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Formation and Evolution of Planetary Systems: Properties of Debris Dust Around Solar-Type Stars
We present Spitzer photometric (IRAC and MIPS) and spectroscopic (IRSlow resolution) observations for 314 stars in the Formation andEvolution of Planetary Systems Legacy program. These data are used toinvestigate the properties and evolution of circumstellar dust aroundsolar-type stars spanning ages from approximately 3 Myr-3 Gyr.We identify 46 sources that exhibit excess infrared emission above thestellar photosphere at 24 μm, and 21 sources with excesses at 70μm. Five sources with an infrared excess have characteristics ofoptically thick primordial disks, while the remaining sources haveproperties akin to debris systems. The fraction of systems exhibiting a24 μm excess greater than 10.2% above the photosphere is 15% for ages< 300 Myr and declines to 2.7% for older ages. The upper envelope tothe 70 μm fractional luminosity appears to decline over a similar agerange. The characteristic temperature of the debris inferred from theIRS spectra range between 60 and 180 K, with evidence for the presenceof cooler dust to account for the strength of the 70 μm excessemission. No strong correlation is found between dust temperature andstellar age. Comparison of the observational data with disk modelscontaining a power-law distribution of silicate grains suggests that thetypical inner-disk radius is gsim 10 AU. Although the interpretation isnot unique, the lack of excess emission shortward of 16 μm and therelatively flat distribution of the 24 μm excess for ages lsim 300Myr is consistent with steady-state collisional models.
| 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).
| The Formation and Evolution of Planetary Systems: Description of the Spitzer Legacy Science Database
We present the science database produced by the Formation and Evolutionof Planetary Systems (FEPS) Spitzer Legacy program. Data reduction andvalidation procedures for the IRAC, MIPS, and IRS instruments aredescribed in detail. We also derive stellar properties for the FEPSsample from available broadband photometry and spectral types, andpresent an algorithm to normalize Kurucz synthetic spectra to opticaland near-infrared photometry. The final FEPS data products include IRACand MIPS photometry for each star in the FEPS sample and calibrated IRSspectra.
| 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.
| Kinematics of the Scorpius-Centaurus OB association
A fine structure related to the kinematic peculiarities of threecomponents of the Scorpius-Centaurus association (LCC, UCL, and US) hasbeen revealed in the UV-velocity distribution of Gould Belt stars. Wehave been able to identify the most likely members of these groups byapplying the method of analyzing the two-dimensional probability densityfunction of stellar UV velocities that we developed. A kinematicanalysis of the identified structural components has shown that, ingeneral, the center-of-mass motion of the LCC, UCL, and US groupsfollows the motion characteristic of the Gould Belt, notably itsexpansion. The entire Scorpius-Centaurus complex is shown to possess aproper expansion with an angular velocity parameter of 46 ± 8 kms‑1 kpc‑1 for the kinematic centerwith l 0 = ‑40° and R 0 = 110 pc found.Based on this velocity, we have estimated the characteristic expansiontime of the complex to be 21 ± 4 Myr. The proper rotationvelocity of the Scorpius-Centaurus complex is lower in magnitude, isdetermined less reliably, and depends markedly on the data quality.
| SIM PlanetQuest Key Project Precursor Observations to Detect Gas Giant Planets around Young Stars
We present a review of precursor observing programs for the SIMPlanetQuest Key Project devoted to detecting Jupiter-mass planets aroundyoung stars. In order to ensure that the stars in the sample are free ofvarious sources of astrometric noise that might impede the detection ofplanets, we have initiated programs to collect photometry, high-contrastimages, interferometric data, and radial velocities for stars in boththe northern and southern hemispheres. We have completed a high-contrastimaging survey of target stars in Taurus and the Pleiades and found nodefinitive common proper motion companions within 1" (140 AU) of the SIMtargets. Our radial velocity surveys have shown that many of the targetstars in Sco-Cen are fast rotators, and a few stars in Taurus and thePleiades may have substellar companions. Interferometric data of a fewstars in Taurus show no signs of stellar or substellar companions withseparations of 5-50 mas. The photometric survey suggests thatapproximately half of the stars initially selected for this program arevariable to a degree (1 σ > 0.1 mag) that would degrade theastrometric accuracy achievable for that star. While the precursorprograms are still a work in progress, we provide a comprehensive listof all targets and rank them according to their viability as a result ofthe observations taken to date. The observable that removes by far themost targets from the SIM young stellar object (YSO) program isphotometric variability.
