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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.
| An X-Ray Imaging Study of the Stellar Population in RCW 49
We present the results of a high-resolution X-ray imaging study of thestellar population in the Galactic massive star-forming region RCW 49and its central OB association Westerlund 2. We obtained a ~40 ks X-rayimage of a ~17'×17' field using the ChandraX-Ray Observatory and deep NIR images using the Infrared Survey Facilityin a concentric ~8.3'×8.3' region. Wedetected 468 X-ray sources and identified optical, NIR, and Spitzer MIRcounterparts for 379 of them. The unprecedented spatial resolution andsensitivity of the X-ray image, enhanced by optical and infrared imagingdata, yielded the following results: (1) The central OB associationWesterlund 2 is resolved for the first time in the X-ray band. X-rayemission is detected from all spectroscopically identified early-typestars in this region. (2) Most (~86%) X-ray sources with optical orinfrared identifications are cluster members in comparison with acontrol field in the Galactic plane. (3) A loose constraint (2-5 kpc)for the distance to RCW 49 is derived from the mean X-ray luminosity ofT Tauri stars. (4) The cluster X-ray population consists of low-masspre-main-sequence and early-type stars as obtained from X-ray and NIRphotometry. About 30 new OB star candidates are identified. (5) Weestimate a cluster radius of 6'-7' based on the X-ray surface numberdensity profiles. (6) A large fraction (~90%) of cluster members areidentified individually using complimentary X-ray and MIR excessemission. (7) The brightest five X-ray sources, two Wolf-Rayet stars andthree O stars, have hard thermal spectra.
| Polarization Shadows of Extragalactic Sources by the Local Magnetoionic Interstellar Medium
We report depolarization of extragalactic sources in the NRAO VLA SkySurvey (NVSS) by local structures in the interstellar medium. The skydensity of polarized sources drops by a factor of 2-4 in regions withangular scales ~10°, implying up to 40% depolarization on averageper source. Some of these polarization shadows are associated with H IIregions, but three are associated with regions of depolarized diffuseGalactic emission. The absence of a correlation between the depth ofpolarization shadows and Hα intensity suggests that some shadowsare related to structure in the magnetic field. At least somepolarization shadows are caused by partial bandwidth depolarization inthe NVSS. Alternatively, some may be caused by regions with small-scale(<~1") variations in rotation measure.
| Star Clusters with the GTC
Star clusters are becoming very useful probes in the studies of manytopics in modern Astronomy. Actually, they are providing invaluableinformation about the origin, structure and star formation processes inextragalactic systems. Otherwise, most of our knowledge on howindividual stars are formed is derived from the analysis of stellarclusters resolved into stars. In this contribution we review the mainscientific objectives in this field, in the light of the firstgeneration instruments for the Gran Telescopio Canarias.
| A Spitzer Space Telescope Study of Disks in the Young σ Orionis Cluster
We report new Spitzer Space Telescope observations, using the IRAC andMIPS instruments, of the young (~3 Myr) σ Orionis cluster. Weidentify 336 stars as members of the cluster, using optical andnear-infrared color-magnitude diagrams. Using the spectral energydistribution slopes in the IRAC spectral range, we place objects intoseveral classes: non-excess stars, stars with optically thick disks(such as classical T Tauri stars), class I (protostellar) candidates,and stars with ``evolved disks'' the last exhibit smaller IRAC excessesthan optically thick disk systems. In general, this classificationagrees with the location expected in IRAC-MIPS color-color diagrams forthese objects. We find that the evolved disk systems are mostly acombination of objects with optically thick but nonflared disks,suggesting grain growth and/or settling, and transition disks, systemsin which the inner disk is partially or fully cleared of small dust. Inall, we identify seven transition disk candidates and three possibledebris disk systems. As in other young stellar populations, the fractionof disks depends on the stellar mass, ranging from ~10% for stars in theHerbig Ae/Be mass range (>2 Msolar) to ~35% for those inthe T Tauri mass range (1-0.1 Msolar). The IRAC infraredexcesses found in stellar clusters and associations with and withoutcentral high-mass stars are similar, suggesting that externalphotoevaporation is not very important in many clusters. Finally, wefind no correlation between the X-ray luminosity and the disk infraredexcess, suggesting that the X-rays are not strongly affected by diskaccretion.
