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X-Ray Observations of the Young Cluster NGC 2264
We present results from an X-ray imaging survey of the young cluster NGC2264, carried out with the European Photon Imaging Cameras (EPIC) onboard the XMM-Newton spacecraft. The X-ray data are merged with extantoptical and near-infrared photometry, spectral classifications, Hαemission strengths, and rotation periods to examine theinterrelationships between coronal and chromospheric activity, rotation,stellar mass, and internal structure for a statistically significantsample of pre-main-sequence stars. A total of 300 distinct X-ray sourcescan be identified with optical or near-infrared counterparts. Thesources are concentrated within three regions of the cluster: in thevicinity of S Mon, within the large emission/reflection nebulositysouthwest of S Mon, and along the broad ridge of molecular gas thatextends from the Cone Nebula to the NGC 2264 IRS 2 field. From theextinction-corrected color-magnitude diagram of the cluster, ages andmasses for the optically identified X-ray sources are derived. A medianage of ~2.5 Myr and an apparent age dispersion of ~5 Myr are suggestedby pre-main-sequence evolutionary models. The X-ray luminosity of thedetected sources appears well correlated with bolometric luminosity,although there is considerable scatter in the relationship. Stellar masscontributes significantly to this dispersion, while isochronal age androtation do not. X-ray luminosity and mass are well correlated such thatLX~(M/Msolar)1.5, similar to therelationship found within the younger Orion Nebula Cluster. No strongevidence is found for a correlation between EH-K, thenear-infrared color excess, and the fractional X-ray luminosity, whichsuggests that optically thick dust disks have little direct influence onthe observed X-ray activity levels. Among the X-ray-detected weak-line TTauri stars, the fractional X-ray luminosity,LX/Lbol, is moderately well correlated with thefractional Hα luminosity, LHα/Lbol,but only at the 2 σ level of significance. The cumulativedistribution functions for the X-ray luminosities of the X-ray-detectedclassical and weak-line T Tauri stars within the cluster are comparable,assuming the demarcation between the two classes is at an Hαequivalent width of 10 Å. However, if the nondetections in X-raysfor the entire sample of Hα emitters known within the cluster aretaken into account, then the cumulative distribution functions of thesetwo groups are clearly different, such that classical T Tauri stars areunderdetected by at least a factor of 2 relative to the weak-line TTauri stars. Examining a small subsample of X-ray-detected stars thatare probable accretors based on the presence of strong Hα emissionand near-infrared excess, we conclude that definitive nonaccretors are~1.6 times more X-ray luminous than their accreting counterparts. Inagreement with earlier published findings for the Orion Nebula Cluster,we find a slight positive correlation (valid at the 2 σ confidencelevel) between LX/Lbol and rotation period in NGC2264. The lack of a strong anticorrelation between X-ray activity androtation period in the stellar population of NGC 2264 suggests thateither the deeply convective T Tauri stars are rotationally saturated orthe physical mechanism responsible for generating magnetic fields inpre-main-sequence stars is distinct from the one that operates inevolved main-sequence stars.
| Tracing mixing in stars: new beryllium observations of the open clusters NGC 2516, Hyades, and M 67
Context: Determinations of beryllium abundance in stars, together withlithium, provide a key tool to investigate the so far poorly understoodextra-mixing processes at work in stellar interiors. Aims: We measuredBe in three open clusters, complementing existing Be surveys, and aimingat gathering a more complete empirical scenario of the evolution of Beas a function of stellar age and temperature. Methods: We analyzedVLT/UVES spectra of members of NGC 2516, theHyades, and M 67 to determinetheir Be and Li abundances. In the first two clusters we focused onstars cooler than 5400 K, while the M 67 sample includes stars warmerthan 6150 K, as well as two subgiants and two blue stragglers. We alsocomputed the evolution of Be for a 0.9 Mȯ star based onstandard evolutionary models. Results: We find different behavioursfor stars in different temperature bins and ages. Stars warmer than 6150K show Be depletion and follow a Be vs. Li correlation, while Be isundepleted in stars in the 6150{-}5600 K range. NGC 2516 members coolerthan 5400 K have not depleted any Be, while older Hyades of similartemperature show some depletion. Be is severely depleted in thesubgiants and blue stragglers. Conclusions: The results for warm starsare in agreement with those of previous studies, supporting thehypothesis that mixing in this temperature regime is driven by rotation.The same holds for the two subgiants that have evolved from the“Li gap”. This mechanism is instead not the dominant one forsolar-type stars. We show that Be depletion of cool Hyades cannot simplybe explained by the effect of increasing depth of the convective zone.Finally, the different Be content of the two blue stragglers suggeststhat they have formed by two different processes (i.e., collisions vs.binary merging).Based on observations collected at ESO-VLT, Paranal Observatory, Chile,Programme numbers 65.L-0427, 68.D-0491, 69.D-0454.
