Contenuti
Immagini
Carica la tua immagine
DSS Images Other Images
Articoli relazionati
Terrestrial Zone Debris Disk Candidates in h and χ Persei
We analyze eight sources with strong mid-infrared excesses in the 13 Myrold double cluster h and χ Persei. New optical spectra and broadbandspectral energy distributions (SEDs; 0.36-8 μm) are consistent withcluster membership. We show that the material with T~300-400 K andLd/L*~10-4 to 10-3 producesthe excesses in these sources. Optically thick blackbody disk models,including those with large inner holes, do not match the observed SEDs.The SEDs of optically thin debris disks produced from terrestrial planetformation calculations match the observations well. Thus, some h andχ Persei stars may have debris from terrestrial zone planetformation.
| Mass and Temperature of the TWA 7 Debris Disk
We present photometric detections of dust emission at 850 and 450 μmaround the pre-main-sequence M1 dwarf TWA 7 using the SCUBA camera onthe James Clerk Maxwell Telescope. These data confirm the presence of acold dust disk around TWA 7, a member of the TW Hydrae Association(TWA). Based on the 850 μm flux, we estimate the mass of the disk tobe 18 Mlunar (0.2 M⊕) assuming a massopacity of 1.7 cm2 g-1 with a temperature of 45 K.This makes the TWA 7 disk (d=55 pc) an order of magnitude more massivethan the disk reported around AU Microscopii (GL 803), the closest (9.9pc) debris disk detected around an M dwarf. This is consistent with TWA7 being slightly younger than AU Mic. We find that the mid-IR andsubmillimeter data require the disk to be comprised of dust at a rangeof temperatures. A model in which the dust is at a single radius fromthe star, with a range of temperatures according to grain size, is aseffective at fitting the emission spectrum as a model in which the dustis of uniform size, but has a range of temperatures according todistance. We discuss this disk in the context of known disks in the TWAand around low-mass stars; a comparison of masses of disks in the TWAreveals no trend in mass or evolutionary state (gas-rich vs. debris) asa function of spectral type.
| An Analysis of the Shapes of Interstellar Extinction Curves. V. The IR-through-UV Curve Morphology
We study the IR-through-UV interstellar extinction curves towards 328Galactic B and late-O stars. We use a new technique which employsstellar atmosphere models in lieu of unreddened "standard" stars. Thistechnique is capable of virtually eliminating spectral mismatch errorsin the curves. It also allows a quantitative assessment of the errorsand enables a rigorous testing of the significance of relationshipsbetween various curve parameters, regardless of whether theiruncertainties are correlated. Analysis of the curves gives the followingresults: (1) In accord with our previous findings, the central positionof the 2175 A extinction bump is mildly variable, its width is highlyvariable, and the two variations are unrelated. (2) Strong correlationsare found among some extinction properties within the UV region, andwithin the IR region. (3) With the exception of a few curves withextreme (i.e., large) values of R(V), the UV and IR portions of Galacticextinction curves are not correlated with each other. (4) The largesightline-to-sightline variation seen in our sample implies that anyaverage Galactic extinction curve will always reflect the biases of itsparent sample. (5) The use of an average curve to deredden a spectralenergy distribution (SED) will result in significant errors, and arealistic error budget for the dereddened SED must include the observedvariance of Galactic curves. While the observed largesightline-to-sightline variations, and the lack of correlation among thevarious features of the curves, make it difficult to meaningfullycharacterize average extinction properties, they demonstrate thatextinction curves respond sensitively to local conditions. Thus, eachcurve contains potentially unique information about the grains along itssightline.