| 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
| Empirical isochrones and relative ages for young stars, and the radiative-convective gap
We have selected pre-main-sequence (PMS) stars in 12 groups of notionalages ranging from 1 to 35 Myr, using heterogeneous membership criteria.Using these members we have constructed empirical isochrones in V, V - Icolour-magnitude diagrams. This allows us to identify clearly the gapbetween the radiative main sequence and the convective PMS (the R-Cgap). We follow the evolution of this gap with age and show that it canbe a useful age indicator for groups less than ~=15 Myr old. We alsoobserve a reduction in absolute spreads about the sequences with age.Finally, the empirical isochrones allow us to place the groups in orderof age, independently of theory. The youngest groups can be collatedinto three sets of similar ages. The youngest set is the ONC, NGC6530and IC5146 (nominally 1 Myr); next Cep OB3b, NGC2362, λ Ori andNGC2264 (nominally 3 Myr); and finally σ Ori and IC348 (nominally4-5 Myr). This suggests Cep OB3b is younger than previously thought, andIC348 older. For IC348 the stellar rotation rate distribution andfraction of stars with discs imply a younger age than we derive. Wesuggest this is because of the absence of O-stars in this cluster, whosewinds and/or ionizing radiation may be an important factor in theremoval of discs in other clusters.
| Characterizing Young Brown Dwarfs Using Low-Resolution Near-Infrared Spectra
We present near-infrared (1.0-2.4 μm) spectra confirming the youthand cool temperatures of six brown dwarfs and low-mass stars withcircumstellar disks toward the Chamaeleon II and Ophiuchus star-formingregions. The spectrum of one of our objects indicates a spectral type of~L1, making it one of the latest spectral type young brown dwarfsidentified to date. Comparing spectra of young brown dwarfs, fielddwarfs, and giant stars, we define a 1.49-1.56 μm H2Oindex capable of determining spectral type to +/-1 subtype, independentof gravity. We have also defined an index based on the 1.14 μm sodiumfeature that is sensitive to gravity, but only weakly dependent onspectral type. Our 1.14 μm Na index can be used to distinguish youngcluster members (τ<~5 Myr) from young field dwarfs, both of whichmay have the triangular H-band continuum shape that persists for atleast tens of Myr. Using Teff values determined from thespectral types of our objects along with luminosities derived from nearand mid-infrared photometry, we place our objects on the H-R diagram andoverlay evolutionary models to estimate the masses and ages of our youngsources. Three of our sources have inferred ages (τ~=10-30 Myr) thatare significantly older than the median stellar age of their parentclouds (1-3 Myr). For these three objects, we derive masses ~3 timesgreater than expected for 1-3 Myr old brown dwarfs with the bolometricluminosities of our sources. The large discrepancies in the inferredmasses and ages determined using two separate, yet reasonable, methodsemphasize the need for caution when deriving or exploiting brown dwarfmass and age estimates.
| The Monitor project: searching for occultations in young open clusters
The Monitor project is a photometric monitoring survey of nine young(1-200Myr) clusters in the solar neighbourhood to search for eclipses byvery low mass stars and brown dwarfs and for planetary transits in thelight curves of cluster members. It began in the autumn of 2004 and usesseveral 2- to 4-m telescopes worldwide. We aim to calibrate the relationbetween age, mass, radius and where possible luminosity, from the Kdwarf to the planet regime, in an age range where constraints onevolutionary models are currently very scarce. Any detection of anexoplanet in one of our youngest targets (<~10Myr) would also provideimportant constraints on planet formation and migration time-scales andtheir relation to protoplanetary disc lifetimes. Finally, we will usethe light curves of cluster members to study rotation and flaring inlow-mass pre-main-sequence stars.The present paper details the motivation, science goals and observingstrategy of the survey. We present a method to estimate the sensitivityand number of detections expected in each cluster, using a simplesemi-analytic approach which takes into account the characteristics ofthe cluster and photometric observations, using (tunable) best-guessassumptions for the incidence and parameter distribution of putativecompanions, and we incorporate the limits imposed by radial velocityfollow-up from medium and large telescopes. We use these calculations toshow that the survey as a whole can be expected to detect over 100 younglow and very low mass eclipsing binaries, and ~3 transiting planets withradial velocity signatures detectable with currently availablefacilities.