| 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%.
| Spitzer Observations of NGC 2362: Primordial Disks at 5 Myr
We present results from a mid-infrared imaging survey of the ~5 Myr oldcluster NGC 2362 carried out with the Infrared Array Camera (IRAC) onboard the Spitzer Space Telescope. The archival mid-infrared data weremerged with extant Hα emission data, optical and near-infraredphotometry, and moderate-resolution optical spectroscopy to identify theremnant disk-bearing population of the cluster and to estimate thefraction of stars that still retain primordial circumstellar disks. Theprincipal sample of 232 suspected cluster members with masses rangingfrom ~10 to 0.3 Msolar (B2-M5 spectral types) was drawn fromknown Hα emission stars, X-ray-detected stars from a single 100 ksarchival Chandra observation, and established lithium-rich stars. Asecond sample of 153 stars over a similar mass range whose membershipstatus was based on optical photometry alone was also examined. Measuredfluxes in the optical and infrared passbands were fitted with synthetic,low-resolution spectra created using the NextGen atmospheric models,permitting the detection of infrared excesses relative to predictedstellar photospheric fluxes. Using the measured slope of the stellarspectral energy distribution through the four IRAC channels tocharacterize disk emission for the 195 out of 232activity/lithium-selected stars and the 105 out of 153 photometricmembership candidates having complete IRAC photometry, we derive anupper limit for the primordial, optically thick disk fraction of NGC2362 of ~7%+/-2%, with another ~12%+/-3% of suspected members exhibitinginfrared excesses indicative of weak or optically thin disk emission.The presence of circumstellar disks among candidate members of NGC 2362is strongly mass-dependent, such that no stars more massive than ~1.2Msolar exhibit significant infrared excess shortward of 8μm. An upper limit for the fraction of stars hosting primordial,optically thick disks peaks near 10.7%+/-4% for stars with massesbetween 1.05 and 0.6 Msolar, but the Spitzer IRAC survey issensitivity-limited below ~0.3 Msolar. From Hαemission-line strengths, an upper limit for the accretion fraction ofthe cluster is estimated at ~5%, with most suspected accretorsassociated with primordial, optically thick disks identified withSpitzer. The presence of primordial disk-bearing stars in NGC 2362, someof which are suspected of still experiencing gaseous accretion, mayimply that even within dense cluster environments, sufficient numbers ofinner disks survive to ages consistent with core accretion models ofgiant planet formation to account for the observed frequency ofexoplanets within 5 AU of all FGKM-type stars.
| 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)].
| Rotation and chemical abundances of Ap/Bp stars in the open cluster NGC 6475
The connection between age, rotation and chemical abundance of magneticAp stars is poorly understood. Using open clusters, we are able to studysamples of stars that are both coeval and co-environmental. Bydetermining rotation and chemical abundance for Ap star members ofclusters with various ages, the variations of these properties as afunction of age and environment can be derived. All four probable Apstar members of the open cluster NGC 6475, as well as one normal late Bstar, were studied using detailed spectrum synthesis of high-resolutionUVES-POP spectra. Probable cluster membership was confirmed for all fivestars; however, chemical abundance anomalies only appear to be presentin spectra of three. Projected rotational velocity and chemicalabundances for 21 elements ranging from C to Eu are presented for thefive stars. In the three peculiar stars, we find overabundances of Si,Cr, Mn, Fe and rare earths, such as Nd, characteristic of Ap stars. Theset of chemically peculiar stars show fairly homogeneous abundancetables, however, notable differences exist for a few elements. Therealso exist appreciable differences in the v sini and main-sequenceevolutionary stage of the chemically peculiar stars. This may hint atthe underlying processes giving rise to the observed abundanceanomalies. With this first detailed study of chemical abundances of acomplete sample of magnetic Ap/Bp stars in an open cluster, we haveinitiated an exploration of the environmental and evolutionary influenceon chemical peculiarity.
| 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.
