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EK Draconis. Magnetic activity in the photosphere and chromosphere
Context: As a young solar analogue, EK Draconis provides an opportunityto study the magnetic activity of the infant Sun. Aims: We presentthree new surface temperature maps of EK Draconis and compare them withprevious results obtained from long-term photometry. Furthermore, wedetermined a set of stellar parameters and compared the determinedvalues with the corresponding solar values. Methods: Atmosphericparameters were determined by comparing observed and synthetic spectracalculated with stellar atmosphere models. Surface temperature maps wereobtained using the Occamian approach inversion technique. Thedifferential rotation of EK Dra was estimated using two differentmethods. Results: A detailed model atmosphere analysis of highresolution spectra of EK Dra has yielded a self-consistent set ofatmospheric parameters: T_eff = 5750 K, log g = 4.5, [M/H] = 0.0,ξt = 1.6 km s-1. The evolutionary models implythat the star is slightly more massive than the Sun and has an agebetween 30-50 Myr, which agrees with the determined lithium abundance oflog N(Li) = 3.02. Moreover, the atmospheric parameters, as well as thewings of the Ca ii 8662 Å, indicate that the photosphere of EK Drais very similar to the one of the present Sun, while their chromospheresdiffer. There also seems to be a correlation between magnetic featuresseen in the photosphere and chromosphere. The temperature images revealspots of only 500 K cooler than the quiet photosphere. The mean spotlatitude varies with time. The obtained differential rotation is verysmall, but the sign of it supports solar type differential rotation onEK Dra.Based on observations made with the Nordic Optical Telescope, operatedon the island of La Palma jointly by Denmark, Finland, Iceland, Norway,and Sweden, in the Spanish Observatorio del Roque de los Muchachos ofthe Instituto de Astrofisica de Canarias. Table [see full textsee fulltextsee full textsee full textsee full textsee full text] and Figs. [seefull textsee full textsee full textsee full textsee full textsee fulltext] and [see full textsee full textsee full textsee full textsee fulltextsee full text] are only available in electronic form athttp://www.aanda.org
| 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%.
| 25 Orionis: A Kinematically Distinct 10 Myr Old Group in Orion OB1a
We report here on the photometric and kinematic properties of awell-defined group of nearly 200 low-mass pre-main-sequence stars,concentrated within ~1° of the early-B star 25 Ori, in the OrionOB1a subassociation. We refer to this stellar aggregate as the 25Orionis group. The group also harbors the Herbig Ae/Be star V346 Ori anda dozen other early-type stars with photometry, parallaxes, and somewith IR excess emission, indicative of group membership. The number ofhigh- and low-mass stars is in agreement with expectations from astandard initial mass function. The velocity distribution for thelow-mass stars shows a narrow peak at 19.7 km s-1, offset~-10 km s-1 from the velocity characterizing the youngerstars of the Ori OB1b subassociation, and -4 km s-1 from thevelocity of widely spread young stars of the Ori OB1a population; thisresult provides new and compelling evidence that the 25 Ori group is adistinct kinematic entity, and that considerable space and velocitystructure is present in the Ori OB1a subassociation. The low-massmembers follow a well-defined band in the color-magnitude diagram,consistent with an isochronal age of ~7-10 Myr. The ~2 time drop in theoverall Li I equivalent widths and accretion fraction between theyounger Ori OB1b and the 25 Ori group is consistent with the latterbeing significantly older. In a simple-minded kinematic evolutionscenario, the 25 Ori group may represent the evolved counterpart of theyounger σ Ori cluster. The 25 Ori stellar aggregate is the mostpopulous ~10 Myr sample yet known within 500 pc, setting it as anexcellent laboratory to study the evolution of solar-like stars andprotoplanetary disks.Based on observations obtained at the Llano del Hato NationalAstronomical Observatory of Venezuela, operated by CIDA for theMinisterio de Ciencia y Tecnología the MMT Observatory, a jointfacility of the Smithsonian Institution and the University of Arizona;and the Fred Lawrence Whipple Observatory of the SmithsonianInstitution.