| A Young Stellar Cluster Surrounding the Peculiar Eruptive Variable V838 Monocerotis
V838 Monocerotis is an unusual variable star that underwent a suddenoutburst in 2002. Unlike a classical nova, which quickly evolves to hightemperatures, V838 Mon remained an extremely cool, luminous supergiantthroughout its eruption. It continues to illuminate a spectacular seriesof light echoes, as the outburst light is scattered from nearbycircumstellar dust. V838 Mon has an unresolved B3 V companion star.During a program of spectroscopic monitoring of V838 Mon, weserendipitously discovered that a neighboring 16th magnitude star isalso of type B. We then carried out a spectroscopic survey of otherstars in the vicinity, revealing two more B-type stars, all within 45"of V838 Mon. We have determined the distance to this sparse, youngcluster based on spectral classification and photometric main-sequencefitting of the three B stars. The cluster distance is found to be6.2+/-1.2 kpc, in excellent agreement with the geometric distance toV838 Mon of 6.1 kpc obtained from Hubble Space Telescope polarimetry ofthe light echoes. An upper limit to the age of the cluster is about 25Myr, and its reddening is E(B-V)=0.85.The absolute luminosity of V838 Mon during its outburst, based on ourdistance measurement, was very similar to that of M31 RV, an object inthe bulge of M31 that was also a cool supergiant throughout its eruptionin 1988. However, there is no young population at the site of M31 RV.Using our distance determination, we show that the B3 V companion ofV838 Mon is sufficient to account for the entire luminosity of thevariable star measured on sky-survey photographs before its outburst.The B3 star is currently, however, about 1 mag fainter than before theeruption, suggesting that it is now suffering extinction due to dustejected from V838 Mon. These results indicate that, whatever the natureof the progenitor object, it was not of high luminosity. Nor does itappear possible to form a nova-like cataclysmic binary system within theyoung age of the V838 Mon cluster. These considerations appear to leavestellar collision or stellar merger scenarios as one of the remainingviable explanations for the outbursts of V838 Mon and M31 RV.Based on observations made with the Small- and Medium-Aperture ResearchTelescope System (SMARTS).
| CCD photometric search for peculiar stars in open clusters. VIII. King 21, NGC 3293, NGC 5999, NGC 6802, NGC 6830, Ruprecht 44, Ruprecht 115, and Ruprecht 120
Context: We continue our survey of magnetic chemically peculiar (CP2)stars in galactic open clusters to shed more light on their origin andevolution. Aims: To study the group of CP2 stars, it is essential tofind these objects in different galactic environments and at a widerange of evolutionary stages. The knowledge of open cluster ages andmetallicities can help for finding a correlation between theseparameters and the (non-)presence of peculiarities, which has to betaken into account in stellar evolution models. Methods: Theintermediate band Δ a photometric system samples the depth of the5200 Å flux depression by comparing the flux at the centre withthe adjacent regions with bandwidths of 110 Å to 230 Å. Itis capable of detecting magnetic CP2 and CP4 stars with high efficiency,but also the groups of (metal-weak) λ Bootis and classicalBe/shell stars can be successfully investigated. In addition, it allowsthe age, reddening, and distance modulus to be determined withappropriate accuracy by fitting isochrones. Results: From the 1677observed members of the eight open clusters, one Ae and twenty-five CP2stars were identified. Furthermore nineteen deviating stars aredesignated as questionable for several reasons. The estimated age,reddening, and distance for the programme clusters were compared withpublished values of the literature and discussed in this context. Conclusions: .The current paper shows that CP2 stars are continuouslypresent in very young (7 Myr) to intermediate age (500 Myr) openclusters at distances greater than 2 kpc from the Sun.Based on observations at CASLEO, CTIO (Proposal 2003A-0057), and OSN.The Observatorio de Sierra Nevada is operated by the Consejo Superior deInvestigaciones Científicas through the Instituto deAstrofísica de Andalucía (Granada, Spain). Photometricdata are only avaialable in electronic form at the CDS via anonymous ftpto cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/462/591 Full Fig. [seefull textsee full textsee full textsee full textsee full text], Tables[see full textsee full textsee full textsee full textsee full text] and[see full textsee full textsee full textsee full textsee full text] areonly available in electronic form at http://www.aanda.org