| Beryllium abundance in turn-off stars of NGC 6752
Aims.To measure the beryllium abundance in two TO stars of the GlobularCluster NGC 6752, one oxygen rich and sodium poor, the other presumablyoxygen poor and sodium rich. Be abundances in these stars are used toput on firmer grounds the hypothesis of Be as cosmochronometer and toinvestigate the formation of Globular Clusters. Methods: We presentnear UV spectra with resolution R˜ 45 000 obtained with the UVESspectrograph on the 8.2 m VLT Kueyen telescope, analysed with spectrumsynthesis based on plane parallel LTE model atmospheres. Results: Be isdetected in the O rich star with log(Be/H) = -12.04 ±0.15, whileBe is not detected in the other star for which we obtain the upper limitlog(Be/H) < -12.2. A large difference in nitrogen abundance (1.6 dex)is found between the two stars. Conclusions: .The Be measurement iscompatible with what found in field stars with the same [Fe/H] and[O/H]. The "Be age" of the cluster is found to be 13.3 Gyr, in excellentagreement with the results from main sequence fitting and stellarevolution. The presence of Be confirms the results previously obtainedfor the cluster NGC 6397 and supports the hypothesis that Be can be usedas a clock for the early formation of the Galaxy. Since only an upperlimit is found for the star with low oxygen abundance, we cannot decidebetween competing scenarios of Globular Cluster formation, but we canexclude that "polluted" stars are substantially younger than"unpolluted" ones. We stress that the Be test might be the onlymeasurement capable of distinguishing between these scenarios.Based on observations collected at the ESO VLT, Paranal Observatory,Chile, program 075.D-0807(A).
| Improved age constraints for the AB Doradus quadruple system. The binary nature of AB Doradus B
We present resolved NACO photometry of the close binary AB Dor B in H-and Ks-band. AB Dor B is itself known to be a wide binary companion toAB Dor A, which in turn has a very low-mass close companion named AB DorC. These four known components make up the young and dynamicallyinteresting system AB Dor, which will likely become a benchmark systemfor calibrating theoretical pre-main sequence evolutionary mass tracksfor low-mass stars. However, for this purpose the actual age has to beknown, and this subject has been a matter of discussion in the recentscientific literature. We compare our resolved photometry of AB Dor Baand Bb with theoretical and empirical isochrones in order to constrainthe age of the system. This leads to an age estimate of about 50 to 100Myr. We discuss the implications of such an age range for the case of ABDor C, and compare with other results in the literature.Based on observations collected at the European Southern Observatory,Chile (NACO SDI commissioning run, February 2004).
| New brown dwarfs in Upper Sco using UKIDSS Galactic Cluster Survey science verification data
We present first results from a deep (J = 18.7), wide-field(6.5deg2) infrared (ZY JHK) survey in the Upper Scoassociation conducted within the science verification phase of theUnited Kingdom Infrared Telescope Infrared Deep Sky Survey GalacticCluster Survey (GCS). Cluster members define a sequence well separatedfrom field stars in the (Z - J, Z) colour-magnitude diagram. We haveselected a total of 164 candidates with J = 10.5-18.7 mag from the (Z -J, Z) and (Y - J, Y) diagrams. We further investigated the location ofthose candidates in the other colour-magnitude and colour-colourdiagrams to weed out contaminants. The cross-correlation of the GCScatalogue with the Two-Micron All-Sky Survey data base confirms themembership of 116 photometric candidates down to 20 Jupiter masses asthey lie within a 2σ circle centred on the association meanmotion. The final list of cluster members contains 129 sources withmasses between 0.3 and 0.007 Msolar. We extracted a dozen newlow-mass brown dwarfs below 20 MJup, the limit of previoussurveys in the region. Finally, we have derived the mass function inUpper Sco over the 0.3-0.01 Msolar mass range, best fit by asingle segment with a slope of index α = 0.6 +/- 0.1, in agreementwith previous determination in open clusters.Based on observations made with the United Kingdom Infrared Telescope,operated by the Joint Astronomy Centre on behalf of the UK ParticlePhysics and Astronomy Research Council.E-mail: nl41@star.le.ac.uk
| Spitzer 24 μm Observations of Open Cluster IC 2391 and Debris Disk Evolution of FGK Stars
We present 24 μm Spitzer MIPS photometric observations of the ~50 Myropen cluster IC 2391. Thirty-four cluster members ranging in spectraltype from B3 to M5 were observed in the central square degree of thecluster. Excesses indicative of debris disks were discovered around oneA star, six FGK stars, and possibly one M dwarf. For the cluster membersobserved to their photospheric limit, we find a debris disk frequency of10+17-3% for B-A stars and31+13-9% for FGK stars using a 15% relative excessthreshold. Relative to a model of decaying excess frequency, thefrequency of debris disks around A-type stars appears marginally low forthe cluster's age while that of FGK stars appears consistent. Scenariosthat may qualitatively explain this result are examined. We concludethat planetesimal activity in the terrestrial region of FGK stars iscommon in the first ~50 Myr and decays on timescales of ~100 Myr.Despite luminosity differences, debris disk evolution does not appear todepend strongly on stellar mass.