| Rotation and Activity of Pre-Main-Sequence Stars
We present a study of rotation (vsini) and chromospheric activity(Hα equivalent width) based on an extensive set of high-resolutionoptical spectra obtained with the MIKE instrument on the 6.5 m MagellanClay telescope. Our targets are 74 F-M dwarfs in four young stellarassociations, spanning ages from 6 to 30 Myr. By comparing Hα EWsin our sample to results in the literature, we see a clear evolutionarysequence: Chromospheric activity declines steadily from the T Tauriphase to the main sequence. Using activity as an age indicator, we finda plausible age range for the Tuc-Hor association of 10-40 Myr. Between5 and 30 Myr, we do not see evidence for rotational braking in the totalsample, and thus angular momentum is conserved, in contrast to youngerstars. This difference indicates a change in the rotational regulationat ~5-10 Myr, possibly because disk braking cannot operate longer thantypical disk lifetimes, allowing the objects to spin up. Therotation-activity relation is flat in our sample; in contrast tomain-sequence stars, there is no linear correlation for slow rotators.We argue that this is because young stars generate their magnetic fieldsin a fundamentally different way from main-sequence stars, and not justthe result of a saturated solar-type dynamo. By comparing our rotationalvelocities with published rotation periods for a subset of stars, wedetermine ages of 13+7-6 and9+8-2 Myr for the η Cha and TWA associations,respectively, consistent with previous estimates. Thus we conclude thatstellar radii from evolutionary models by Baraffe et al. (1998) are inagreement with the observed radii to within +/-15%.
| Unraveling the Origins of Nearby Young Stars
A systematic search for close conjunctions and clusterings in the pastof nearby stars younger than the Pleiades is undertaken, which mayreveal the time, location, and mechanism of formation of these oftenisolated, disconnected from clusters and star-forming regions, objects.The sample under investigation includes 101 T Tauri, post-TT, andmain-sequence stars and stellar systems with signs of youth, culled fromthe literature. Their Galactic orbits are traced back in time and nearapproaches are evaluated in time, distance, and relative velocity.Numerous clustering events are detected, providing clues to the originof very young, isolated stars. Each star's orbit is also matched withthose of nearby young open clusters, OB and TT associations andstar-forming molecular clouds, including the Ophiuchus, Lupus, CoronaAustralis, and Chamaeleon regions. Ejection of young stars from openclusters is ruled out for nearly all investigated objects, but thenearest OB associations in Scorpius-Centaurus, and especially, the denseclouds in Ophiuchus and Corona Australis have likely played a major rolein the generation of the local streams (TWA, Beta Pic, andTucana-Horologium) that happen to be close to the Sun today. The core ofthe Tucana-Horologium association probably originated from the vicinityof the Upper Scorpius association 28 Myr ago. A few proposed members ofthe AB Dor moving group were in conjunction with the coeval Cepheus OB6association 38 Myr ago.