| Chandra Observations of the Eagle Nebula. I. Embedded Young Stellar Objects near the Pillars of Creation
We present and analyze the first high-resolution X-ray images everobtained of the Eagle Nebula star-forming region. On 2001 July 30 theChandra X-Ray Observatory obtained a 78 ks image of the Eagle Nebula(M16) that includes the core of the young galactic cluster NGC 6611 andthe dark columns of dust and cold molecular gas in M16 known as the``Pillars of Creation.'' We find a total of 1101 X-ray sources in the17'×17' ACIS-I field of view. Most of theX-ray sources are low-mass pre-main-sequence or high-mass main-sequencestars in this young cluster. A handful of hard X-ray sources in thepillars are spatially coincident with deeply embedded young stellarobjects seen in high-resolution near-infrared images recently obtainedwith the VLT (McCaughrean & Andersen). In this paper, we focus onthe 40 X-ray sources in and around pillars 1-4 at the heart of the EagleNebula. None of the X-ray sources are associated with the evaporatinggaseous globules (EGGs) first observed by Hester and coworkers) in HSTWFPC2 images of M16, implying either that the EGGs do not containprotostars or that the protostars have not yet become X-ray active.Eight X-ray counts are coincident with the Herbig-Haro object HH 216,implying logLX~30.0.
| Canis Major the trusty Dog.
Not Available
| The young star cluster NGC 2362: low-mass population and initial mass function from a Chandra X-ray observation
Context: . We study the stellar population of the very young cluster NGC2362, using a deep Chandra ACIS-I X-ray observation. This cluster, only5 Myr old, has already cleared most of its inter- and circumstellardust, and with its small and uniform reddening offers a uniqueopportunity of studying its pre-main-sequence stellar population withminimal disturbance from a dense interstellar medium. Aims: .Ourmain purposes are to select cluster members down to low masses and tostudy their properties as a population (spatial properties, initial massfunction, and coronal properties). Methods: .We compare existingdeep optical photometry and Hα data with new X-ray data. We usecombined optical and X-ray criteria to select cluster members.Results: .We detect 387 X-ray sources down to log LX = 29.0(erg/s), and identify most of them (308) with star-like objects. Themajority (88%) of optically identified X-ray sources are found to bevery good candidate low-mass pre-main-sequence stars, with minimalfield-object contamination. This increases the known cluster census by asubstantial amount at low masses, with respect to previous optical/IRstudies. The fraction of stars with active accretion is found to be inthe range 5-9%. We find a significantly wider spatial distribution forlow-mass stars than for massive stars (mass segregation). We find only asmall spread around the low-mass cluster sequence in the HR diagram,indicating that star formation lasted only about 1-2 Myr. We havederived the cluster initial mass function, which appears to flatten (onthe low-mass side) at higher masses with respect to other very youngclusters. The quiescent X-ray emission of low-mass cluster stars isfound to be rather strictly correlated with the stellar bolometricluminosity: the small spread in this correlation puts an upper bound onthe amplitude of X-ray variability on time scales longer than one day(e.g., activity cycles) in such young coronal sources. We findsignificant X-ray spectral differences between low-mass stars brighterand fainter than log LX ˜ 30.3 (erg/s), respectively,with X-ray brighter stars showing hotter components (kT ˜ 2 keV),absent in fainter stars.
| The population in the background of open clusters: tracer of the Norma-Cygnus arm
We present colour-magnitude diagrams of open clusters, located in therange 112° < l < 252°, manifesting stellar populations inthe background of clusters. Some of the populations are found to belocated beyond the Perseus arm and may be the part of the Norma-Cygnus(outer) arm. The outer arm seems to be continued from l ~ 120° to~235°. The background populations follow the downward warp of theGalactic plane around l ~ 240°.