| On the common origin of the AB Doradus moving group and the Pleiades cluster
AB Doradus (AB Dor) is the nearest identified moving group. As withother such groups, the age is important for understanding of several keyquestions. It is important, for example, in establishing the origin ofthe group and also in comparative studies of the properties of planetarysystems, eventually surrounding some of the AB Dor group members, withthose existing in other groups. For AB Dor two rather differentestimates for its age have been proposed: the first one, of the order of50 Myr, by Zuckerman and coworkers from a comparison with theTucana/Horologium moving group and a second one of about 100-125 Myr byLuhman and coworkers from colour-magnitude diagrams. Using this lastvalue and the closeness in velocity space of AB Dor and the Pleiadesgalactic cluster, Luhman and coworkers suggested coevality for thesesystems. Because strictly speaking such a closeness does not stillguarantee coevality, here we address this problem by computing andcomparing the full 3D orbits of AB Dor, Pleiades, α Persei and IC2602. The latter two open clusters have estimated ages of about 85-90and 50 Myr. The resulting age 119 +/- 20 Myr is consistent with AB Dorand Pleiades being coeval. Our solution and the scenario of open clusterformation proposed by Kroupa and collaborators suggest that the AB Dormoving group may be identified with the expanding subpopulation (GroupI) present in this scenario. We also discuss other related aspects asiron and lithium abundances, eventual stellar mass segregation duringthe formation of the systems and possible fraction of debris discs inthe AB Dor group.
| The Keele-Exeter young cluster survey - I. Low-mass pre-main-sequence stars in NGC 2169
We have used RCIC CCD photometry from the IsaacNewton telescope and intermediate-resolution spectroscopy from theGemini North telescope to identify and characterize low-mass (0.15
| Old Stars in Young Clusters: Lithium-depleted Low-Mass Stars of the Orion Nebula Cluster
We measured lithium in a sample of low-mass stars (~0.1-0.3Msolar) of the Orion Nebula cluster. We find evidence forsignificant Li depletion in four high-probability members, correspondingto nuclear ages between ~15 and 30 Myr. In two cases, there is excellentagreement between the mass and age based on models of Li burning andthose derived from the H-R diagram, reinforcing our early findings. Forthe two other stars, the nuclear age is significantly larger than theisochronal one. Several Li-depleted stars display accretion activity,veiling, and emission lines. We discuss empirical evidence in favor ofthe old nuclear age and the implications on the star formation historyof the Orion cluster.Based on data collected at ESO-VLT, Paranal Observatory, Chile [ID074.C-0757(A)].
| 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.
| New catalogue of blue stragglers in open clusters
We present a catalogue of blue-straggler candidates in galactic openclusters. It is based on the inspection of the colour-magnitude diagramsof the clusters, and it updates and supersedesthe first version(Ahumada & Lapasset 1995). A new bibliographical search was made foreach cluster, and the resulting information is organised into twotables. Some methodological aspects have been revised, in particularthose concerning the delimitation of the area in the diagrams where thestragglers are selected.A total of 1887 blue-straggler candidates have been found in 427 openclusters of all ages, doubling the original number. The catalogued starsare classified into two categories mainly according to membershipinformation.The whole catalogue (Tables 8, 9, notes, and references) is onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/463/789
| VLT/Flames observations of the star forming region NGC 6530
Context: Mechanisms regulating the evolution of pre-main sequence starscan be understood by studying stellar properties such as rotation, diskaccretion, internal mixing and binarity. To investigate such properties,we studied a sample of 332 candidate members of the massive and populousstar forming region NGC 6530. Aims: We select cluster members usingdifferent membership criteria, to study the properties of pre-mainsequence stars with or without circumstellar disks. Methods: We useintermediate resolution spectra including the Li I 6707.8 Å lineto derive radial and rotational velocities, binarity and to measure theEquivalent Width of the lithium line; these results are combined withX-ray data to study the cluster membership. Optical-IR data and Hαspectra, these latter available for a subsample of our targets, are usedto classify CTTS and WTTS and to compare the properties of stars withand without disks. Results: We find a total of 237 certain membersincluding 53 binaries. The rotational velocity distributions of starswith IR excesses are statistically different from that of stars withoutIR excesses, while the fraction of binaries with disks is significantlysmaller than that of single stars. Stars with evidence of accretion showcircumstellar disks; youth of cluster members is confirmed by thelithium abundance consistent with the initial content. Conclusions: .Asindicated by the disk-locking picture, stars with disks in general haverotational velocities lower than stars without disks. Binaries in NGC6530 seem have undergone significant disk evolution.Full Tables 1 and 3 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/462/123