| CCD BV survey of 42 open clusters
Aims.We present results of a photometric survey whose aim was to derivestructural and astrophysical parameters for 42 open clusters. While oursample is definitively not representative of the total open clustersample in the Galaxy, it does cover a wide range of cluster parametersand is uniform enough to allow for simple statistical considerations. Methods: BV wide-field CCD photometry was obtained for open clusters forwhich photometric, structural, and dynamical evolution parameters weredetermined. The limiting and core radii were determined by analyzingradial density profiles. The ages, reddenings, and distances wereobtained from the solar metallicity isochrone fitting. The mass functionwas used to study the dynamical state of the systems, mass segregationeffect and to estimate the total mass and number of cluster members. Results: This study reports on the first determination of basicparameters for 11 out of 42 observed open clusters. The angular sizesfor the majority of the observed clusters appear to be several timeslarger than the catalogue data indicate. The core and limiting clusterradii are correlated and the latter parameter is 3.2 times larger onaverage. The limiting radius increases with the cluster's mass, and boththe limiting and core radii decrease in the course of dynamicalevolution. For dynamically not advanced clusters, the mass functionslope is similar to the universal IMF slope. For more evolved systems,the effect of evaporation of low-mass members is clearly visible. Theinitial mass segregation is present in all the observed young clusters,whereas the dynamical mass segregation appears in clusters older thanabout log({age}) = 8. Low-mass stars are deficient in the cores ofclusters older than log({age}) = 8.5 and not younger than one relaxationtime.Tables 1-5 are only available in electronic form at http://www.aanda.org
| Spitzer IRAC and JHKs Observations of h and χ Persei: Constraints on Protoplanetary Disk and Massive Cluster Evolution at ~107 Years
We describe IRAC 3.6-8 μm observations and ground-based near-IRJHKs photometry from Mimir and 2MASS of the massive doublecluster h and χ Persei complete to J=15.5 (M~1.3 Msolar).Within 25' of the cluster centers we detect ~11,000 sourceswith J<=15.5, ~7000 sources with [4.5]<=15, and ~5000 sources with[8]<=14.5. In both clusters the surface density profiles derived fromthe 2MASS data decline with distance from the cluster centers asexpected for a bound cluster. Within 15' of the clustercenters, ~50% of the stars lie on a reddened ~13 Myr isochrone; at15'-25' from the cluster centers, ~40% lie on thisisochrone. Thus, the optical/2MASS color-magnitude diagrams indicatethat h and χ Per are accompanied by a halo population with roughlythe same age and distance as the two dense clusters. The double clusterlacks any clear IR excess sources for J<=13.5 (~2.7Msolar). Therefore, disks around high-mass stars disperseprior to ~107 yr. At least 2%-3% of the fainter cluster starshave strong IR excess at both [5.8] and [8]. About 4%-8% of sourcesslightly more massive than the Sun (~1.4 Msolar) have IRexcesses at [8]. Combined with the lack of detectable excesses forbrighter stars, this result suggests that disks around lower mass starshave longer lifetimes. The IR excess population also appears to belarger at longer IRAC bands ([5.8], [8]) than at shorter IRAC/2MASSbands (Ks, [4.5]), a result consistent with an inside-outclearing of disks.
| 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.
| Rotational Velocities for B0-B3 Stars in Seven Young Clusters: Further Study of the Relationship between Rotation Speed and Density in Star-Forming Regions
We present the results of a study aimed at assessing the differences inthe distribution of rotation speeds N(vsini) among young (1-15 Myr) Bstars spanning a range of masses 6 Msolar>1 Msolar pc-3)ensembles that will survive as rich, bound stellar clusters for ageswell in excess of 108 yr. Our results demonstrate (1) thatindependent of environment, the rotation rates for stars in this massrange do not change by more than 0.1 dex over ages t~1 to ~15 Myr; and(2) that stars formed in high-density regions lack the cohort of slowrotators that dominate the low-density regions and young field stars. Wesuggest that the differences in N(vsini) between low- and high-densityregions may reflect a combination of initial conditions andenvironmental effects: (1) the higher turbulent speeds that characterizemolecular gas in high-density, cluster-forming regions; and (2) thestronger UV radiation fields and high stellar densities thatcharacterize such regions. Higher turbulent speeds may lead to highertime-averaged accretion rates during the stellar assembly phase. In thecontext of stellar angular momentum regulation via ``disk-locking,''higher accretion rates lead to both higher initial angular momenta andevolution-driven increases in surface rotation rates as stars contractfrom the birth line to the zero-age main sequence (ZAMS). Stronger UVradiation fields and higher densities may lead to shorter disk lifetimesin cluster-forming regions. If so, B stars formed in dense clusters aremore likely to be ``released'' from their disks early during theirpre-main-sequence lifetimes and evolve into rapid rotators as theyconserve angular momentum and spin up in response to contraction. Bycontrast, the majority of their brethren in low-density,association-forming regions can retain their disks for much or all oftheir pre-main-sequence lifetimes, are ``locked'' by their disks torotate at constant angular speed, and lose angular momentum as theycontract toward the ZAMS, and thus arrive on the ZAMS as relativelyslowly rotating stars.