| On the current status of open-cluster parameters
We aim to characterize the current status of knowledge on the accuracyof open-cluster parameters such as the age, reddening and distance.These astrophysical quantities are often used to study the globalcharacteristics of the Milky Way down to the very local stellarphenomena. In general, the errors of these quantities are neglected orset to some kind of heuristic standard value. We attempt to give somerealistic estimates for the accuracy of available cluster parameters byusing the independently derived values published in the literature. Intotal, 6437 individual estimates for 395 open clusters were used in ourstatistical analysis. We discuss the error sources depending ontheoretical as well as observational methods and compare our resultswith those parameters listed in the widely used catalogue by Dias et al.In addition, we establish a list of 72 open clusters with the mostaccurate known parameters which should serve as a standard table in thefuture for testing isochrones and stellar models.
| Stellar Rotation in Young Clusters. II. Evolution of Stellar Rotation and Surface Helium Abundance
We derive the effective temperatures and gravities of 461 OB stars in 19young clusters by fitting the Hγ profile in their spectra. We usesynthetic model profiles for rotating stars to develop a method toestimate the polar gravity for these stars, which we argue is a usefulindicator of their evolutionary status. We combine these results withprojected rotational velocity measurements obtained in a previous paperon these same open clusters. We find that the more massive B starsexperience a spin-down as predicted by the theories for the evolution ofrotating stars. Furthermore, we find that the members of binary starsalso experience a marked spin-down with advanced evolutionary state dueto tidal interactions. We also derive non-LTE-corrected heliumabundances for most of the sample by fitting the He Iλλ4026, 4387, 4471 lines. A large number of heliumpeculiar stars are found among cooler stars withTeff<23,000 K. The analysis of the high-mass stars (8.5Msolar
| Stellar Rotation in Young Clusters. I. Evolution of Projected Rotational Velocity Distributions
Open clusters offer us the means to study stellar properties in sampleswith well-defined ages and initial chemical composition. Here we presenta survey of projected rotational velocities for a large sample of mainlyB-type stars in young clusters to study the time evolution of therotational properties of massive stars. The survey is based onmoderate-resolution spectra made with the WIYN 3.5 m and CTIO 4 mtelescopes and Hydra multi-object spectrographs, and the target starsare members of 19 young open clusters with an age range of approximately6-73 Myr. We made fits of the observed lines He I λλ4026,4387, 4471, and Mg II λ4481, using model theoretical profiles tofind projected rotational velocities for a total of 496 OB stars. Wefind that there are fewer slow rotators among the cluster B-type starsrelative to nearby B stars in the field. We present evidence consistentwith the idea that the more massive B stars (M>9 Msolar)spin down during their main-sequence phase. However, we also find thatthe rotational velocity distribution appears to show an increase in thenumbers of rapid rotators among clusters with ages of 10 Myr and higher.These rapid rotators appear to be distributed between the zero age andterminal age main-sequence locations in the Hertzsprung-Russell diagram,and thus only a minority of them can be explained as the result of aspin-up at the terminal age main sequence due to core contraction. Wesuggest instead that some of these rapid rotators may have been spun upthrough mass transfer in close binary systems.
| 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.
| Multiwavelength Analysis of the Young Open Cluster NGC 2362
We present a multiwavelength analysis of the young open cluster NGC2362; UBVRCIC CCD photometric observations,together with available data in the Chandra database, near-infrared datafrom the Two Micron All Sky Survey (2MASS), and recently publishedHα spectroscopy were used to get information about theevolutionary stage of the cluster and the main physical properties ofits stellar content. Cluster membership is estimated for everyindividual star by means of zero-age main sequence (ZAMS) and isochronefitting. The cluster is confirmed to host a rich population ofpre-main-sequence (PMS) objects and to contain a large number ofX-ray-emitting stars, which reach from the PMS members of GK spectraltype up to the most luminous OB main-sequence (MS) members. The PMScluster members show no significant age spread, and the comparison toboth PMS and post-MS isochrones suggests an homogeneous age for allcluster members. The analysis allows us to assess the validity ofcurrently used PMS evolutionary models and supports the suggestion of awell-defined positive correlation of the X-ray emission from PMS starswith their bolometric luminosity. Clear differences are found betweenthe X-ray activity properties of MS and PMS cluster members, both in therelation between X-ray luminosity and bolometric luminosity, and inspectral properties as well.
