comprehensive listof all targets and rank them according to their viability as a result ofthe observations taken to date. The observable that removes by far themost targets from the SIM young stellar object (YSO) program isphotometric variability.
| Rotation and Activity of Pre-Main-Sequence Stars
We present a study of rotation (vsini) and chromospheric activity(Hα equivalent width) based on an extensive set of high-resolutionoptical spectra obtained with the MIKE instrument on the 6.5 m MagellanClay telescope. Our targets are 74 F-M dwarfs in four young stellarassociations, spanning ages from 6 to 30 Myr. By comparing Hα EWsin our sample to results in the literature, we see a clear evolutionarysequence: Chromospheric activity declines steadily from the T Tauriphase to the main sequence. Using activity as an age indicator, we finda plausible age range for the Tuc-Hor association of 10-40 Myr. Between5 and 30 Myr, we do not see evidence for rotational braking in the totalsample, and thus angular momentum is conserved, in contrast to youngerstars. This difference indicates a change in the rotational regulationat ~5-10 Myr, possibly because disk braking cannot operate longer thantypical disk lifetimes, allowing the objects to spin up. Therotation-activity relation is flat in our sample; in contrast tomain-sequence stars, there is no linear correlation for slow rotators.We argue that this is because young stars generate their magnetic fieldsin a fundamentally different way from main-sequence stars, and not justthe result of a saturated solar-type dynamo. By comparing our rotationalvelocities with published rotation periods for a subset of stars, wedetermine ages of 13+7-6 and9+8-2 Myr for the η Cha and TWA associations,respectively, consistent with previous estimates. Thus we conclude thatstellar radii from evolutionary models by Baraffe et al. (1998) are inagreement with the observed radii to within +/-15%.
| 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.
| Accretion Disks around Young Stars: Lifetimes, Disk Locking, and Variability
We report the findings of a comprehensive study of disk accretion andrelated phenomena in four of the nearest young stellar associationsspanning 6-30 million years in age, an epoch that may coincide with thelate stages of planet formation. We have obtained ~650 multiepochhigh-resolution optical spectra of 100 low-mass stars that are likelymembers of the η Chamaeleontis (~6 Myr), TW Hydrae (~8 Myr), βPictoris (~12 Myr), and Tucanae-Horologium (~30 Myr) groups. Our datawere collected over 12 nights between 2004 December and 2005 July on theMagellan Clay 6.5 m telescope. Based on Hα line profiles, alongwith a variety of other emission lines, we find clear evidence ofongoing accretion in 3 out of 11 η Cha stars and 2 out of 32 TWHydrae members. None of the 57 β Pic or Tuc-Hor members showsmeasurable signs of accretion. Together, these results imply significantevolution of the disk-accretion process within the first several Myr ofa low-mass star's life. While a few disks can continue to accrete for upto ~10 Myr, our findings suggest that disks accreting for beyond thattimescale are rather rare. This result provides an indirect constrainton the timescale for gas dissipation in inner disks and, in turn, ongas-giant planet formation. All accretors in our sample are slowrotators, whereas nonaccretors cover a large range in rotationalvelocities. This may hint at rotational braking by disks at ages up to~8 Myr. Our multiepoch spectra confirm that emission-line variability iscommon even in somewhat older T Tauri stars, among which accretors tendto show particularly strong variations. Thus, our results indicate thataccretion and wind activity undergo significant and sustained variationsthroughout the lifetime of accretion disks.
| Contributions to the Nearby Stars (NStars) Project: Spectroscopy of Stars Earlier than M0 within 40 pc-The Southern Sample
We are obtaining spectra, spectral types, and basic physical parametersfor the nearly 3600 dwarf and giant stars earlier than M0 in theHipparcos catalog within 40 pc of the Sun. Here we report on resultsfor 1676 stars in the southern hemisphere observed at Cerro TololoInter-American Observatory and Steward Observatory. These resultsinclude new, precise, homogeneous spectral types, basic physicalparameters (including the effective temperature, surface gravity, andmetallicity [M/H]), and measures of the chromospheric activity of ourprogram stars. We include notes on astrophysically interesting stars inthis sample, the metallicity distribution of the solar neighborhood, anda table of solar analogs. We also demonstrate that the bimodal nature ofthe distribution of the chromospheric activity parameterlogR'HK depends strongly on the metallicity, andwe explore the nature of the ``low-metallicity'' chromosphericallyactive K-type dwarfs.
| Differential Radial Velocities and Stellar Parameters of Nearby Young Stars
Radial velocity searches for substellar-mass companions have focusedprimarily on stars older than 1 Gyr. Increased levels of stellaractivity in young stars hinders the detection of solar system analogs,and therefore until recently there has been a prejudice againstinclusion of young stars in radial velocity surveys. Adaptive opticssurveys of young stars have given us insight into the multiplicity ofyoung stars, but only for massive, distant companions. Understanding thelimit of the radial velocity technique, restricted to high-mass,close-orbiting planets and brown dwarfs, we began a survey of youngstars of various ages. While the number of stars needed to carry outfull analysis of the problems of planetary and brown dwarf populationand evolution is large, the beginning of such a sample is included here.We report on 61 young stars ranging in age from the β Pictorisassociation (~12 Myr) to the Ursa Major association (~300 Myr). Thisinitial search resulted in no stars showing evidence of companionslarger than ~1MJup-2MJup in short-period orbits atthe 3 σ level. We also present derived stellar parameters, as mosthave unpublished values. The chemical homogeneity of a cluster, andpresumably of an association, may help to constrain true membership, sowe present [Fe/H] abundances for the stars in our sample.
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| Csillagkép: | Aranyhal |
| Rektaszcenzió: | 05h00m47.13s |
| Deklináció: | -57°15'25.5" |
| Vizuális fényesség: | 10.112 |
| Távolság: | 26.261 parszek |
| RA sajátmozgás: | 36.2 |
| Dec sajátmozgás: | 72.6 |
| B-T magnitude: | 11.737 |
| V-T magnitude: | 10.247 |
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