| WIYN open cluster study. XXVI. Improved kinematic membership and spectroscopy of IC 2391
Context: Young open clusters provide important clues to the interfacebetween the main sequence and pre-main-sequence phases of stellarevolution. The young and nearby open cluster IC 2391 is well-suited tostudies of these two evolutionary phases. Aims: We establish a bonafide set of cluster members and then analyze this set in terms of binaryfrequency, projected rotational velocities, [Fe/H], and lithiumabundance. In the wake of the Hipparcos distance controversy for thePleiades, we compare the main-sequence fitting distance modulus to theHipparcos mean parallax for IC 2391. Methods: We have obtained newproper motions for 6991 stars down to V ˜ 13-16 over a~9-deg2 area of the sky comprising IC 2391. The precision ofproper motions, σ_μ=1.7 mas yr-1, allowed us tocalculate reliable membership probabilities. We also obtained preciseradial velocity and v sin i measurements with Coravel and FEROS for 76probable cluster members. The cluster's mean radial velocity is+14.8±0.7 km s-1. The FEROS high-resolution spectrawere used to determine both the [Fe/H] abundance in the four mainsequence dwarfs of IC 2391 and the Li abundance in 47 stars. Inaddition, new BV CCD photometry was obtained for the majority ofprobable cluster members. Results: The proper-motion survey covers a 6times larger sky area than the prior targeted searches for clustermembers in IC 2391. A total of 66 stars are considered bona fide clustermembers down to a mass equivalent to 0.5 Mȯ. A quarterof them have been newly identified with many in the F2-K5 spectralrange, which is crucial for a main-sequence fit. We find a mean [Fe/H]value of +0.06±0.06, when a solar abundance of log ɛ(Fe)= 7.45 is adopted. The main sequence fitting yields a distance modulusthat is 0.19 mag larger than that derived from Hipparcos parallaxes;thus this offset nearly has the size of a similar offset found for thePleiades. The Li abundance pattern is similar to the earlier findingsand is typical for a 40 Myr old open cluster. Conclusions: .A varietyof new data on the probable members of IC 2391 improve essentially allobservational parameters of this young open cluster.
| 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.
| Debris Disk Evolution around A Stars
We report 24 and/or 70 μm measurements of ~160 A-type main-sequencestars using the Multiband Imaging Photometer for Spitzer (MIPS). Theirages range from 5 to 850 Myr, based on estimates from the literature(cluster or moving group associations) or from the H-R diagram andisochrones. The thermal infrared excess is identified by comparing thedeviation (~3% and ~15% at the 1 σ level at 24 and 70 μm,respectively) between the measurements and the synthetic Kuruczphotospheric predictions. Stars showing excess infrared emission due tostrong emission lines or extended nebulosity seen at 24 μm areexcluded from our sample; therefore, the remaining infrared excesses arelikely to arise from circumstellar debris disks. At the 3 σconfidence level, the excess rate at 24 and 70 μm is 32% and >=33%(with an uncertainty of 5%), considerably higher than what has beenfound for old solar analogs and M dwarfs. Our measurements placeconstraints on the fractional dust luminosities and temperatures in thedisks. We find that older stars tend to have lower fractional dustluminosity than younger ones. While the fractional luminosity from theexcess infrared emission follows a general 1/t relationship, the valuesat a given stellar age vary by at least 2 orders of magnitude. We alsofind that (1) older stars possess a narrow range of temperaturedistribution peaking at colder temperatures, and (2) the disk emissionat 70 μm persists longer than that at 24 μm. Both results suggestthat the debris disk clearing process is more effective in the innerregions.
| Search for associations containing young stars (SACY). I. Sample and searching method
We report results from a high-resolution optical spectroscopic surveyaimed to search for nearby young associations and young stars amongoptical counterparts of ROSAT All-Sky Survey X-ray sources in theSouthern Hemisphere. We selected 1953 late-type (B-V~≥~0.6),potentially young, optical counterparts out of a total of 9574 1RXSsources for follow-up observations. At least one high-resolutionspectrum was obtained for each of 1511 targets. This paper is the firstin a series presenting the results of the SACY survey. Here we describeour sample and our observations. We describe a convergence method in the(UVW) velocity space to find associations. As an example, we discuss thevalidity of this method in the framework of the β Pic Association.
