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Collisional processes and size distribution in spatially extended debris discs
Context: New generations of instruments provide, or are about toprovide, pan-chromatic images of debris discs and photometricmeasurements, that require new generations of models, in particular toaccount for their collisional activity. Aims: We present a newmulti-annulus code for the study of collisionally evolving extendeddebris discs. We first aim to confirm and extend our preliminary resultobtained for a single-annulus system, namely that the size distributionin realistic debris discs always departs from the theoreticalcollisional “equilibrium” dN ∝ R-3.5 dRpower law, especially in the crucial size range of observable particles(R⪉ 1 cm), where it displays a characteristic wavy pattern. We alsostudy how debris discs density distributions, scattered light luminosityprofiles, and Spectral Energy Distributions (SEDs) are affected by thecoupled effect of collisions and radial mixing due to radiation pressureaffected small grains. Methods: The size distribution evolution ismodeled over 10 orders of magnitude, going from μm-sized grains to 50km-sized bodies. The model takes into account the crucial influence ofradiation pressure-affected small grains. We consider the collisionalevolution of a fiducial, idealized a = 120 AU radius disc with aninitial surface density Σ(a) ∝ aα. Severalkey parameters are explored: surface density profile, system's dynamicalexcitation, total dust mass, collision outcome prescriptions. Results:We show that the system's radial extension plays a crucial role and thatthe waviness of the size distribution is amplified by inter-annuliinteractions: in most regions the collisional and size evolution of thedust is imposed by small particles on eccentric or unbound orbitsproduced further inside the disc. Moreover, the spatial distribution ofall grains ⪉1 cm departs significantly from the initial profile inΣ(a) ∝ aα, while the bigger objects,containing most of the system's mass, still follow the initialdistribution. This has consequences on the scattered-light radialprofiles which get significantly flatter. We propose an empirical law totrace back the distribution of large unseen parent bodies from theobserved profiles. We also show that the the waviness of the sizedistribution has a clear observable signature in the far-infrared and at(sub-)millimeter wavelengths. This suggests a test of our collisionmodel, which requires observations with future facilities such asHerschel, SOFIA, SCUBA-2 and ALMA. Finally, we provide empiricalformulae for the collisional size distribution and collision timescalewhich can be used for future debris disc modeling.Appendices A and B are only available in electronic form athttp://www.aanda.org
| Steady State Evolution of Debris Disks around A Stars
This paper confronts a simple analytical model for the steady stateevolution of debris disks due to collisions with Spitzer observations ofdust around main-sequence A stars. It is assumed that every star has aplanetesimal belt, the initial mass and radius of which are drawn fromdistributions. In the model disk mass is constant until the largestplanetesimals reach collisional equilibrium, whereupon mass falls~t-1age. We find that the detection statistics andtrends seen at 24 and 70 μm can be fitted well by the model. Whilethere is no need to invoke stochastic evolution or delayed stirring toexplain the statistics, a moderate rate of stochastic events is notruled out. Potentially anomalous systems are identified by a high dustluminosity compared with the maximum permissible in the model (HD 3003,HD 38678, HD 115892, HD 172555); their planetesimals may have unusualproperties (high strength or low eccentricity), or this dust could betransient. The overall success of our model, which assumes planetesimalsin all belts have the same strength, eccentricity, and maximum size,suggests the outcome of planet formation is reasonably uniform. Thedistribution of planetesimal belt radii, once corrected for detectionbias, follows N(r)~r-0.8+/-0.3 for 3-120 AU. Since beltboundaries may be attributed to unseen planets, this provides a uniqueconstraint on A star planetary systems. It is also shown that P-R dragmay sculpt the inner edges of A star disks close to the Spitzerdetection threshold (HD 2262, HD 19356, HD 106591, HD 115892). Thismodel can be readily applied to the interpretation of future surveys,and predictions for the upcoming SCUBA-2 survey include that 17% of Astar disks should be detectable at 850 μm.
