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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.
| 25 Orionis: A Kinematically Distinct 10 Myr Old Group in Orion OB1a
We report here on the photometric and kinematic properties of awell-defined group of nearly 200 low-mass pre-main-sequence stars,concentrated within ~1° of the early-B star 25 Ori, in the OrionOB1a subassociation. We refer to this stellar aggregate as the 25Orionis group. The group also harbors the Herbig Ae/Be star V346 Ori anda dozen other early-type stars with photometry, parallaxes, and somewith IR excess emission, indicative of group membership. The number ofhigh- and low-mass stars is in agreement with expectations from astandard initial mass function. The velocity distribution for thelow-mass stars shows a narrow peak at 19.7 km s-1, offset~-10 km s-1 from the velocity characterizing the youngerstars of the Ori OB1b subassociation, and -4 km s-1 from thevelocity of widely spread young stars of the Ori OB1a population; thisresult provides new and compelling evidence that the 25 Ori group is adistinct kinematic entity, and that considerable space and velocitystructure is present in the Ori OB1a subassociation. The low-massmembers follow a well-defined band in the color-magnitude diagram,consistent with an isochronal age of ~7-10 Myr. The ~2 time drop in theoverall Li I equivalent widths and accretion fraction between theyounger Ori OB1b and the 25 Ori group is consistent with the latterbeing significantly older. In a simple-minded kinematic evolutionscenario, the 25 Ori group may represent the evolved counterpart of theyounger σ Ori cluster. The 25 Ori stellar aggregate is the mostpopulous ~10 Myr sample yet known within 500 pc, setting it as anexcellent laboratory to study the evolution of solar-like stars andprotoplanetary disks.Based on observations obtained at the Llano del Hato NationalAstronomical Observatory of Venezuela, operated by CIDA for theMinisterio de Ciencia y Tecnología the MMT Observatory, a jointfacility of the Smithsonian Institution and the University of Arizona;and the Fred Lawrence Whipple Observatory of the SmithsonianInstitution.
| 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.
| Remarks on Rapid vs. Slow Star Formation
We discuss problems with some observational estimates indicating longprotostellar core lifetimes and large stellar age spreads in molecularclouds. We also point out some additional observational constraintswhich suggest that protostellar cores do not have long lifetimes beforecollapsing. For external galaxies, we argue that the widths of spiralarms do not imply a long star-formation process, since the formation ofmassive stars will disrupt molecular clouds, move material around,compress it in other regions which produce new star-forming clouds.Thus, it seems unavoidable that this cyclical process will result in anextended period of enhanced star formation, which does not represent thesurvival time of any individual molecular cloud. We argue that the rapidstar formation indicated observationally is also easier to understandtheoretically than the traditional scenario of slow quasi-staticcontraction with ambipolar diffusion.
| 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
| Silicate Dust in Evolved Protoplanetary Disks: Growth, Sedimentation, and Accretion
We present the Spitzer IRS spectra for 33 young stars in Tr 37 and NGC7160. The sample includes the high- and intermediate-mass stars withMIPS 24 μm excess, the only known active accretor in the 12 Myr oldcluster NGC 7160, and 19 low-mass stars with disks in the 4 Myr oldcluster Tr 37. We examine the 10 μm silicate feature, present in thewhole sample of low-mass stars and in three of the high- andintermediate-mass targets, and we find that PAH emission is detectableonly in the Herbig Be star. We analyze the composition and size of thewarm photospheric silicate grains by fitting the 10 μm silicatefeature and study the possible correlations between the silicatecharacteristics and the stellar and disk properties (age, SED slope,accretion rate, and spectral type). We find indications of dust settlingwith age and of the effect of turbulent enrichment of the diskatmosphere with large grains. Crystalline grains are only smallcontributors to the total silicate mass in all disks and do not seem tocorrelate with any other property, except maybe binarity. We alsoobserve that spectra with very weak silicate emission are at least 3times more frequent among M stars than among earlier spectral types,which may be evidence of inner disk evolution. Finally, we find thatfive of the high- and intermediate-mass stars have SEDs and IRS spectraconsistent with debris disk models involving planet formation, whichcould indicate debris disk formation at ages as early as 4 Myr.
| 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.
| High-Resolution Spectroscopy in Tr 37: Gas Accretion Evolution in Evolved Dusty Disks
Using the Hectochelle multifiber spectrograph, we have obtainedhigh-resolution (R~34,000) spectra in the Hα region for a largenumber of stars in the 4 Myr old cluster Tr 37, containing 146previously known members and 26 newly identified ones. We present theHα line profiles of all members, compare them to our IRobservations of dusty disks (Two Micron All Sky Survey JHK + IRAC + MIPS24 μm), use the radial velocities as a membership criterion, andcalculate the rotational velocities. We find a good correlation betweenthe accretion-broadened profiles and the presence of protoplanetarydisks, noting that a small fraction of the accreting stars presentsbroad profiles with Hα equivalent widths smaller than thecanonical limit separating classical T Tauri stars (CTTSs) andweak-lined T Tauri stars (WTTSs). The number of strong accretors appearsto be lower than in younger regions, and a large number of CTTSs havevery small accretion rates (M˙<=10-9 Msolaryr-1). Taking into account that the spectral energydistributions are consistent with dust evolution (grain growth/settling)in the innermost disk, this suggests a parallel evolution of the dustyand gaseous components. We also observe that about half of the``transition objects'' (stars with no IR excesses at λ<=6μm) do not show any signs of active accretion, whereas the other halfis accreting with accretion rates <=10-9 Msolaryr-1. These zero or very low accretion rates reveal importantgas evolution and/or gas depletion in the innermost disk, which could berelated to grain growth up to planetesimal or even planet sizes.Finally, we examine the rotational velocities of accreting andnonaccreting stars, finding no significant differences that couldindicate disk locking at these ages.Observations reported here were obtained at the MMT Observatory, a jointfacility of the Smithsonian Institution and the University of Arizona.