| On the origin of the ultramassive white dwarf GD50
We argue on the basis of astrometric and spectroscopic data that theultramassive white dwarf GD50 is associated with the star formationevent that created the Pleiades and is possibly a former member of thiscluster. Its cooling age (~60 Myr) is consistent with it having evolvedessentially as a single star from a progenitor with a mass M >6Msolar, so we find no need to invoke a white dwarf-whitedwarf binary merger scenario to account for its existence. This resultmay represent the first direct observational evidence that single-starevolution can produce white dwarfs with M > 1.1Msolar, aspredicted by some stellar evolutionary theories. On the basis of itstangential velocity, we also provisionally identify the ultramassive (M~ 1.2Msolar) white dwarf PG0136 + 251 as being related to thePleiades. These findings may help to alleviate the difficulties inreconciling the observed number of hot nearby ultramassive white dwarfswith the smaller number predicted by binary evolution models under theassumption that they are the products of white dwarf mergers.Based on observations collected at the European Southern Observatory,Chile. ESO No. 072.D-0362.E-mail: pdd@star.le.ac.uk
| On the age of the TW Hydrae association and 2M1207334-393254
Aims.We have estimated the age of the young moving group TW HydraeAssociation, a cohort of a few dozen stars and brown dwarfs located nearthe Sun which share the same kinematic properties and, presumably, thesame origin and age. Methods: .The chronology has been determinedby analyzing different properties (magnitudes, colors, activity,lithium) of its members and comparing them with several well-known starforming regions and open clusters, as well as theoretical models. Inaddition, by using medium-resolution optical spectra of two M8 membersof the association (2M1139 and 2M1207 - an accreting brown dwarf with aplanetary mass companion), we have derived spectral types and measuredHα and lithium equivalent widths. We have also estimated theireffective temperature and gravity, which were used to produce anindependent age estimation for these two brown dwarfs. We have alsocollected spectra of 2M1315, a candidate member with a L5 spectral typeand measured its Hα equivalent width. Results: .Our ageestimate for the association, 10+10-7 Myr, agreeswith previous values cited in the literature. In the case of the twobrown dwarfs, we have derived an age of 15+15-10Myr, which also agree with our estimate for the whole group.Conclusions: .We compared our results with recent articles published onthe same subject using other techniques, and discuss the limits of theage-dating techniques.
| Testing the Disk Regulation Paradigm with Spitzer Observations. I. Rotation Periods of Pre-Main-Sequence Stars in the IC 348 Cluster
We present 105 stellar rotation periods in the young cluster IC 348, 75of which are new detections, increasing the total number of knownperiods in this cluster to 143. The period distribution resembles thatseen in the heart of the Orion Nebula cluster by Herbst and colleagues.Stars estimated to be less massive than 0.25 Msolar show aunimodal distribution of fast rotators (P~1-2 days) and a tail of slowerrotators, while stars estimated to be more massive than 0.25Msolar show a bimodal distribution with peaks at ~2 and ~8days. We combine all published rotation periods in IC 348 with Spitzermid-IR (3.6, 4.5, 5.8, and 8.0 μm) photometry, an unprecedentedlyefficient and reliable disk indicator, in order to test the disk-brakingparadigm. We find no evidence that the tail of slow rotators in low-massstars or the long-period peak in high-mass stars are preferentiallypopulated by objects with disks as might be expected based on thecurrent disk-braking model. We find some indication that the diskfraction decreases significantly for stars with very short periods(P<~1.5 days), which is the only feature of our sample that couldpotentially be interpreted as evidence for disk braking. It has beenproposed that the observational signatures of disk braking might besignificantly masked by the intrinsic breadth of the initial perioddistribution. We argue that more rigorous modeling of angular momentumevolution and a quantitative analysis of the observational data arerequired before the disk-braking model can be regarded as inconsistentwith observations.