| 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%.
| Discovery of Extreme Asymmetry in the Debris Disk Surrounding HD 15115
We report the first scattered light detection of a dusty debris disksurrounding the F2 V star HD 15115 using the Hubble Space Telescope inthe optical and Keck adaptive optics in the near-infrared. The mostremarkable property of the HD 15115 disk relative to other debris disksis its extreme length asymmetry. The east side of the disk is detectedto ~315 AU radius, whereas the west side of the disk has radius >550AU. We find a blue optical to near-infrared scattered light colorrelative to the star that indicates grain scattering properties similarto the AU Mic debris disk. The existence of a large debris disksurrounding HD 15115 adds further evidence for membership in the βPic moving group, which was previously argued based on kinematics alone.Here we hypothesize that the extreme disk asymmetry is due to dynamicalperturbations from HIP 12545, an M star east of HD 15115 that shares acommon proper motion vector, heliocentric distance, galactic spacevelocity, and age.
| 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.
| Formation of Narrow Dust Rings in Circumstellar Debris Disks
Narrow dust rings observed around some young stars (e.g., HR 4796A) needto be confined. We present a possible explanation for the formation andconfinement of such rings in optically thin circumstellar disks, withoutinvoking shepherding planets. If an enhancement of dust grains (e.g.,due to a catastrophic collision) occurs somewhere in the disk,photoelectric emission from the grains can heat the gas to temperatureswell above that of the dust. The gas orbits with super (sub)-Keplerianspeeds inward (outward) of the associated pressure maximum. This tendsto concentrate the grains into a narrow region. The rise in dust densityleads to further heating and a stronger concentration of grains. Anarrow dust ring forms as a result of this instability. We show thatthis mechanism not only operates around early-type stars that have highUV fluxes, but also around stars with spectral types as late as K. Thisimplies that this process is generic and may have occurred during thelifetime of each circumstellar disk. We examine the stringent upperlimit on the H2 column density in the HR 4796A disk and findit to be compatible with the presence of a significant amount ofhydrogen gas in the disk. We also compute the O I and C II infrared linefluxes expected from various debris disks and show that these will beeasily detectable by the upcoming Herschel mission. Herschel will beinstrumental in detecting and characterizing gas in these disks.
| 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.
| Discovery of an 86 AU Radius Debris Ring around HD 181327
HST NICMOS PSF-subtracted coronagraphic observations of HD 181327 haverevealed the presence of a ringlike disk of circumstellar debris seen in1.1 μm light scattered by the disk grains, surrounded by a diffuseouter region of lower surface brightness. The annular disk appears to beinclined by 31.7d+/-1.6d from face-on, with the disk major-axis P.A. at107deg+/-2deg. The total 1.1 μm flux density ofthe light scattered by the disk (at 1.2"
| Spitzer IRS Spectroscopy of IRAS-discovered Debris Disks
We have obtained Spitzer Space Telescope Infrared Spectrograph (IRS)5.5-35 μm spectra of 59 main-sequence stars that possess IRAS 60μm excess. The spectra of five objects possess spectral features thatare well-modeled using micron-sized grains and silicates withcrystalline mass fractions 0%-80%, consistent with T Tauri and HerbigAeBe stars. With the exception of η Crv, these objects are youngwith ages <=50 Myr. Our fits require the presence of a cool blackbodycontinuum, Tgr=80-200 K, in addition to hot, amorphous, andcrystalline silicates, Tgr=290-600 K, suggesting thatmultiple parent body belts are present in some debris disks, analogousto the asteroid and Kuiper belts in our solar system. The spectra forthe majority of objects are featureless, suggesting that the emittinggrains probably have radii a>10 μm. We have modeled the excesscontinua using a continuous disk with a uniform surface densitydistribution, expected if Poynting-Robertson and stellar wind drag arethe dominant grain removal processes, and using a single-temperatureblackbody, expected if the dust is located in a narrow ring around thestar. The IRS spectra of many objects are better modeled with asingle-temperature blackbody, suggesting that the disks possess innerholes. The distribution of grain temperatures, based on our blackbodyfits, peaks at Tgr=110-120 K. Since the timescale for icesublimation of micron-sized grains with Tgr>110 K is afraction of a Myr, the lack of warmer material may be explained if thegrains are icy. If planets dynamically clear the central portions ofdebris disks, then the frequency of planets around other stars isprobably high. We estimate that the majority of debris disk systemspossess parent body masses, MPB<1 M⊕. Thelow inferred parent body masses suggest that planet formation is anefficient process.Based on observations with the NASA Spitzer Space Telescope, which isoperated by the California Institute of Technology for NASA.