| On the difference between nuclear and contraction ages
Context: .Ages derived from low mass stars still contracting onto themain sequence often differ from ages derived from the high mass onesthat have already evolved away from it. Aims: .We investigate thegeneral claim of disagreement between these two independent agedeterminations by presenting UBVRI photometry for the young galacticopen clusters NGC 2232, NGC 2516, NGC 2547 and NGC 4755, spanning theage range ~10-150 Myr Methods: .We derived reddenings, distances,and nuclear ages by fitting ZAMS and isochrones to color-magnitudes andcolor-color diagrams. To derive contraction ages, we used four differentpre-main sequence models, with an empirically calibratedcolor-temperature relation to match the Pleiades cluster sequence.Results: .When exclusively using the V vs. V-I color-magnitude diagramand empirically calibrated isochrones, there is consistency betweennuclear and contraction ages for the studied clusters. Although thecontraction ages seem systematically underestimated, in none of thecases do they deviate by more than one standard deviation from thenuclear ages.
| 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.
| Effects of metallicity, star-formation conditions, and evolution in B and Be stars. I. Large Magellanic Cloud, field of NGC 2004
Aims.To statistically study the effects of the metallicity,star-formation conditions, and evolution on the behaviour of massivestars and, more particularly, of B and Be stars, we observed largesamples of stars in the Magellanic Clouds for the first time. In thisarticle we present the first part of this study. Methods:.Spectroscopic observations of hot stars belonging to the young clusterLMC-NGC 2004 and its surrounding region were carried out with theVLT-GIRAFFE facilities in MEDUSA mode. We determined the fundamentalparameters (T_eff, log~g, V sin i, and radial velocity) for all B and Bestars in the sample thanks to a code developed in our group. The effectof fast rotation (stellar flattening and gravitational darkening) aretaken into account in this study. We also determined the age of observedclusters. We then compared the mean V sin i obtained for field andcluster B and Be stars in the Large Magellanic Cloud (LMC) with the onesin the Milky Way (MW). Results: .We find, in particular, that Bestars rotate faster in the LMC than in the MW, in the field as well asin clusters. We discuss the relations between V sin i, metallicity,star-formation conditions, and stellar evolution by comparing the LMCwith the MW. We conclude that Be stars began their main sequence lifewith an initial rotational velocity higher than the one for B stars. Itis probable that only part of the B stars, those with a sufficientinitial rotational velocity, can become Be stars. This result mayexplain the differences in the proportion of Be stars in clusters withsimilar ages.
| Spiral structure of the third galactic quadrant and the solution to the Canis Major debate
With the discovery of the Sagittarius dwarf spheroidal, a galaxy caughtin the process of merging with the Milky Way, the hunt for other suchaccretion events has become a very active field of astrophysicalresearch. The identification of a stellar ring-like structure inMonoceros, spanning more than 100°, and the detection of anoverdensity of stars in the direction of the constellation of CanisMajor (CMa), apparently associated to the ring, has led to thewidespread belief that a second galaxy being cannibalized by the MilkyWay had been found. In this scenario, the overdensity would be theremaining core of the disrupted galaxy and the ring would be the tidaldebris left behind. However, unlike the Sagittarius dwarf, which is wellbelow the Galactic plane and whose orbit, and thus tidal tail, is nearlyperpendicular to the plane of the Milky Way, the putative CMa galaxy andring are nearly co-planar with the Galactic disc. This severelycomplicates the interpretation of observations. In this Letter, we showthat our new description of the Milky Way leads to a completelydifferent picture. We argue that the Norma-Cygnus spiral arm defines adistant stellar ring crossing Monoceros and the overdensity is simply aprojection effect of looking along the nearby local arm. Our perspectivesheds new light on a very poorly known region, the third Galacticquadrant, where CMa is located.