| 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.
| Search for cold debris disks around M-dwarfs
Debris disks are believed to be related to planetesimals left overaround stars after planet formation has ceased. The frequency of debrisdisks around M-dwarfs which account for 70% of the stars in the Galaxyis unknown while constrains have already been found for A- to K-typestars. We have searched for cold debris disks around 32 field M-dwarfsby conducting observations at λ = 850~μm with the SCUBAbolometer array camera at the JCMT and at λ = 1.2 mm with theMAMBO array at the IRAM 30-m telescopes. This is the first survey of alarge sample of M-dwarfs conducted to provide statistical constraints ondebris disks around this type of stars. We have detected a new debrisdisk around the M0.5 dwarf GJ 842.2 at λ = 850~μm, providingevidence for cold dust at large distance from this star (~300 AU). Bycombining the results of our survey with the ones of Liu et al. (2004),we estimate for the first time the detection rate of cold debris disksaround field M-dwarfs with ages between 20 and 200 Myr. This detectionrate is 13+6-8% and is consistent with thedetection rate of cold debris disks (9-23%) around A- to K-type mainsequence stars of the same age. This is an indication that cold disksmay be equally prevalent across stellar spectral types.
| 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.
| HD 203030B: An Unusually Cool Young Substellar Companion near the L/T Transition
We present the discovery of a brown dwarf companion to the star HD203030: a G8 V Solar analog with an estimated age between 130 and 400Myr old G8 V. Separated by 11.9" (487 AU in projection) from its hoststar, HD 203030B has an estimated mass of0.023+0.008-0.011 Msolar. The K-bandspectral type of L7.5+/-0.5 places HD 203030B near the critical L/Ttransition in brown dwarfs, which is characterized by the rapiddisappearance of dust in substellar photospheres. From a comparativeanalysis with well-characterized field L/T transition dwarfs, we findthat, despite its young age, HD 203030B has a bolometric luminositysimilar to the >1 Gyr old field dwarfs. Adopting a radius fromcurrent models of substellar evolution, we hence obtain that theeffective temperature of HD 203030B is only1206+74-116 K, markedly lower than the ~1440 Keffective temperatures of field L/T transition dwarfs. The temperaturediscrepancy can be resolved if either (1) the ages of field brown dwarfshave been overestimated by a factor of ~1.5, leading to underestimatedradii, or (2) the lower effective temperature of HD 203030B is relatedto its young age, implying that the effective temperature at the L/Ttransition is gravity dependent.
| 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.
| The Nyquist frequency for irregularly spaced time-series: a calculation formula
Eyer & Bartholdi showed that the Nyquist frequency of irregularlysampled time-series can be very high. In this paper, a calculationformula for the Nyquist frequency is presented. In practice there is anupper limit of 0.5/Δ on this frequency, where Δ is the bestaccuracy with which time is recorded.
| OB Stars in the Solar Neighborhood. II. Kinematics
Using the spatial classification method and the structural parametersestimated for the Gould Belt (GB) and the local Galactic disk (LGD) froma previous paper, we have evaluated spatial membership probabilities fora sample of O and B stars from the Hipparcos catalog (Perryman andcoworkers) with available proper motions and radial velocity data. Thus,being able to study the space velocity fields of both systems, weconclude that the GB and the LGD present different statisticaldistributions, both in velocity space and in phase space. In light oftheir possible existence as distinct systems, we analyze differentkinematic aspects in the vicinity of the Sun, and we find the GB to beresponsible for the highly negative vertex deviation found for the OBstars in the solar neighborhood. We also find that the GB noticeablyalters the estimation of the Oort constants in the Galactic disk; thus,in order to establish comparisons with other kinematic studies based onolder stellar populations, a careful pruning of the GB members must beperformed. Further analysis of the GB velocity field and the movinggroups that contribute to it suggest the possibility that the GB can bethe result of a chance superposition of moving groups. We propose forfuture investigations the dynamical analysis of these moving groups inorder to probe the origin of the GB.