| On the AU Microscopii debris disk. Density profiles, grain properties, and dust dynamics
Context: . AU Mic is a young M-type star surroundedby an edge-on optically thin debris disk that shares many commonobservational properties with the disk around βPictoris. In particular, the scattered light surfacebrightness profile falls off as r-5 outside 120 AU forβ Pictoris and 35 AU for AUMic. In both cases, the disk color rises as the distanceincreases beyond these reference radii. Aims: . In this paper, wepresent the first comprehensive analysis of the AUMic disk properties since the system was resolved by Kalas etal. (2004, Science, 303, 1990). We explore whether the dynamical model,which successfully reproduces the β Pictoris brightness profile(e.g., Augereau et al. 2001, A&A, 370, 447), could apply toAU Mic. Methods: . We calculate the surfacedensity profile of the AU Mic disk by performing theinversion of the near-IR and visible scattered light brightness profilesmeasured by Liu (2004, Science, 305, 1442) and Krist et al. (2005, AJ,129, 1008), respectively. We discuss the grain properties by analysingthe blue color of the disk in the visible (Krist et al. 2005) and byfitting the disk spectral energy distribution. Finally, we evaluate theradiation and wind forces on the grains. The impact of the recurrentX-ray and UV-flares on the dust dynamics is also discussed. Results: . We show that irrespective of the mean scattering asymmetryfactor of the grains, most of the emission arises from an asymmetric,collisionally-dominated region that peaks close to the surfacebrightness break around 35 AU. The elementary scatterers at visiblewavelengths are found to be sub-micronic, but the inferred sizedistribution underestimates the number of large grains, resulting insub-millimeter emissions that are too low compared to the observations.From our inversion procedure, we find that the V- to H-band scatteringcross sections ratio increases outside 40 AU, in line with the observedcolor gradient of the disk. This behavior is expected if the grains havenot been produced locally, but placed in orbits of high eccentricity bya size-dependent pressure force, resulting in a paucity of large grainsbeyond the outer edge of the parent bodies' disk. Because of the lowluminosity of AU Mic, radiation pressure isinefficient to diffuse the smallest grains in the outer disk, even whenthe flares are taken into account. Conversely, we show that a standard,solar-like stellar wind generates a pressure force onto the dustparticles that behaves much like a radiation pressure force. With anassumed dot{M} ≃ 3×102 dot{M}ȯ,the wind pressure overcomes the radiation pressure, and this effect isenhanced by the stellar flares. This greatly contributes to populatingthe extended AU Mic debris disk and explains thesimilarity between the β Pictoris andAU Mic brightness profiles. In both cases, the colorgradient beyond 120 AU for β Pictoris and 35 AUfor AU Mic, is believed to be a direct consequence ofthe dust dynamics.