| Proper-motion binaries in the Hipparcos catalogue. Comparison with radial velocity data
Context: .This paper is the last in a series devoted to the analysis ofthe binary content of the Hipparcos Catalogue. Aims: .Thecomparison of the proper motions constructed from positions spanning ashort (Hipparcos) or long time (Tycho-2) makes it possible to uncoverbinaries with periods of the order of or somewhat larger than the shorttime span (in this case, the 3 yr duration of the Hipparcos mission),since the unrecognised orbital motion will then add to the propermotion. Methods: .A list of candidate proper motion binaries isconstructed from a carefully designed χ2 test evaluatingthe statistical significance of the difference between the Tycho-2 andHipparcos proper motions for 103 134 stars in common between the twocatalogues (excluding components of visual systems). Since similar listsof proper-motion binaries have already been constructed, the presentpaper focuses on the evaluation of the detection efficiency ofproper-motion binaries, using different kinds of control data (mostlyradial velocities). The detection rate for entries from the NinthCatalogue of Spectroscopic Binary Orbits (S_B^9) is evaluated, as wellas for stars like barium stars, which are known to be all binaries, andfinally for spectroscopic binaries identified from radial velocity datain the Geneva-Copenhagen survey of F and G dwarfs in the solarneighbourhood. Results: .Proper motion binaries are efficientlydetected for systems with parallaxes in excess of ~20 mas, and periodsin the range 1000-30 000 d. The shortest periods in this range(1000-2000 d, i.e., once to twice the duration of the Hipparcos mission)may appear only as DMSA/G binaries (accelerated proper motion in theHipparcos Double and Multiple System Annex). Proper motion binariesdetected among S_B9 systems having periods shorter than about400 d hint at triple systems, the proper-motion binary involving acomponent with a longer orbital period. A list of 19 candidate triplesystems is provided. Binaries suspected of having low-mass(brown-dwarf-like) companions are listed as well. Among the 37 bariumstars with parallaxes larger than 5 mas, only 7 exhibit no evidence forduplicity whatsoever (be it spectroscopic or astrometric). Finally, thefraction of proper-motion binaries shows no significant variation amongthe various (regular) spectral classes, when due account is taken forthe detection biases.Full Table [see full textsee full text] is only available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/464/377
| The Formation and Evolution of Planetary Systems: Placing Our Solar System in Context with Spitzer
We provide an overview of the Spitzer Legacy Program, Formation andEvolution of Planetary Systems, that was proposed in 2000, begun in2001, and executed aboard the Spitzer Space Telescope between 2003 and2006. This program exploits the sensitivity of Spitzer to carry outmid-infrared spectrophotometric observations of solar-type stars. With asample of ~328 stars ranging in age from ~3 Myr to ~3 Gyr, we trace theevolution of circumstellar gas and dust from primordial planet-buildingstages in young circumstellar disks through to older collisionallygenerated debris disks. When completed, our program will help define thetimescales over which terrestrial and gas giant planets are built,constrain the frequency of planetesimal collisions as a function oftime, and establish the diversity of mature planetary architectures. Inaddition to the observational program, we have coordinated a concomitanttheoretical effort aimed at understanding the dynamics of circumstellardust with and without the effects of embedded planets, dust spectralenergy distributions, and atomic and molecular gas line emission.Together with the observations, these efforts will provide anastronomical context for understanding whether our solar system-and itshabitable planet-is a common or a rare circumstance. Additionalinformation about the FEPS project can be found on the team Web site.
| 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.
| Formation and Evolution of Planetary Systems: Upper Limits to the Gas Mass in Disks around Sun-like Stars
We have carried out a sensitive search for gas emission lines at IR andmillimeter wavelengths for a sample of 15 young Sun-like stars selectedfrom our dust disk survey with Spitzer. We have used mid-IR lines totrace the warm (300-100 K) gas in the inner disk and millimetertransitions of 12CO to probe the cold (~20 K) outer disk. Wereport no gas line detections from our sample. Line flux upper limitsare first converted to warm and cold gas mass limits using simpleapproximations allowing a direct comparison with values from theliterature. We also present results from more sophisticated modelsfollowing Gorti & Hollenbach that confirm and extend our simpleanalysis. These models show that the [S I] 25.23 μm line can setconstraining limits on the gas surface density at the disk inner radiusand traces disk regions up to a few AU. We find that none of the 15systems have more than 0.04MJ of gas within a few AU from thedisk inner radius for disk radii from 1 to ~40 AU. These gas mass upperlimits even in the eight systems younger than ~30 Myr suggest that mostof the gas is dispersed early. The gas mass upper limits in the 10-40 AUregion, which is mainly traced by our CO data, are <2M⊕. If these systems are analogs of the solar system,they either have already formed Uranus- and Neptune-like planets or willnot form them beyond 100 Myr. Finally, the gas surface density upperlimits at 1 AU are smaller than 0.01% of the minimum mass solar nebulafor most of the sources. If terrestrial planets form frequently andtheir orbits are circularized by gas, then circularization occurs early.