| 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.
| 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
| A Hubble Space Telescope Survey of the Disk Cluster Population of M31. I. WFPC2 Pointings
As a follow-up to the automated cluster search carried out by Williams& Hodge, we have examined 39 Hubble Space Telescope Wide FieldPlanetary Camera 2 (WFPC2) pointings to locate and study a comprehensivecollection of disk clusters. The Williams technique was effective infinding young clusters, but not intermediate-age or old clusters. Oursearches have shown that M31 has large numbers of these intermediate andolder open clusters, most of them undetected by both the Williams surveyand other ground-based searches. We present a catalog of 343 clustersdetected on the WFPC2 images. Extrapolation from our data indicates thatthe entire disk of M31 contains approximately 80,000 star clusters. Wehave carried out integrated multicolor photometry of these clusters toascertain their properties and to compare their properties with clustersystems of other galaxies. We show the cluster luminosity function, thecolor-magnitude diagram, the formation function, and the sizedistribution. Cluster densities and colors show trends with diskposition. An age distribution is derived and, although the ages are veryuncertain for the fainter clusters, there is evidence for clusterdynamical destruction at about the same rate as in our Galaxy.Based on observations with the NASA/ESA Hubble Space Telescope obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555.
| 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.
| The VLT-FLAMES survey of massive stars: stellar parameters and rotational velocities in NGC 3293, NGC 4755 and NGC 6611
An analysis is presented of VLT-FLAMES spectroscopy for three Galacticclusters, NGC 3293, NGC 4755 and NGC 6611. Non-LTE model atmospherecalculations have been used to estimate effective temperatures (fromeither the helium spectrum or the silicon ionization equilibrium) andgravities (from the hydrogen spectrum). Projected rotational velocitieshave been deduced from the helium spectrum (for fast and moderaterotators) or the metal line spectrum (for slow rotators). The origin ofthe low gravity estimates for apparently near main sequence objects isdiscussed and is related to the stellar rotational velocity. Theatmospheric parameters have been used to estimate cluster distances(which are generally in good agreement with previous determinations) andthese have been used to estimate stellar luminosities and evolutionarymasses. The observed Hertzsprung-Russell diagrams are compared withtheoretical predictions and some discrepancies including differences inthe main sequence luminosities are discussed. Cluster ages have beendeduced and evidence for non-coeval star formation is found for allthree of the clusters. Projected rotational velocities for targets inthe older clusters, NGC 3293 and NGC 4755, have been found to besystematically larger than those for the field, confirming recentresults in other similar age clusters. The distribution of projectedrotational velocities are consistent with a Gaussian distribution ofintrinsic rotational velocities. For the relatively unevolved targets inthe older clusters, NGC 3293 and NGC 4755, the peak of the velocitydistribution would be 250 km s-1 with afull-width-half-maximum of approximately 180 km s-1. For NGC6611, the sample size is relatively small but implies a lower meanrotational velocity. This may be evidence for the spin-down effect dueto angular momentum loss through stellar winds, although our results areconsistent with those found for very young high mass stars. For allthree clusters we deduce present day mass functions with Γ-valuesin the range of -1.5 to -1.8, which are similar to other young stellarclusters in the Milky Way.
| 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.