| Spitzer Far-Infrared Detections of Cold Circumstellar Disks
Observations at 70 μm with the Spitzer Space Telescope have detectedseveral stellar systems within 65 pc of the Sun. Of 18 presumably youngsystems detected in this study, as many as 15 have 70 μm emission inexcess of that expected from their stellar photospheres. Five of thesystems with excesses are members of the Tucanae association. The 70μm excesses range from a factor of ~2 to nearly 30 times the expectedphotospheric emission from these stars. In contrast to the 70 μmproperties of these systems, there is evidence for an emission excess at24 μm for only HD 3003, confirming previous results for this star.The lack of a strong 24 μm excess in most of these systems suggeststhat the circumstellar dust producing the IR excesses is relatively cool(Tdust<~150 K) and that there is little IR-emittingmaterial within the inner few AU of the primary stars. Many of thesesystems lie close enough to Earth that the distribution of the dustproducing the IR excesses might be imaged in scattered light at opticaland near-IR wavelengths.
| Nearby Debris Disk Systems with High Fractional Luminosity Reconsidered
By searching the IRAS and ISO databases, we compiled a list of 60 debrisdisks that exhibit the highest fractional luminosity values(fd>10-4) in the vicinity of the Sun (d<120pc). Eleven out of these 60 systems are new discoveries. Special carewas taken to exclude bogus disks from the sample. We computed thefractional luminosity values using available IRAS, ISO, and Spitzer dataand analyzed the Galactic space velocities of the objects. The resultsrevealed that stars with disks of high fractional luminosity oftenbelong to young stellar kinematic groups, providing an opportunity toobtain improved age estimates for these systems. We found thatpractically all disks with fd>5×10-4 areyounger than 100 Myr. The distribution of the disks in the fractionalluminosity versus age diagram indicates that (1) the number of oldsystems with high fd is lower than was claimed before, (2)there exist many relatively young disks of moderate fractionalluminosity, and (3) comparing the observations with a currenttheoretical model of debris disk evolution, a general good agreementcould be found.
| Optical polarimetry of infrared excess stars
We present UBRVI polarimetry measurements for a group of 38 IRASinfrared excess stars and complement these observations with V-band datataken from the literature for 87 additional objects. After correctingthe observed values by the interstellar contribution, we find that 48%of the analyzed sample has polarization excess. In addition, thepolarization of these stars may correlate with infrared color excesses,particularly at 60 and 100 μm. We caution, however, that poor IRASdata quality at longer wavelengths affects this correlation. We analyzethe wavelength dependence of the linear polarization of 15 polarizedobjects in relation to Serkowski's empirical interstellar law. We findthat for 6 to 7 objects (depending on the interstellar model) themeasured polarization differs significantly from the empiricalinterstellar law, suggesting an intrinsic origin. We analyze thepolarimetry distribution of IRAS infrared excess objects in relation tothe Exoplanet host stars (i.e., stars associated with at least onelikely planetary mass object). The corresponding polarimetrydistributions are different within a high confidence level. Finally, wecompare the metallicity distributions of F and G IRAS infrared excess,Exoplanet host and field main sequence stars, and find that F-G IRASinfrared excess objects have metallicities quite similar (although notidentical) to field main sequence stars and significantly different fromthe Exoplanet host group.
| Effective temperature scale and bolometric corrections from 2MASS photometry
We present a method to determine effective temperatures, angularsemi-diameters and bolometric corrections for population I and II FGKtype stars based on V and 2MASS IR photometry. Accurate calibration isaccomplished by using a sample of solar analogues, whose averagetemperature is assumed to be equal to the solar effective temperature of5777 K. By taking into account all possible sources of error we estimateassociated uncertainties to better than 1% in effective temperature andin the range 1.0-2.5% in angular semi-diameter for unreddened stars.Comparison of our new temperatures with other determinations extractedfrom the literature indicates, in general, remarkably good agreement.These results suggest that the effective temperaure scale of FGK starsis currently established with an accuracy better than 0.5%-1%. Theapplication of the method to a sample of 10 999 dwarfs in the Hipparcoscatalogue allows us to define temperature and bolometric correction (Kband) calibrations as a function of (V-K), [m/H] and log g. Bolometriccorrections in the V and K bands as a function of T_eff, [m/H] and log gare also given. We provide effective temperatures, angularsemi-diameters, radii and bolometric corrections in the V and K bandsfor the 10 999 FGK stars in our sample with the correspondinguncertainties.