| The Monitor project: rotation of low-mass stars in the open cluster M34
We report on the results of a V- and i-band time-series photometricsurvey of M34 (NGC 1039) using the Wide Field Camera (WFC) on the IsaacNewton Telescope (INT), achieving better than 1 per cent precision perdata point for 13 <~ i <~ 17. Candidate cluster members wereselected from a V versus V - I colour-magnitude diagram over 14 < V< 24 (0.12 <~ M/Msolar <~ 1.0), finding 714candidates, of which we expect ~400 to be real cluster members (takinginto account contamination from the field). The mass function wascomputed, and found to be consistent with a lognormal distribution indN/d logM. Searching for periodic variable objects in the candidatemembers gave 105 detections over the mass range 0.25 ~ 5d), consistent with the work ofother authors at very low masses. Our results are interpreted in thecontext of previous work, finding that we reproduce the same generalfeatures in the rotational period distributions. A number of rapidrotators were found with velocities ~ a factor of 2 lower than in thePleiades, consistent with models of angular momentum evolution assumingsolid body rotation without needing to invoke core-envelope decoupling.
| Identification of New M Dwarfs in the Solar Neighborhood
We present the results from a spectroscopic study of 1080 nearby activeM dwarfs, selected by correlating the Two Micron All Sky Survey andROSAT catalogs. We have derived the spectral types and estimateddistances for all of our stars. The spectral types range between K5 andM6. Nearly half of our stars lie within 50 pc. We have measured theequivalent width of the Hα emission line. Our targets show anincrease in chromospheric activity from early to midspectral types, witha peak in activity around M5. Using the count rate and hardness ratiosobtained from the ROSAT catalog, we have derived the X-ray luminosities.Our stars display a ``saturation-type'' relation between thechromospheric and coronal activity. The relation is such thatlogLX/Lbol remains ``saturated'' at a value ofapproximately -3 for varying Hα equivalent width. We have found568 matches in the USNO-B catalog and have derived the tangentialvelocities for these stars. There is a slight trend of decreasingchromospheric activity with age, such that the stars with highervtan have lower Hα equivalent widths. The coronalemission, however, remains saturated at a value oflogLX/Lbol~-3 for varying tangential velocities,suggesting that the coronal activity remains saturated with age. We donot find any break in the saturation-type relation at the spectral typeat which stars become fully convective (~M3.5). Most of the stars in oursample show more coronal emission than the dMe stars in the Hyades andPraesepe and have vtan<40 km s-1, suggesting ayoung population.
| Study of the nearest open clusters and the associated moving clusters by numerical simulations
In our previous papers, we showed that at the final phases of thedynamical evolution of an open cluster, an extended population of starselongated along its Galactic orbit, the stellar tail of the cluster, isformed. The tail stars that escaped from the cluster at different timesmove in a common orbit with low relative velocities. Experiencing a weakinteraction with Galactic field stars, these objects, the relics of openclusters, can exist for a fairly long time. In this paper, weinvestigate the structures of such stellar tails in the nearest openclusters: Hyades, Pleiades, Praesepe, Alpha Persei, Coma, IC 2391, andIC 2602. To this end, we performed several numerical simulations of thedynamical evolution of these clusters in the tidal field of the Galaxy.Our computations of the dynamical evolution were based on known clusterage estimates and real Galactic orbits. The initial conditions werechosen in such a way that the parameters of the simulated clusterscorresponded to their observed parameters. As a result, we obtainedmodels of the stellar tails for the nearest open clusters and estimatedsuch parameters of the tails as their sizes, densities, locationsrelative to the solar neighborhood, and others.
| The effect of heavy element opacity on pre-main sequence Li depletion
Context: .Recent 3-D analysis of the solar spectrum data suggests asignificant change of the solar chemical composition. This may affectthe temporal evolution of the surface abundance of light elements sincethe extension of the convective envelope is largely affected by theinternal opacity value. Aims: .We analyse the influence of theadopted solar mixture on the opacity in the convective envelope ofpre-main sequence (PMS) stars and thus on PMS lithium depletion. Thesurface Li abundance depends on the relative efficiency of severalprocesses, some of them still not known with the required precision;this paper thus analyses one of the aspects of this "puzzle".Methods: .Focusing on PMS evolution, where the largest amount of Liburning occurs, we computed stellar models for three selected masses(0.8, 1.0 and 1.2 Mȯ, with Z=0.013, Y=0.27, α=1.9)by varying the chemical mixture, that is the internal elementdistribution in Z. We analysed the contribution of the single elementsto the opacity at the temperatures and densities of interest for Lidepletion. Several mixtures were obtained by varying the abundance ofthe most important elements one at a time; we then calculated thecorresponding PMS Li abundance evolution. Results: .We found thata mixture variation does change the Li abundance: at fixed totalmetallicity, the Li depletion increases when increasing the fraction ofelements heavier than O.