| NGC 7419: a young open cluster with a number of very young intermediate mass pre-MS stars
We present a photometric and spectroscopic study of the young opencluster NGC 7419, which is known to host a large number of classical Bestars for reasons not well understood. Based on CCD photometricobservations of 327 stars in UBV passbands, we estimated the clusterparameters as, reddening [E(B - V)] = 1.65 +/- 0.15 mag and distance =2900 +/- 400 pc. The turn-off age of the cluster was estimated as 25 +/-5 Myr using isochrone fits. UBV data of the stars were combined with theJHK data from Two-Micron All-Sky Survey (2MASS) and were used to createthe near-infrared (NIR) (J - H) versus (H - K) colour-colour diagram. Alarge fraction of stars (42 per cent) was found to have NIR excess andtheir location in the diagram was used to identify them as intermediatemass pre-main-sequence (MS) stars. The isochrone fits to pre-MS stars inthe optical colour-magnitude diagram showed that the turn-on age of thecluster is 0.3-3 Myr. This indicates that there has been a recentepisode of star formation in the vicinity of the cluster.Slitless spectra were used to identify 27 stars which showed Hα inemission in the field of the cluster, of which six are newidentifications. All these stars were found to show NIR excess and arelocated closer to the region populated by Herbig Ae/Be stars in the (J -H) versus (H - K) diagram. Slit spectra of 25 stars were obtained in theregion 3700-9000 Å. The spectral features were found to be verysimilar to those of Herbig Be stars. These stars were found to be morereddened than the main-sequence stars by 0.4 mag, on an average. Thus,the emission-line stars found in this cluster are more similar to theHerbig Be-type stars where the circumstellar material is the remnant ofthe accretion disc. We conclude that the second episode of starformation has led to the formation of a large number of Herbig Be starsas well as intermediate mass pre-MS stars in the field of NGC 7419, thusexplaining the presence of emission-line stars in this cluster. Thiscould be one of the young open clusters with the largest number ofHerbig Be stars.
| Accretion in young low intermediate mass stars
We present preliminary results of a study of the evolution of accretionrates in some young members of open clusters: NGC1502(1), NGC884(3),Trumpler37(2), Biurakan2(3), Berkeley87(1). Accretion rates werecomputed using UVI photometry and the Gullbring relation. In addition wefound that accretion rates of three members of the MBM12 youngassociation ( ˜ 2Myr) are in agreement with the expected valuesfor CTTS.
| 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.
| Discovery of an Extraordinarily Massive Cluster of Red Supergiants
We report the discovery of an extraordinarily massive young cluster ofstars in the Galaxy, having an inferred total initial cluster masscomparable to the most massive young clusters in the Galaxy. UsingIRMOS, 2MASS, and Spitzer observations, we conclude that there are 14red supergiants in the cluster, compared with five, in what waspreviously thought to be the richest Galactic cluster of such stars. Weinfer spectral types from near-infrared spectra that reveal deep CObandhead absorption that can only be fit by red supergiants. We identifya gap of ΔKs~4 mag between the stars and the bulk ofthe other stars in the region that can only be fit by models if thebrightest stars in the cluster are red supergiants. We estimate adistance of 5.8 kpc to the cluster by associating an OH maser with theenvelope of one of the stars. We also identify a ``yellow'' supergiantof G6 I type in the cluster. Assuming a Salpeter IMF, we infer aninitial cluster mass of 20,000-40,000 Msolar for cluster agesof 7-12 Myr. Continuing with these assumptions, we find that 80% of theinitial mass and 99% of the number of stars remain at the present time.We associate the cluster with an X-ray source (detected by ASCA andEinstein), a recently discovered very high energy γ-ray source(detected by INTEGRAL and HESS), and several nonthermal radio sources,finding that these objects are likely related to recent supernovae inthe cluster. In particular, we claim that the cluster has produced atleast one recent supernova remnant with properties similar to the CrabNebula.
| 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.
| Polycyclic Aromatic Hydrocarbons Orbiting HD 233517, an Evolved Oxygen-rich Red Giant
We report spectra obtained with the Spitzer Space Telescope in theλ = 5-35 μm range of HD 233517, an evolved K2 III giant withcircumstellar dust. For λ > 13 μm, the flux is a smoothcontinuum that varies approximately as ν-5/3. For λ< 13 μm, although the star is oxygen-rich, PAH features producedby carbon-rich species at 6.3, 8.2, 11.3, and 12.7 μm are detectedalong with likely broad silicate emission near 20 μm. These resultscan be explained if there is a passive, flared disk orbiting HD 233517.Our data support the hypothesis that organic molecules in orbiting disksmay be synthesized in situ as well as having been incorporated from theinterstellar medium.Based on observations with the NASA Spitzer Space Telescope, which isoperated by the California Institute of Technology for NASA.
| Monitor: transiting planets and brown dwarfs in star forming regions and young open clusters
The Monitor project is a large scale photometric monitoring survey often star forming regions and open clusters aged between 1 and 200 Myrusing wide-field optical cameras on 2-4 m telescopes worldwide. Theprimary goal of the project is to search for close-in planets and browndwarfs at young ages through the detection of transit events. Suchdetections would provide unprecedented constraints on planet formationand migration time-scales, as well as on evolutionary models of planetsand brown dwarfs in an age range where such constraints are very scarce.Additional science goals include rotation period measurements and theanalysis of flares and accretion-related variability.Workshop ``Ultralow-mass star formation and evolution'', see AN 326, No. 10www.ast.cam.ac.uk/suz/monitor/monitor.php
| Eclipsing binary stars in open clusters.