| 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.
| Constraining the Lifetime of Circumstellar Disks in the Terrestrial Planet Zone: A Mid-Infrared Survey of the 30 Myr old Tucana-Horologium Association
We have conducted an N-band survey of 14 young stars in the ~30 Myr oldTucana-Horologium association to search for evidence of warm,circumstellar dust disks. Using the MIRAC-BLINC camera on the Magellan I(Baade) 6.5 m telescope, we find that none of the stars have astatistically significant N-band excess compared to the predictedstellar photospheric flux. Using three different sets of assumptions,this null result rules out the existence of the following around thesepost-T Tauri stars: (1) optically thick disks with inner hole radii of<~0.1 AU, (2) optically thin disks with masses of less than10-6 M⊕ (in ~1 μm sized grains) within<~10 AU of these stars, and (3) scaled-up analogs of the solar systemzodiacal dust cloud with more than 4000 times the emitting area. Oursurvey was sensitive to dust disks in the terrestrial planet zone withfractional luminosity oflog(Ldust/L*)~10-2.9, yet none werefound. Combined with results from previous surveys, these data suggestthat circumstellar dust disks become so optically thin as to beundetectable at N band before age ~20 Myr. We also present N-bandphotometry for several members of other young associations and asubsample of targets that will be observed with the Spitzer SpaceTelescope by the Formation and Evolution of Planetary Systems LegacyScience Program. Finally, we present an absolute calibration ofMIRAC-BLINC for four filters (L, N, 11.6, and Qs) on theCohen-Walker-Witteborn system.
| The Geneva-Copenhagen survey of the Solar neighbourhood. Ages, metallicities, and kinematic properties of 14 000 F and G dwarfs
We present and discuss new determinations of metallicity, rotation, age,kinematics, and Galactic orbits for a complete, magnitude-limited, andkinematically unbiased sample of 16 682 nearby F and G dwarf stars. Our63 000 new, accurate radial-velocity observations for nearly 13 500stars allow identification of most of the binary stars in the sampleand, together with published uvbyβ photometry, Hipparcosparallaxes, Tycho-2 proper motions, and a few earlier radial velocities,complete the kinematic information for 14 139 stars. These high-qualityvelocity data are supplemented by effective temperatures andmetallicities newly derived from recent and/or revised calibrations. Theremaining stars either lack Hipparcos data or have fast rotation. Amajor effort has been devoted to the determination of new isochrone agesfor all stars for which this is possible. Particular attention has beengiven to a realistic treatment of statistical biases and errorestimates, as standard techniques tend to underestimate these effectsand introduce spurious features in the age distributions. Our ages agreewell with those by Edvardsson et al. (\cite{edv93}), despite severalastrophysical and computational improvements since then. We demonstrate,however, how strong observational and theoretical biases cause thedistribution of the observed ages to be very different from that of thetrue age distribution of the sample. Among the many basic relations ofthe Galactic disk that can be reinvestigated from the data presentedhere, we revisit the metallicity distribution of the G dwarfs and theage-metallicity, age-velocity, and metallicity-velocity relations of theSolar neighbourhood. Our first results confirm the lack of metal-poor Gdwarfs relative to closed-box model predictions (the ``G dwarfproblem''), the existence of radial metallicity gradients in the disk,the small change in mean metallicity of the thin disk since itsformation and the substantial scatter in metallicity at all ages, andthe continuing kinematic heating of the thin disk with an efficiencyconsistent with that expected for a combination of spiral arms and giantmolecular clouds. Distinct features in the distribution of the Vcomponent of the space motion are extended in age and metallicity,corresponding to the effects of stochastic spiral waves rather thanclassical moving groups, and may complicate the identification ofthick-disk stars from kinematic criteria. More advanced analyses of thisrich material will require careful simulations of the selection criteriafor the sample and the distribution of observational errors.Based on observations made with the Danish 1.5-m telescope at ESO, LaSilla, Chile, and with the Swiss 1-m telescope at Observatoire deHaute-Provence, France.Complete Tables 1 and 2 are only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/418/989
| Dusty Debris Disks as Signposts of Planets: Implications for Spitzer Space Telescope
Submillimeter and near-infrared images of cool dusty debris disks andrings suggest the existence of unseen planets. At dusty but nonimagedstars, semimajor axes of associated planets can be estimated from thedust temperature. For some young stars these semimajor axes are greaterthan 1" as seen from Earth. Such stars are excellent targets forsensitive near-infrared imaging searches for warm planets. To probe thefull extent of the dust and hence of potential planetary orbits, Spitzerobservations should include measurements with the 160 μm filter.