| 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.
| Dynamical Evolution of the TW Hydrae Association
Using Galactic dynamics we have determined the age of the low-masspost-T Tauri stars in the TW Hya Association (TWA). To do so we appliedthe method of Ortega and coworkers to five stars of the association withHipparcos-measured distances (TWA 1, TWA 4, TWA 9, TWA 11, and TWA 19).The method is based on the calculation of the past three-dimensionalorbits of the stars. Of these stars, only TWA 9 presents a quitedifferent orbit so that it does not appear to be a dynamical member ofthe TWA. The four remaining stars have their first maximum orbitalconfinement at the age of -8.3+/-0.8 Myr, which is considered thedynamical age of the TWA. This confinement fixes the probablethree-dimensional forming region of the TWA within a mean radius of 14.5pc. This region is related to the older subgroups of the Sco-Cen OBassociation, Lower Centaurus Crux and Upper Centaurus Lupus, both with amean age of about 18 Myr. This dynamical age of the TWA and that of theβ Pic Moving Group, 11 Myr, also discussed here, introduce a moreprecise temporal scale for studies of disk evolution and planetaryformation around some stars of these associations. Using the retracedorbit of the runaway star HIP 82868 we examine the possibility that theformation of the TWA was triggered by a supernova explosion. It is shownthat for the four considered TWA stars, the expansion in volume is afactor of 5 from their origin to the present state. This is mainly dueto the currently more distant star TWA 19.
| Formation and Evolution of Planetary Systems (FEPS): Primordial Warm Dust Evolution from 3 to 30 Myr around Sun-like Stars
We present data obtained with the Infrared Array Camera (IRAC) aboardthe Spitzer Space Telescope (Spitzer) for a sample of 74 young (t<30Myr old) Sun-like (0.7
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| Expansion of the TW Hydrae association and the encounter with Vega
We investigate the paths of several probable members of the youngassociation around the star TW Hydrae (TWA) with accurate distances,proper motions and radial velocities. We find that three of thepreviously identified members, TWA 1, TWA 4 and TWA 11, together withtwo other young nearby stars, HD 139084 and HD 220476, form a rapidlyexpanding association with an expansion age of 4.7 +/- 0.6 Myr. Initialvelocities of member stars with respect to the common centre of massrange from 4 to 10km s-1. A characteristic size of theassociation in the initial configuration is 21 pc, which may be somewhatbiased upwards due to the uncertainties in the observational data. TheLower Centaurus Crux (LCC) OB association passed near TWA, at a distanceof 36 +/- 6 pc, 11 Myr ago. A plausible scenario, which accounts for thedifference between the isochrone age (~=10 Myr) and expansion age (5Myr), is that star formation was stimulated in the TWA progenitor cloudby the near passage of the LCC, but that the newly formed stars were notreleased from the cloud until a subsequent collision with one of theother molecular clouds in the North Ophiuchus region. Vega was insidethe TWA association, and close to its centre of gravity, at the time ofmaximum compression 4.7 Myr ago. If this alignment is a chanceencounter, the powerful particular disc around Vega could have beenenhanced by the passage through the TWA progenitor cloud at 8 kms-1.