Not Available
| NGC 146: a young open cluster with a Herbig Be star and intermediate mass pre-main sequence stars
We present UBV CCD photometry and low-resolution spectra of stars in thefield of the young open cluster NGC 146. UBV photometry of 434 starswere used to estimate the E(B-V) reddening of 0.55 ± 0.04 mag andBV photometry of 976 stars were used to estimate a distance modulus of(m-M)0 = 12.7 ± 0.2 mag, corresponding to a distanceof 3470+335-305 pc. We estimated 10-16 Myr as theturn-off age for the upper main sequence of the cluster using isochronesand synthetic colour magnitude diagrams. We identified two B type starswith Hα in emission and located on the MS using slit-less spectra.A higher resolution spectrum of the brighter Be star indicated thepresence of a number of emission lines, with some lines showing thesignature of gas infall. This star was found to be located in the regionof Herbig Ae/Be stars in the (J-H) vs. (H-K) colour-colour diagram.Thus, we identify this star as a Herbig Be star. On the other hand, 54stars were found to show near infrared excess, of which 17 were found tobe located in the region of Herbig Ae/Be stars and 18 stars were foundto be located in the region of Be stars in the NIR colour-colourdiagram. Thus NGC 146 is a young cluster with a large number ofintermediate mass pre-main sequence stars. The turn-on age of thecluster is found to be ~3 Myr. Though NGC 146 shows an older turn off,the bulk of stars in this cluster seems to belong to the youngerpopulation of 3 Myr.
| The Effective Temperature Scale of Galactic Red Supergiants: Cool, but Not As Cool As We Thought
We use moderate-resolution optical spectrophotometry and the new MARCSstellar atmosphere models to determine the effective temperatures of 74Galactic red supergiants (RSGs). The stars are mostly members of OBassociations or clusters with known distances, allowing a criticalcomparison with modern stellar evolutionary tracks. We find we canachieve excellent matches between the observations and the reddenedmodel fluxes and molecular transitions, although the atomic lines Ca Iλ4226 and Ca II H and K are found to be unrealistically strong inthe models. Our new effective temperature scale is significantly warmerthan those in the literature, with the differences amounting to 400 Kfor the latest type M supergiants (i.e., M5 I). We show that the newlyderived temperatures and bolometric corrections give much betteragreement with stellar evolutionary tracks. This agreement provides acompletely independent verification of our new temperature scale. Thecombination of effective temperature and bolometric luminosities allowsus to calculate stellar radii; the coolest and most luminous stars (KWSgr, Case 75, KY Cyg, HD 206936=μ Cep) have radii of roughly 1500Rsolar (7 AU), in excellent accordance with the largeststellar radii predicted from current evolutionary theory, althoughsmaller than that found by others for the binary VV Cep and for thepeculiar star VY CMa. We find that similar results are obtained for theeffective temperatures and bolometric luminosities using only thedereddened V-K colors, providing a powerful demonstration of theself-consistency of the MARCS models.
| Astrophysical parameters of Galactic open clusters
We present a catalogue of astrophysical data for 520 Galactic openclusters. These are the clusters for which at least three most probablemembers (18 on average) could be identified in the ASCC-2.5, a catalogueof stars based on the Tycho-2 observations from the Hipparcos mission.We applied homogeneous methods and algorithms to determine angular sizesof cluster cores and coronae, heliocentric distances, mean propermotions, mean radial velocities, and ages. For the first time we derivedistances for 200 clusters, radial velocities for 94 clusters, and agesof 196 clusters. This homogeneous new parameter set is compared withearlier determinations, where we find, in particular, that the angularsizes were systematically underestimated in the literature.