| An infrared imaging search for low-mass companions to members of the young nearby β Pic and Tucana/Horologium associations
We present deep high dynamic range infrared images of young nearby starsin the Tucana/Horologium and β Pic associations, all 10 to 35Myrs young and at 10 to 60 pc distance. Such young nearby starsare well-suited for direct imaging searches for brown dwarf and evenplanetary companions, because young sub-stellar objects are stillself-luminous due to contraction and accretion. We performed ourobservations at the ESO 3.5m NTT with the normal infrared imagingdetector SofI and the MPE speckle camera Sharp-I. Three arc sec north ofGSC 8047-0232 in Horologium a promising brown dwarf companion candidateis detected, which needs to be confirmed by proper motion and/orspectroscopy. Several other faint companion candidates are alreadyrejected by second epoch imaging. Among 21 stars observed inTucana/Horologium, there are not more than one to five brown dwarfcompanions outside of 75 AU (1.5'' at 50 pc); most certainly only <=5% of the Tuc/HorA stars have brown dwarf companions (13 to 78 Jupitermasses) outside of 75 AU. For the first time, we can report an upperlimit for the frequency of massive planets ( 10 Mjup) atwide separations ( 100 AU) using a meaningfull and homogeneoussample: Of 11 stars observed sufficiently deep in β Pic (12 Myrs),not more than one has a massive planet outside of 100 AU, i.e.massive planets at large separations are rare (<= 9%).Based on observations obtained on La Silla, Chile, in ESO programs65.L-0144(B), 66.D-0135, 66.C-0310(A), 67.C-0209(B), 67.C-0213(A),68.C-0008(A), and 68.C-0009(A)} }
| Adaptive optics imaging survey of the Tucana-Horologium association
We present the results of an adaptive optics (AO) imaging survey of thecommon associations of Tucana and Horologium, carried out at the ESO 3.6m telescope with the ADONIS/SHARPII system. Based on our observations oftwo dozen probable association members, HIP 1910 and HIP 108422 appearto have low-mass stellar companions, while HIP 6856 and GSC 8047-0232have possible sub-stellar candidate companions. Astrometricmeasurements, performed in November 2000 and October 2001, indicate thatHIP 1910 B likely is bound to its primary, but are inconclusive in thecase of the candidate companion to HIP 6856. Additional observations areneeded to confirm the HIP 6856 companionship as well as for HIP 108422and GSC 8047-0232.
| Discovery of Reflection Nebulosity around Five Vega-like Stars
Coronagraphic optical observations of six Vega-like stars revealreflection nebulosities, five of which were previously unknown. Thenebulosities illuminated by HD 4881, HD 23362, HD 23680, HD 26676, andHD 49662 resemble that of the Pleiades, indicating an interstellarorigin for dust grains. The reflection nebulosity around HD 123160 has adouble-arm morphology, but no disklike feature is seen as close as 2.5"from the star in K-band adaptive optics data. We demonstrate that auniform density dust cloud surrounding HD 23362, HD 23680, and HD 123160can account for the observed 12-100 μm spectral energy distributions.For HD 4881, HD 26676, and HD 49662, an additional emission source, suchas from a circumstellar disk or nonequilibrium grain heating, isrequired to fit the 12-25 μm data. These results indicate that insome cases, particularly for Vega-like stars located beyond the LocalBubble (>100 pc), the dust responsible for excess thermal emissionmay originate from the interstellar medium rather than from a planetarydebris system.