| X-ray variability of NGC 2516 stars in the XMM-Newton observations
We present the characteristics of the X-ray variability of stars in thecluster NGC 2516 as derived from XMM-Newton/EPIC/pn data. The X-rayvariations on short (hours), medium (months), and long (years) timescales have been explored. We detected 303 distinct X-ray sources byanalysing six EPIC/pn observations; 194 of them are members of thecluster. Stars of all spectral types, from the early-types to the late-Mdwarfs, were detected. The Kolmogorov-Smirnov test applied to the X-rayphoton time series shows that, on short time scales, only a relativelysmall fraction (ranging from 6% to 31% for dG and dF, respectively) ofthe members of NGC 2516 are variable with a confidence level ≥99%;however, it is possible that the fraction is small only because of thepoor statistics. The time X-ray amplitude distribution functions (XAD)of a set of dF7-dK2 stars, derived on short (hours) and medium (months)time scales, seem to suggest that medium-term variations, if present,have a much smaller amplitude than those on short time scales; a similarresult is also obtained for dK3-dM stars. The amplitude variations oflate-type stars in NGC 2516 are consistent with those of the coevalPleiades stars. Comparing these data with those of ROSAT/PSPC, collected7-8 years earlier, and of ROSAT/HRI, just 4-5 years earlier, we find noevidence of significant variability on the related time scales,suggesting that long-term variations due to activity cycles similar tothe solar cycle are not common among young stars. Indications ofspectral variability was found in one star whose spectra at three epochswere available.
| Non-gray rotating stellar models and the evolutionary history of the Orion Nebular Cluster
Context: .Rotational evolution in the pre-main sequence is describedwith new sets of pre-MS evolutionary tracks including rotation, non-grayboundary conditions (BCs) and either low (LCE) or high convectionefficiency (HCE). Aims: .Using observational data and ourtheoretical predictions, we aim at constraining (1) the differencesobtained for the rotational evolution of stars within the ONC by meansof these different sets of new models; (2) the initial angular momentumof low mass stars, by means of their templates in the ONC. Methods: .We discuss the reliability of current stellar models for thepre-MS. While the 2D radiation hydrodynamic simulations predict HCE inpre-MS, semi-empirical calibrations either seem to require thatconvection is less efficient in pre-MS than in the following MS phase(lithium depletion) or are still contradictory (binary masses). Wederive stellar masses and ages for the ONC by using both LCE and HCE. Results: .The resulting mass distribution for the bulk of the ONCpopulation is in the range 0.2-0.4 M_ȯ for our new non-gray modelsand, as in previous analyse, in the range 0.1-0.3 M_ȯ for modelshaving gray BCs. In agreement with Herbst et al. (2002) we find that alarge percentage (~70%) of low-mass stars (M ⪉ 0.5 M_ȯ for LCE;M ⪉ 0.35 M_ȯ for HCE) in the ONC appears to be fast rotators(P<4 days). Three possibilities are open: 1) ~70% of the ONC low massstars lose their disk at early evolutionary phases; 2) their "lockingperiod" is shorter; 3) the period evolution is linked to a differentmorphology of the magnetic fields of the two groups of stars. We alsoestimate the range of initial angular momentum consistent with theobserved periods. Conclusions: .The comparisons made indicatethat a second parameter is needed to describe convection in the pre-MS,possibly related to the structural effect of a dynamo magnetic field.
| 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.