| Structure and Mass Segregation in h and χ Persei
We use V magnitudes and spectral types to examine the density structureof h and χ Per. We describe an automatic method for derivingspectral types and compare classifications for observations made at twodifferent facilities. With these data, we measure an extinction to theclusters of E(B-V)=0.52+/-0.07, consistent with other authors. However,there appears to be a correlation between the spectral types of thestars used and the resulting value of the extinction. We compareextinction values measured by different authors using different numbersof stars and reproduce their values by imposing different cuts in the Vmagnitude. This variation in color excess versus spectral type suggeststhat the standard intrinsic colors for the earliest type stars are bluerthan the stars in h and χ Per. We measure centers for h and χPer at α(2000)=2h18m56.4s+/-3.0s,δ(2000)=57deg8'25''+/-23''and α(2000)=2h22m4.3s+/-2.9s,δ(2000)=57deg8'35''+/-25'',respectively. We fit the density structure of the clusters and find coreradii of 1.9 and 2.4 pc, respectively. Integration of the Miller-Scaloinitial mass function suggests overall cluster masses of 5500 and 4300Msolar and central densities of 27 and 50 Msolarpc-3, respectively. We find strong evidence of masssegregation in h Per but not in χ Per. Examination of the dynamicaltimescales, as well as comparisons between the two clusters, suggestthat the mass segregation is partly primordial.
| On the massive stellar population of the super star cluster Westerlund 1
We present new spectroscopic and photometric observations of the youngGalactic open cluster Westerlund 1 (Wd 1) that reveala unique population of massive evolved stars. We identify ~200 clustermembers and present spectroscopic classifications for ~25% of these. Wefind that all stars so classified are unambiguously post-Main Sequenceobjects, consistent with an apparent lack of an identifiable MainSequence in our photometric data to V 20. We are able to identifyrich populations of Wolf Rayet stars, OB supergiants and short livedtransitional objects. Of these, the latter group consists of both hot(Luminous Blue Variable and extreme B supergiant) and cool (YellowHypergiant and Red Supergiant) objects - we find that half the knownGalactic population of YHGs resides within Wd 1. We obtain a meanV-MV ~ 25 mag from the cluster Yellow Hypergiants, implying aMain Sequence turnoff at or below MV =-5 (O7 V or later).Based solely on the masses inferred for the 53 spectroscopicallyclassified stars, we determine an absolute minimum mass of ~1.5 ×10^3~Mȯ for Wd 1. However, considering the completephotometrically and spectroscopically selected cluster population andadopting a Kroupa IMF we infer a likely mass for Wd 1 of~10^5~Mȯ, noting that inevitable source confusion andincompleteness are likely to render this an underestimate. As such, Wd 1is the most massive compact young cluster yet identified in the LocalGroup, with a mass exceeding that of Galactic Centre clusters such asthe Arches and Quintuplet. Indeed, the luminosity, inferred mass andcompact nature of Wd 1 are comparable with those of Super Star Clusters- previously identified only in external galaxies - and is consistentwith expectations for a Globular Cluster progenitor.
| CCD uvbyβ photometry of the young open cluster NGC 663
NGC 663 is a young, moderately rich open cluster, known to contain oneof the largest fractions of Be stars among all galactic clusters. Inthis work we present CCD uvbyβ photometry for stars in its centralarea. We have used these data to obtain the main cluster physicalparameters. We find that the reddening is highly variable, with valuesranging from E(b-y) = 0.639 +/- 0.032 in the central part to E(b-y) =0.555 +/- 0.038 in the south-east. The distance modulus is found to be11.6 +/- 0.1 mag (2.1 kpc) and the age logt= 7.4 +/- 0.1 yr(25+7-5 Myr). The age obtained is consistent withthe interpretation of the Be phenomenon as an evolutionary effect.