| L-band (3.5 μm) IR-excess in massive star formation. II. RCW 57/NGC 3576
Context: .We present a JHKsL survey of the massive star forming regionRCW 57 (NGC 3576) based on L-band data at 3.5 μm taken with SPIREX(South Pole Infrared Explorer), and 2MASS JHKs data at 1.25-2.2 μm.This is the second of two papers, the first one concerning a similarJHKsL survey of 30 Doradus. Aims: .Colour-colour andcolour-magnitude diagrams are used to detect sources with infraredexcess. This excess emission is interpreted as coming from circumstellardisks, and hence gives the cluster disk fraction (CDF). Based on the CDFand the age of RCW 57, it is possible to draw conclusions on theformation and early evolution of massive stars. Methods: .Theinfrared excess is detected by comparing the locations of sources inJHKsL colour-colour and L vs. (K_s-L) colour-magnitude diagrams to thereddening band due to interstellar extinction. Results: .A totalof 251 sources were detected. More than 50% of the 209 sources includedin the diagrams have an infrared excess. Conclusions: .Comparisonwith other JHKsL surveys, including the results on 30 Doradus from thefirst paper, support a very high initial disk fraction (>80%) evenfor massive stars, although there is an indication of a possible fasterevolution of circumstellar disks around high mass stars. 33 sources onlyfound in the L-band indicate the presence of heavily embedded, massiveClass I protostars. We also report the detection of diffuse PAHsemission throughout the RCW 57 region.
| Formation and Evolution of Planetary Systems (FEPS): Primordial Warm Dust Evolution from 3 to 30 Myr around Sun-like Stars
We present data obtained with the Infrared Array Camera (IRAC) aboardthe Spitzer Space Telescope (Spitzer) for a sample of 74 young (t<30Myr old) Sun-like (0.7
| The Pre-Main-Sequence Population of L988
L988 is a large (~0.5d×0.7d) dark cloud complex at about 600 pcthat contains several bright pre-main-sequence objects (such as V1331Cyg and LkHα 321), but this paper deals in detail only with asmall region on its eastern edge, near the HAeBe star LkHα 324.That star and its distant companion LkHα 324SE lie at the apex ofa V-shaped area apparently excavated from the edge of L988, and are thebrightest members of a small cluster containing about 50Hα-emission stars. A median age of about 0.6 Myr (with largedispersion) is inferred from its color-magnitude diagram, constructedfrom VRI photometry to V=22. Keck HIRES spectra show that LkHα324SE is probably also an HAeBe. Its image is nonstellar, and within 3"to the northwest are three condensations having complex [S II] and [O I]profiles and radial velocities up to -200 km s-1. Theyprobably originate in an outflow from LkHα 324SE. A bright Ap starwith strong Si II lines is embedded in the heavy obscuration 8' to thewest. It illuminates a small reflection nebulosity, has several faintHα emitters nearby, and shares the radial velocity of L988, soclearly it was formed in that cloud. It demonstrates again that suchchemical peculiarities can be established very early in young stars ofmoderate mass.
| Disk Evolution in Cep OB2: Results from the Spitzer Space Telescope
We present the results of an infrared imaging survey of two clusters inthe Cep OB2 Association, Tr 37 and NGC 7160, using the IRAC and MIPSinstruments on board the Spitzer Space Telescope. Our observations coverthe wavelength range from 3.6 to 24 μm, allowing us to detect diskemission over a typical range of radii ~0.1 to ~20 AU from the centralstar. In Tr 37, with an age of about 4 Myr, about 48% of the low-massstars exhibit detectable disk emission in the IRAC bands. Roughly 10% ofthe stars with disks may be ``transition'' objects, with essentiallyphotospheric fluxes at wavelengths <=4.5 μm but with excesses atlonger wavelengths, indicating an optically thin inner disk. The medianoptically thick disk emission in Tr 37 is lower than the correspondingmedian for stars in the younger Taurus region; the decrease in infraredexcess is larger at 6-8 μm than at 24 μm, suggesting that graingrowth and/or dust settling has proceeded faster at smaller disk radii,as expected on general theoretical grounds. Only about 4% of thelow-mass stars in the 10 Myr old cluster NGC 7160 show detectableinfrared disk emission. We also find evidence for 24 μm excessesaround a few intermediate-mass stars, which may represent so-called``debris disk'' systems. Our observations provide new constraints ondisk evolution through an important age range.
| Proper motion determination of open clusters based on the UCAC2 catalogue
We present the kinematics of hundreds of open clusters, based on theUCAC2 Catalogue positions and proper motions. Membership probabilitieswere obtained for the stars in the cluster fields by applying astatistical method uses stellar proper motions. All open clusters withknown distance were investigated, and for 75 clusters this is the firstdetermination of the mean proper motion. The results, including the DSSimages of the cluster's fields with the kinematic members marked, areincorporated in the Open Clusters Catalogue supported on line by ourgroup.
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