| Kinematics of the Open Cluster System in the Galaxy
Absolute proper motions and radial velocities of 202 open clusters inthe solar neighborhood, which can be used as tracers of the Galacticdisk, are used to investigate the kinematics of the Galaxy in the solarvicinity, including the mean heliocentric velocity components(u1,u2,u3) of the open cluster system,the characteristic velocity dispersions(σ1,σ2,σ3), Oortconstants (A,B) and the large-scale radial motion parameters (C,D) ofthe Galaxy. The results derived from the observational data of propermotions and radial velocities of a subgroup of 117 thin disk young openclusters by means of a maximum likelihood algorithm are:(u1,u2,u3) =(-16.1+/-1.0,-7.9+/-1.4,-10.4+/-1.5) km s-1,(σ1,σ2,σ3) =(17.0+/-0.7,12.2+/-0.9,8.0+/-1.3) km s-1,(A,B) =(14.8+/-1.0,-13.0+/-2.7) km s-1 kpc-1, and (C,D) =(1.5+/-0.7,-1.2+/-1.5) km s-1 k pc-1. A discussionon the results and comparisons with what was obtained by other authorsis given.
| The Nearest Young Moving Groups
The latest results in the research of forming planetary systems have ledseveral authors to compile a sample of candidates for searching forplanets in the vicinity of the Sun. Young stellar associations areindeed excellent laboratories for this study, but some of them are notclose enough to allow the detection of planets through adaptive opticstechniques. However, the existence of very close young moving groups cansolve this problem. Here we have compiled the members of the nearestyoung moving groups, as well as a list of new candidates from ourcatalog of late-type stars that are possible members of young stellarkinematic groups, studying their membership through spectroscopic andphotometric criteria.
| 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.
| Deep X-ray survey of the young open cluster NGC 2516 with XMM-Newton
Aims.We report a deep X-ray survey of the young (~140 Myr), rich opencluster NGC 2516 obtained with the EPIC camera on board the XMM-Newtonsatellite. Methods: .By combining data from six observations, ahigh sensitivity, greater than a factor of 5 with respect to recentChandra observations, has been achieved. Kaplan-Meier estimators of thecumulative X-ray luminosity distribution are built, statisticallycorrected for non members contaminants and compared to those of thenearly coeval Pleiades. The EPIC spectra of the X-ray brightest starsare fitted using optically thin model plasma with one or two thermalcomponents. Results: .We detected 431 X-ray sources and 234 ofthem have as optical counterparts cluster stars spanning the entire NGC2516 Main Sequence. On the basis of X-ray emission and opticalphotometry, we indicate 20 new candidate members of the cluster; at thesame time we find 49 X-ray sources without known optical or infraredcounterpart. The X-ray luminosities of cluster stars span the range logLX (erg s-1) = 28.4-30.8. The representativetemperatures span the 0.3-0.6 keV (3.5-8 MK) range for the coolcomponent and 1.0-2.0 keV (12-23 MK) for the hot one; similar values arefound in other young open clusters like the Pleiades, IC 2391, andBlanco 1. While no significant differences are found in X-ray spectra,NGC 2516 solar type stars are definitely less luminous in X-rays thanthe nearly coeval Pleiades. The comparison with a previous ROSAT surveyreveals the lack of variability amplitudes larger than a factor of 2 insolar type stars in a 11 yr time scale of the cluster and thusactivity cycles like in the Sun are probably absent or have a differentperiod and amplitude in young stars.