| A Search for Hot Massive Extrasolar Planets around Nearby Young Stars with the Adaptive Optics System NACO
We report on a survey devoted to the search of exoplanets around youngand nearby stars carried out with NACO at the VLT. The detection limitfor 28 among the best available targets versus the angular separationfrom the star is presented. The nondetection of any planetary masscompanion in our survey is used to derive, for the first time, thefrequency of the upper limit of the projected planet-star separation. Inparticular, we find that in 50% of the cases, no 5MJ (or moremassive) planet has been detected at projected separations larger than14 AU, and no 10MJ (or more massive) planet has been detectedat projected separations larger than 8.5 AU. In 100% of the cases, thesevalues increase to 36 and 65 AU, respectively. The excellent sensitivityreached by our study leads to a much lower upper limit of the projectedplanet-star separation compared with previous studies. For example, forthe β Pictoris group (~12 Myr), we did not detect any10MJ planet at distances larger than 15 AU. A previous studycarried out with 4 m class telescopes put an upper limit for10MJ planets at ~60 AU. For our closest target (V2306 Oph;d=4.3 pc), it is shown that it would be possible to detect a10MJ planet at a minimum projected separation from the starof 1 AU and a 5MJ planet at a minimum projected separation of3.7 AU. Our results are discussed with respect to mechanisms explainingplanet formation and migration and forthcoming observational strategiesand future planet-finder observations from the ground.Based on observations collected at the European Southern Observatory,Chile. Program 70.C-0777D, 70.C-0777E, and 71.C-0029A.
| Statistical Constraints for Astrometric Binaries with Nonlinear Motion
Useful constraints on the orbits and mass ratios of astrometric binariesin the Hipparcos catalog are derived from the measured proper motiondifferences of Hipparcos and Tycho-2 (Δμ), accelerations ofproper motions (μ˙), and second derivatives of proper motions(μ̈). It is shown how, in some cases, statistical bounds can beestimated for the masses of the secondary components. Two catalogs ofastrometric binaries are generated, one of binaries with significantproper motion differences and the other of binaries with significantaccelerations of their proper motions. Mathematical relations betweenthe astrometric observables Δμ, μ˙, and μ̈ andthe orbital elements are derived in the appendices. We find a remarkabledifference between the distribution of spectral types of stars withlarge accelerations but small proper motion differences and that ofstars with large proper motion differences but insignificantaccelerations. The spectral type distribution for the former sample ofbinaries is the same as the general distribution of all stars in theHipparcos catalog, whereas the latter sample is clearly dominated bysolar-type stars, with an obvious dearth of blue stars. We point outthat the latter set includes mostly binaries with long periods (longerthan about 6 yr).
| Evolution of Cold Circumstellar Dust around Solar-type Stars
We present submillimeter (Caltech Submillimeter Observatory 350 μm)and millimeter (Swedish-ESO Submillimetre Telescope [SEST] 1.2 mm, OwensValley Radio Observatory [OVRO] 3 mm) photometry for 127 solar-typestars from the Formation and Evolution of Planetary Systems SpitzerLegacy program that have masses between ~0.5 and 2.0 Msolarand ages from ~3 Myr to 3 Gyr. Continuum emission was detected towardfour stars with a signal-to-noise ratio>=3: the classical T Tauristars RX J1842.9-3532, RX J1852.3-3700, and PDS 66 with SEST, and thedebris-disk system HD 107146 with OVRO. RX J1842.9-3532 and RXJ1852.3-3700 are located in projection near the R CrA molecular cloud,with estimated ages of ~10 Myr (Neuhäuser et al.), whereas PDS 66is a probable member of the ~20 Myr old Lower Centaurus-Crux subgroup ofthe Scorpius-Centaurus OB association (Mamajek et al.). The continuumemission toward these three sources is unresolved at the 24" SESTresolution and likely originates from circumstellar accretion disks,each with estimated dust masses of ~5×10-5Msolar. Analysis of the visibility data toward HD 107146(age~80-200 Myr) indicates that the 3 mm continuum emission is centeredon the star within the astrometric uncertainties and resolved with aGaussian-fit FWHM size of (6.5"+/-1.4")×(4.2"+/-1.3"), or185AU×120 AU. The results from our continuum survey are combinedwith published observations to quantify the evolution of dust mass withtime by comparing the mass distributions for samples with differentstellar ages. The frequency distribution of circumstellar dust massesaround solar-type stars in the Taurus molecular cloud (age~2 Myr) isdistinguished from that around 3-10 Myr and 10-30 Myr old stars at asignificance level of ~1.5 and ~3 σ, respectively. These resultssuggest a decrease in the mass of dust contained in small dust grainsand/or changes in the grain properties by stellar ages of 10-30 Myr,consistent with previous conclusions. Further observations are needed todetermine if the evolution in the amount of cold dust occurs on evenshorter timescales.