| Late-type members of young stellar kinematic groups - I. Single stars
This is the first paper of a series aimed at studying the properties oflate-type members of young stellar kinematic groups. We concentrate ourstudy on classical young moving groups such as the Local Association(Pleiades moving group, 20-150Myr), IC 2391 supercluster (35Myr), UrsaMajor group (Sirius supercluster, 300Myr), and Hyades supercluster(600Myr), as well as on recently identified groups such as the Castormoving group (200Myr). In this paper we compile a preliminary list ofsingle late-type possible members of some of these young stellarkinematic groups. Stars are selected from previously established membersof stellar kinematic groups based on photometric and kinematicproperties as well as from candidates based on other criteria such astheir level of chromospheric activity, rotation rate and lithiumabundance. Precise measurements of proper motions and parallaxes takenfrom the Hipparcos Catalogue, as well as from the Tycho-2 Catalogue, andpublished radial velocity measurements are used to calculate theGalactic space motions (U, V, W) and to apply Eggen's kinematic criteriain order to determine the membership of the selected stars to thedifferent groups. Additional criteria using age-dating methods forlate-type stars will be applied in forthcoming papers of this series. Afurther study of the list of stars compiled here could lead to a betterunderstanding of the chromospheric activity and their age evolution, aswell as of the star formation history in the solar neighbourhood. Inaddition, these stars are also potential search targets for directimaging detection of substellar companions.
| The β Pictoris Moving Group
Following the 1983 IRAS detection and subsequent imaging of itsextensive dusty circumstellar disk, β Pictoris became theprototypical and most studied example of a potential forming planetarysystem. Here we report the identification of 17 star systems, each withone or more characteristics indicative of extreme youth, that are movingthrough space together with β Pic. This diverse set of ~12 millionyr old star systems, which includes a ~35 Jupiter mass brown dwarf, anda wide assortment of dusty circumstellar disks, is the comoving,youthful group closest to Earth. Their unique combination of youth andproximity to Earth makes group members-many of which have masses similarto that of the Sun-prime candidates for imaging of warm planets anddusty circumstellar disks with ground- and space-based telescopes.
| Tucana Association
Among star clusters, only the sparse Ursa Major nucleus is closer toEarth than the recently identified Tucana association. Based on newphotometric VRI magnitudes, we construct a color-magnitude diagramcomposed of likely and possible Tucana members. The implied age of theTucana association stars, <~40 Myr, is consistent with the agedetermined from a previous analysis by Stelzer & Neuhäuser ofthe X-ray luminosities of plausible association members. Based primarilyon space motions and X-ray fluxes, we identify potential new members ofthe Tucana association not considered in earlier studies. Torres andcoworkers recently identified a group of post-T Tauri stars, theHorologium association, which is located near the Tucana association inright ascension and declination. Because the Horologium stars have thesame space motions, age, distance from Earth, volume density, and rangeof spectral types as Tucana stars, we suggest that, rather than beingcharacterized as a separate group, it would be appropriate andeconomical to subsume the Horologium association stars into the Tucanastream.