| Searching for links between magnetic fields and stellar evolution. I. A survey of magnetic fields in open cluster A- and B-type stars with FORS1
Context: .About 5% of upper main sequence stars are permeated by astrong magnetic field, the origin of which is still matter of debate. Aims: . With this work we provide observational material to studyhow magnetic fields change with the evolution of stars on the mainsequence, and to constrain theory explaining the presence of magneticfields in A and B-type stars. Methods: . Using FORS1 inspectropolarimetric mode at the ESO VLT, we have carried out a survey ofmagnetic fields in early-type stars belonging to open clusters andassociations of various ages. Results: . We have measured themagnetic field of 235 early-type stars with a typical uncertainty of 100 G. In our sample, 97 stars are Ap or Bp stars. For thesetargets, the median error bar of our field measurements was 80 G.A field has been detected in about 41 of these stars, 37 of which werenot previously known as magnetic stars. For the 138 normal A and B-typestars, the median error bar was 136 G, and no field was detected in anyof them.
| 51 Eridani and GJ 3305: A 10-15 Myr old Binary Star System at 30 Parsecs
Following the suggestion of Zuckerman and coworkers, we consider theevidence that 51 Eri (spectral type F0) and GJ 3305 (M0), historicallyclassified as unrelated main-sequence stars in the solar neighborhood,are instead a wide physical binary system and members of the youngβ Pic moving group (BPMG). The BPMG is the nearest (d<~50 pc) ofseveral groups of young stars with ages around 10 Myr that arekinematically convergent with the Oph-Sco-Cen association (OSCA), thenearest OB star association. Combining South African AstronomicalObservatory optical photometry, Hobby-Eberly Telescope high-resolutionspectroscopy, Chandra X-Ray Observatory data, and Second US NavalObservatory CCD Astrograph Catalog kinematics, we confirm with highconfidence that the system is indeed extremely young. GJ 3305 itselfexhibits very strong magnetic activity but has rapidly depleted most ofits lithium. The 51 Eri/GJ 3305 system is the westernmost known memberof the OSCA, lying 110 pc from the main subgroups. The system is similarto the BPMG wide binary HD 172555/CD -64 1208 and the HD 104237 quintet,suggesting that dynamically fragile multiple systems can survive theturbulent environments of their natal giant molecular cloud complexes,while still having high dispersion velocities imparted. Nearby youngsystems such as these are excellent targets for evolved circumstellardisk and planetary studies, having stellar ages comparable to that ofthe late phases of planet formation.
| Fundamental properties of pre-main sequence stars in young, southern star forming regions: metallicities
Aims. The primary motivation for this project is to search formetal-rich star forming regions, in which, stars of super-solarmetallicity will be created, as hopefully, will be extra-solar planetsorbiting them! The two aims of this project are: 1) to show that oursample stars are young, lithium rich, magnetically active andnon-accreting kinematic members of their respective regions. 2) Tomeasure the metallicity for such members. Methods. The feroséchelle spectrograph together with eso's 2.2 m telescope, wasused to obtain high resolution (R = 32 000) spectra for each of ourweak-lined T-Tauri target stars. The wavelength range of the spectra is≃4000{-}8000 Å. Results. We find (pre-main sequence)model-dependent isochronal ages of the Lupus, Chamaeleon and CrA targetsto be 9.1 ± 2.1 Myr, 4.5 ± 1.6 Myr and 9.0 ± 3.9Myr respectively. The majority of the stars have Li i 6707.8 Åequivalent widths similar to, or above those of, their similar massPleiades counterparts, confirming their youthfulness. Most stars arekinematic members, either single or binary, of their regions. We find amean radial velocity for objects in the Lupus cloud to be RV=+2.6± 1.8 km s-1, for the Chamaeleon i & ii clouds,RV=+12.8 ± 3.6 km s-1 whereas for the CrA cloud, wefind RV=-1.1 ± 0.5 km s-1. All stars are coronally andchromospherically active, exhibiting X-ray and Hα emission levelsmarginally less, approximately equal or superior to that of their olderIC 2602/2391 and/or Pleiades counterparts. All bar three of the targetsshow little or no signature of accretion from a circumstellarenvironment, according to their positions in a J-K/H-K'diagram. For the higher quality spectra, we have performed an iron-linemetallicity analysis for five (5) stars in Chamaeleon, four (4) stars inLupus and three (3) stars in the CrA star forming regions. These resultsshow that all three regions are slightly metal-poor, with marginallysub-solar metallicities, with <[Fe/H]> = -0.11 ± 0.14,-0.10 ± 0.04 and -0.04 ± 0.05 respectively. Conclusions. Asample of stars in several nearby, young star-forming regions has beenestablished, the majority of which is young, lithium rich, magneticallyactive and are non-accreting kinematic members of their respectiveclouds. Within the errors, each region is essentially of solarmetallicity.