| Searching for massive extrasolar planets around young and nearby stars: from NACO to CHEOPS
We report on a survey devoted to the search of exo-planets around youngand nearby stars carried out with NACO at the VLT. The detection limitfor 28 among the best available targets vs. the angular separation fromthe star is presented. The non-detection of any planetary mass companionin our survey is used to derive, for the first time, the frequency ofthe upper limit of the projected separation planet/stars. In particular,we find that in 50% of cases, no 5MJ planet (or more massive)has been detected at projected separations larger than 14 AU and no10MJ planet (or more massive) has been detected at projectedseparations larger than 8.5 AU. The excellent sensitivity reached by ourstudy leads to a much lower upper limit of the projected separationplanet-star compared with previous studies. For our closest target(V2306 Oph - d = 4.3 pc) it is shown that it would be possible to detecta 10MJ planet at a minimum projected separation from the starof 1 AU and a 5MJ planet at a minimum projected separation of3.7 AU. Our results are discussed with respect to forthcomingobservational strategies (Simultaneous Differential Imaging technique)and future planet finder observations from the ground.
| 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.
| Exoplanet Recognition Using a Wavelet Analysis Technique
The recognition of a planet orbiting around a star (and in general of afaint companion in a binary system) in direct imaging observations islimited at distances smaller than around 1" by the speckle and photonnoise. There are also other sources of noises as artifacts produced bythe adaptive optics (AO) systems that make the identification of aplanet even more difficult. Techniques to recognize a planet in anautomatic (and objective) way are therefore quite useful. In thisLetter, we test the ability of a ``wavelet analysis'' technique forrecognizing exoplanets in deep images obtained with ground-basedtelescopes and AO facilities. We present the automatic proceduredeveloped for the detection of exoplanets, and we validate it on a deepimage of a young, nearby star, which has been one of the targets of aplanet search survey done with NACO/VLT. Preliminary tests show thatwith the proposed algorithm, it is indeed possible to recognize, in anautomatic way, a planet of 12MJ at a distance of 0.5", aplanet of 5MJ at a distance of 0.7", and a planet of3MJ at a distance of 1" from the star. We also show that astar/planet systems with a maximum ΔM=4.7 mag contrast in the Kband at 0.2" can be recognized automatically. This is a typical contrastfor a 5MJ-10MJ planet and a late-type star (late Mspectral type).
| New Aspects of the Formation of the β Pictoris Moving Group
In a previous work, we explored the possibility that the β Pictorismoving group (BPMG), consisting of low-mass post-T Tauri stars, wasformed near the Scorpius-Centaurus OB association. The cause of theformation could be a Type II supernova exploding either in LowerCentaurus Crux (LCC) or the Upper Centaurus Lupus (UCL), the two oldersubgroups of that association. Here we present new results for BPMG. Amore detailed analysis of the orbit confinement in this group leads to astar distribution pattern at birth that can be considered as arepresentation of the density distribution in the natal cloud. We alsopropose a plausible origin for the supernova that could have triggeredthe star formation in BPMG by finding the past position of the runawaystar HIP 46950. We find that this scenario is capable of explaining theorigin of all the members of BPMG proposed by Zuckerman and coworkersand by Song and coworkers, with the exception of HIP 79881, which isprobably an old main-sequence interloper.
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