| X-ray emission from young stars in the Tucanae association
We report on X-ray emission from members of the recently discoveredTucanae association, a group of stars with youth signatures and similarspace motion. The Tucanae association is the nearest known region ofrecent star formation ( ~ 45 pc) far from molecular clouds(\cite{Zuckerman00.1}). We have made use of the ROSAT Data Archive andsearched for X-rays from Tucanae stars in both ROSAT All-Sky Survey(RASS) and pointed observations. While the RASS provides completecoverage of the sky, only three potential Tucanae members have beenobserved during PSPC pointings. All three stars have been detected. Forthe RASS the percentage of detections is 59%. The comparison of theX-ray luminosity function of Tucanae to that of other star formingregions may provide clues to the uncertain age of the association. Wefind that the distribution of X-ray luminosities is very similar to theones derived for the TW Hya association, the Taurus-Auriga T TauriStars, and the IC 2602 cluster, but significantly brighter than theluminosity distribution of the Pleiades. We conclude that the stars inTucanae are most likely young, on the order of 10-30 Myr. Strongvariability of most stars emerges from the X-ray lightcurves whereseveral flares and irregular variations are observed.
| Identification of a Nearby Stellar Association in theHipparcos Catalog: Implications for Recent, Local Star Formation
The TW Hydrae Association (~55 pc from Earth) is the nearest knownregion of recent star formation. Based primarily on the Hipparcoscatalog, we have identified a group of nine or 10 comoving star systemsat a common distance (~45 pc) from Earth that appear to compriseanother, somewhat older association (``the Tucanae Association'').Together with ages and motions recently determined for some nearby fieldstars, the existence of the Tucanae and TW Hydrae Associations suggeststhat the Sun is now close to a region that was the site of substantialstar formation only 10-50 Myr ago. The TW Hydrae Association representsa final chapter in the local star formation history.
| Optical, infrared and millimetre-wave properties of Vega-like systems - IV. Observations of a new sample of candidate Vega-like sources
Photometric observations at optical and near-infrared wavelengths arepresented for members of a new sample of candidate Vega-like systems, ormain sequence stars with excess infrared emission due to circumstellardust. The observations are combined with IRAS fluxes to define thespectral energy distributions of the sources. Most of the sources showonly photospheric emission at near-IR wavelengths, indicating a lack ofhot (~1000K) dust. Mid-infrared spectra are presented for four sourcesfrom the sample. One of them, HD 150193, shows strong silicate emission,while another, HD 176363, was not detected. The spectra of two starsfrom our previous sample of Vega-like sources both show UIR-bandemission, attributed to hydrocarbon materials. Detailed comparisons ofthe optical and IRAS positions suggest that in some cases the IRASsource is not physically associated with the visible star. Alternativeassociations are suggested for several of these sources. Fractionalexcess luminosities are derived from the observed spectral energydistributions. The values found are comparable to those measuredpreviously for other Vega-like sources.
| Candidate Main-Sequence Stars with Debris Disks: A New Sample of Vega-like Sources
Vega-like sources are main-sequence stars that exhibit IR fluxes inexcess of expectations for stellar photospheres, most likely due toreradiation of stellar emission intercepted by orbiting dust grains. Wehave identified a large sample of main-sequence stars with possibleexcess IR radiation by cross-correlating the Michigan Catalog ofTwo-dimensional Spectral Types for the HD Stars with the IRAS FaintSource Survey Catalog. Some 60 of these Vega-like sources were not foundduring previous surveys of the IRAS database, the majority of whichemployed the lower sensitivity Point Source Catalog. Here, we providedetails of our search strategy, together with a preliminary examinationof the full sample of Vega-like sources.
| The chemical evolution of the solar neighborhood. I - A bias-free reduction technique and data sample
The possible ways of measuring the age-metallicity relation for thegalactic disk in the neighborhood of the sun are discussed. It is shownthat the use of a field star sample chosen on the basis of effectivetemperature introduces a bias which results in a monotonic increase inthe metal abundance of the disk with time. However, if theage-metallicity relation for the disk can be shown to satisfy certaincriteria, the bias introduced in such a sample can be neglected: thegalactic disk apparently satisfies the criteria. It is concluded that asample analyzed through the use of uvby and H(beta) photometry inconjunction with a self-consistent set of theoretical isochronesprovides the least biased, most accurate estimate of the age-metallicityrelation for the disk.
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