| X-ray emission from T Tauri stars in the Lupus 3 star-forming region
Aims.In this paper, I present analysis results of an {XMM-Newton}observation of the Lupus 3 region that contains a high proportion ofyoung low mass (M < 0.3 Mȯ) T Tauri stars in theLupus star-forming complex. Methods: .The detection of X-raysources in 0.5 to 4.5 keV images of the Lupus 3 core was performed usingthe standard source detection method of the {XMM-Newton} ScienceAnalysis Software. The detected sources were correlated with a list ofHerbig-Haro objects and Hα emission stars that contains mainlyclassical T Tauri stars, with a catalogue of weak-line T Tauri Stars andwith a recent list of new low-mass members of the Lupus 3 dark cloudfound in a visible-light spectroscopic survey at the center of the Lupus3 star-forming core. The light curves and spectra of the brightest X-raysources with known T Tauri star counterparts were analysed.Results: .One hundred and two X-ray sources were detected in the30´ diameter field-of-view of the EPIC cameras, of which 25 havevisible or near-IR counterparts that are known as pre-main sequencestars. Their X-ray luminosity ranges from 3 × 1028 to 3× 1030 erg s-1. Two of these objects withmass estimates lower than 0.075 Mȯ have an X-rayluminosity of about 4-7 × 1028 erg s-1,comparable with that of flaring young brown dwarfs. A linear correlationis found between the X-ray luminosity and the mass or volume of thestars that is qualitatively expected from some models of distributedturbulent dynamos. The EPIC spectra of the X-ray brightest sources canbe fitted using optically thin plasma emission models with twocomponents at temperatures in the ranges 3-9 × 106 Kand 1-50 × 107 K, respectively. The large emissionmeasure of hot plasma may be caused by disruptions of magnetic fieldsassociated with an intense flaring activity, while the X-ray emissionfrom the "cool" plasma components may result from solar-type activeregions. The emission measures of the plasma components are of the orderof 1052 cm-3, typical of the values expected fromcoronal plasmas in T Tauri stars, post-T Tauri stars, and activelate-type dwarfs in close binary systems. One property of the X-raybrightest stars in Lupus 3 that seems common among pre-main sequencestars is the low abundance of Fe.
| Methods for analysing structure in molecular clouds
We have previously reported a dimensionless measure, , which can bothquantify, and distinguish between, the extent to which a star cluster iscentrally concentrated, and the extent to which it contains small-scalesubclusters. is the ratio of the normalized correlation length, , (i.e.the mean projected separation between stars, divided by the overallradius of the cluster), to the mean length, , of the segments of aminimal spanning tree (MST) joining all star positions: .In this paper, we attempt to adapt the correlation-length method to thecharacterization of gas clouds, with a view to comparing directly thestructures of gas clouds and star clusters. We also compare the resultsof the correlation-length method with fractal dimensions estimated usingthe more familiar perimeter-area method whereby the lengths of closedcontours are plotted against the areas they enclose, on a log-log plot.We find that the normalized correlation length, when modified to dealwith pixellated grey-scale data, is a robust indicator of either centralconcentration or fractal subclustering of gas clouds, but cannotdistinguish between the two types of structure. It is, however,extremely reliable, easy to implement and works accurately at all scalesand over all dynamic ranges, even with poorly sampled data. Itimplicitly incorporates edge effects, so all the data in the completecloud are used, and it therefore provides a useful method for comparingthe structures of molecular clouds and star clusters.The normalized correlation length produces comparable results to theperimeter-area method when used on molecular cloud data. However, theperimeter-area method is unable to distinguish the degree of clusteringin three-dimensional objects with fractal dimensions greater than 2.0.It also suffers from measurement noise and lack of objectivity,particularly if only a few contours are selected for analysis. It cannotbe used to compare clouds with star clusters.It is not found possible to construct an MST algorithm which worksreliably for grey-scale data and is immune to scaling problems. Thepreviously reported parameter is therefore not useful when consideringgas clouds.
| 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.
| X-ray and optical bursts and flares in YSOs: results from a 5-day XMM-Newton monitoring campaign of L1551
We present the results of a five-day monitoring campaign with XMM-Newtonof six X-ray bright young stellar objects (YSOs) in the star-formingcomplex L1551 in Taurus. All stars present significant variability onthe five-day time scale. Modulation of the light curve on time scalescomparable with the star's rotational period appeared to be present inthe case of one weak-lined T Tauri star. Significant spectral variationsbetween the 2000 and the 2004 observations were detected in the(unresolved) classical T Tauri binary system XZ Tau: a hot plasmacomponent which was present in the X-ray spectrum in 2000 hadsignificantly weakened in 2004. As XZ Tau N was undergoing a strongoptical outburst in 2000, which had terminated since then, we speculateon the possible relationship between episodic, burst accretion, andX-ray heating. The transition object HL Tau underwent a strong flarewith a complex temperature evolution, which is indicative of an eventconfined within a very large magnetic structure (few stellar radii),similar to the ones found in YSOs in the Orion Nebula Cluster.
| Near-infrared time-series photometry of six fields in the young open cluster IC 2391
Fields containing targets of spectral types later than M5 were monitoredin JHKs for about 7h each. None of the targets showedvariability at levels exceeding 0.02 (or smaller, in some cases). Sevennew variable stars were discovered serendipitously. The large number ofmeasurements (more than 50) of each field allows the construction ofaccurate colour-magnitude and colour-colour diagrams. These could beused to identify a small number of candidate very cool objects.
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