| The T Tauri Star Population of the Young Cluster NGC 2264
An Hα emission survey of the young cluster NGC 2264 in the Mon OB1association resulted in the detection of 490 Hα emission stars ina 25'×40' field approximately centeredbetween the O7 V multiple star S Mon and the Cone Nebula. The survey wascarried out with the wide-field grism spectrograph (WFGS) on theUniversity of Hawaii 2.2 m telescope on Mauna Kea. X-ray observationsmade with the European Photon Imaging Camera on board the European SpaceAgency's XMM-Newton satellite observatory will be discussed in asubsequent paper. Optical (BVRCIC) photometry wasobtained for selected fields to supplement similar data from theliterature. Spectra covering the 6000-8000 Å region at aresolution of R~3000 (adequate for the determination of Li Iλ6708 line strengths) were obtained for 150 Hα and X-rayemission sources with the Gemini Multi-Object Spectrograph.Near-infrared spectra (1-2.5 μm) of a number of T Tauri stars (TTSs),X-ray sources, and LHα25 (W90) were also obtained using SpeX onthe Infrared Telescope Facility. Ages and masses for the Hαemitters were inferred from the isochrones and evolutionary tracks ofD'Antona & Mazzitelli. The median age for the TTS population isabout 1.1 Myr, but a considerable dispersion, from 0.1 to 5 Myr, existsfor individual objects. Several fields in the cluster were observed withthe WFGS on more than one occasion, permitting an examination ofHα variability over long baselines in time. About 90% of theclassical T Tauri stars (CTTSs) showed changes in W(Hα) of atleast 10%, while 57% varied at levels of 50% or more. No evidence wasfound for a significant pool of dormant Hα emitters. Summing themasses of the TTSs and the OB stellar population of NGC 2264, a lowerlimit for the total stellar mass content of the cluster is about 430Msolar. This is less than 1% of the total mass of the atomicand molecular gas believed to be associated with NGC 2264. Evidence forhierarchical structure within the cluster is suggested by the spatialdistribution of TTSs. Four concentrations of Hα emitters areevident: two near S Mon and two near the Cone Nebula. The median age ofthe TTSs in the immediate vicinity of S Mon was found to be greater thanthat of the TTSs near Allen's infrared source (IRS-1), but a significantdispersion is present. From the rotational data of Lamm et al. andMakidon et al., 241 of the TTSs are periodic variables, 150 weak-line TTauri stars (WTTSs) and 91 CTTSs, while 123 stars are irregularvariables (30 WTTSs and 93 CTTSs). A weak-to-moderate positivecorrelation is found between H-K color and Prot for theCTTSs, in the sense that stars having longer periods tend to have largerH-K colors. A similar positive correlation is found betweenLHα and Prot among the CTTSs. Nostatistically significant correlation is found between Protand theoretical age or between Prot and LX. Othertopics discussed include the fraction of Hα emitters that areWTTSs, f(WTTS)=N(WTTS)/N(TTS), for clusters of different ages; therelative detectability of Hα emission using WFGS and narrowbandfilter imaging techniques; and the correlation of W(Li I) withTe, age, H-K color, and W(Hα).
| B Star Rotational Velocities in h and χ Persei: A Probe of Initial Conditions during the Star Formation Epoch?
Projected rotational velocities (vsini) have been measured for 216 B0-B9stars in the rich, dense h and χ Persei double cluster and comparedwith the distribution of rotational velocities for a sample of fieldstars having comparable ages (t~12-15 Myr) and masses (M~4-15Msolar). For stars that are relatively little evolved fromtheir initial locations on the zero-age main sequence (ZAMS) (those withmasses M~4-5 Msolar), the mean vsini measured for the h andχ Per sample is slightly more than 2 times larger than the meandetermined for field stars of comparable mass, and the cluster and fieldvsini distributions differ with a high degree of significance. Forsomewhat more evolved stars with masses in the range 5-9Msolar, the mean vsini in h and χ Per is 1.5 times thatof the field; the vsini distributions differ as well, but with a lowerdegree of statistical significance. For stars that have evolvedsignificantly from the ZAMS and are approaching the hydrogen exhaustionphase (those with masses in the range 9-15 Msolar), thecluster and field star means and distributions are only slightlydifferent. We argue that both the higher rotation rates and the patternof rotation speeds as a function of mass that differentiatemain-sequence B stars in h and χ Per from their field analogs werelikely imprinted during the star formation process rather than a resultof angular momentum evolution over the 12-15 Myr cluster lifetime. Wespeculate that these differences may reflect the effects of the higheraccretion rates that theory suggests are characteristic of regions thatgive birth to dense clusters, namely, (1) higher initial rotationspeeds; (2) higher initial radii along the stellar birth line, resultingin greater spin-up between the birth line and the ZAMS; and (3) a morepronounced maximum in the birth line radius-mass relationship thatresults in differentially greater spin-up for stars that become mid- tolate-B stars on the ZAMS.
|
Sottometti un nuovo articolo
Link relazionati
Sottometti un nuovo link
Membro dei seguenti gruppi:
|
Osservazione e dati astrometrici
Cataloghi e designazioni:
|