| Spectral synthesis analysis and radial velocity study of the northern F-, G- and K-type flare stars
In this paper, we present a study of the general physical and chemicalproperties and radial velocity monitoring of young active stars. Wederive temperatures, logg, [Fe/H], v sini and Rspec valuesfor eight stars. The detailed analysis reveals that the stars are nothomogeneous in their principal physical parameters or in the agedistribution. In 4/5, we found a periodic radial velocity signal whichoriginates in surface features; the fifth is surprisingly inactive andshows little variation.
| Rotation- and temperature-dependence of stellar latitudinal differential rotation
More than 600 high resolution spectra of stars with spectral type F andlater were obtained in order to search for signatures of differentialrotation in line profiles. In 147 stars the rotation law could bemeasured, with 28 of them found to be differentially rotating.Comparison to rotation laws in stars of spectral type A reveals thatdifferential rotation sets in at the convection boundary in theHR-diagram; no star that is significantly hotter than the convectionboundary exhibits the signatures of differential rotation. Four lateA-/early F-type stars close to the convection boundary and at v sin{i}≈ 100 km s-1 show extraordinarily strong absolute shear atshort rotation periods around one day. It is suggested that this is dueto their small convection zone depth and that it is connected to anarrow range in surface velocity; the four stars are very similar inTeff and v sin{i}. Detection frequencies of differentialrotation α = ΔΩ/Ω > 0 were analyzed in starswith varying temperature and rotation velocity. Measurable differentialrotation is more frequent in late-type stars and slow rotators. Thestrength of absolute shear, ΔΩ, and differential rotationα are examined as functions of the stellar effective temperatureand rotation period. The highest values of ΔΩ are found atrotation periods between two and three days. In slower rotators, thestrongest absolute shear at a given rotation rateΔΩmax is given approximately byΔΩmax ∝ P-1, i.e.,αmax ≈ const. In faster rotators, bothαmax and ΔΩmax diminish lessrapidly. A comparison with differential rotation measurements in starsof later spectral type shows that F-stars exhibit stronger shear thancooler stars do and the upper boundary in absolute shear ΔΩwith temperature is consistent with the temperature-scaling law found inDoppler Imaging measurements.
| Revisiting the population of Galactic open clusters
We present results of a study of the galactic open cluster populationbased on the all-sky catalogue ASCC-2.5 (I/280A) compiled from Tycho-2,Hipparcos and other catalogues. The sample of optical clusters fromASCC-2.5 is complete up to about 850 pc from the Sun. The symmetry planeof the clusters' distribution is determined to be at Z_0=-22±4pc, and the scale height of open clusters is only 56±3 pc. Thetotal surface density and volume density in the symmetry plane areΣ= 114 kpc-2 and D(Z_0)=1015 kpc-3,respectively. We find the total number of open clusters in the Galacticdisk to be of order of 105 at present. Fluctuations in thespatial and velocity distributions are attributed to the existence offour open cluster complexes (OCCs) of different ages containing up to afew tens of clusters. Members in an OCC show the same kinematicbehaviour, and a narrow age spread. We find, that the youngest clustercomplex, OCC 1 (log t<7.9), with 19 deg inclination to the Galacticplane, is apparently a signature of Gould's Belt. The most abundant OCC2 complex has moderate age (log t≈8.45). The clusters of thePerseus-Auriga group, having the same age as OCC 2, but differentkinematics are seen in breaks between Perseus-Auriga clouds. The oldest(log t≈8.85) and sparsest group was identified due to a large motionin the Galactic anticentre direction. Formation rate and lifetime ofopen clusters are found to be 0.23±0.03 kpc-2Myr-1 and 322±31 Myr, respectively. This implies atotal number of cluster generations in the history of the Galaxy between30 to 40. We estimate that less than about 10% of the total Galacticstellar disk population has ever passed an open cluster membership.
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