| 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.
| 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.
| Contact Binaries with Additional Components. I. The Extant Data
We have attempted to establish observational evidence for the presenceof distant companions that may have acquired and/or absorbed angularmomentum during the evolution of multiple systems, thus facilitating orenabling the formation of contact binaries. In this preliminaryinvestigation we use several techniques (some of themdistance-independent) and mostly disregard the detection biases ofindividual techniques in an attempt to establish a lower limit to thefrequency of triple systems. While the whole sample of 151 contactbinary stars brighter than Vmax=10 mag gives a firm lowerlimit of 42%+/-5%, the corresponding number for the much better observednorthern-sky subsample is 59%+/-8%. These estimates indicate that mostcontact binary stars exist in multiple systems.
| 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.
| Spitzer Observations of IC 348: The Disk Population at 2-3 Million Years
We present near- and mid-infrared photometry obtained with the SpitzerSpace Telescope of ~300 known members of the IC 348 cluster. We mergethis photometry with existing ground-based optical and near-infraredphotometry in order to construct optical-infrared spectral energydistributions (SEDs) for all the cluster members and present a completeatlas of these SEDs. We employ these observations to investigate boththe frequency and nature of the circumstellar disk population in thecluster. The Spitzer observations span a wavelength range between 3.6and 24 μm, corresponding to disk radii of ~0.1-5 AU from the centralstar. The observations are sufficiently sensitive to enable the firstdetailed measurement of the disk frequency for very low mass stars atthe peak of the stellar initial mass function. Using measurements ofinfrared excess between 3.6 and 8.0 μm, we find the total frequencyof disk-bearing stars in the cluster to be 50%+/-6%. However, only30%+/-4% of the member stars are surrounded by optically thick,primordial disks, while the remaining disk-bearing stars are surroundedby what appear to be optically thin, anemic disks. Both these values arebelow previous estimates for this cluster. The disk fraction appears tobe a function of spectral type and stellar mass. The fraction of starswith optically thick disks ranges from 11%+/-8% for stars earlier thanK6 to 47%+/-12% for K6-M2 stars to 28%+/-5% for M2-M6 stars. The disklongevity and thus conditions for planet formation appear to be mostfavorable for the K6-M2 stars, which are objects of comparable mass tothe Sun for the age of this cluster. The optically thick disks aroundlater type (>M4) stars appear to be less flared than the disks aroundearlier type stars. This may indicate a greater degree of dust settlingand a more advanced evolutionary state for the late M disk population.Finally, we find that the presence of an optically thick dust disk iscorrelated with gaseous accretion, as measured by the strength ofHα emission. A large fraction of stars classified as classical TTauri stars possess robust, optically thick disks, and very few suchstars are found to be diskless. The majority (64%) of stars classifiedas weak-lined T Tauri stars are found to be diskless. However, asignificant fraction (12%) of these stars are found to be surrounded bythick, primordial disks. These results suggest that it is more likelyfor dust disks to persist in the absence of active gaseous accretionthan for active accretion to persist in the absence of dusty disks.
| 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.
| Spitzer/IRAC Photometry of the η Chameleontis Association
We present IRAC 3.6, 4.5, 5.8, and 8 μm photometry for the 17 A-, K-,and M-type members of the η Chameleontis association. These datashow infrared excesses toward six of the 15 K and M stars, indicatingthe presence of circumstellar disks around 40% of the stars with massesof 0.1-1 Msolar. The two A stars show no infrared excesses.The excess emission around one of the stars is comparable to the medianexcess for classical T Tauri stars in the Taurus association; theremaining five show comparatively weak excess emission. Taking intoaccount published Hα spectroscopy that shows that five of the sixstars are accreting, we argue that the disks with weak mid-infraredexcesses are disks in which the inner disks have been largely depletedof small grains by grain growth or, in one case, the small grains havesettled to the midplane. This suggests that η Cha has a much higherfraction of disks caught in the act of transitioning into optically thindisks than that measured in younger clusters and associations.
| Cloud Structure and Physical Conditions in Star-forming Regions from Optical Observations. II. Analysis
To complement the optical absorption line survey of diffuse moleculargas in Paper I, we obtained and analyzed far-ultraviolet H2and CO data on lines of sight toward stars in Cep OB2 and Cep OB3.Possible correlations between column densities of different species forindividual velocity components, not total columns along a line of sightas in the past, were examined and were interpreted in terms of cloudstructure. The analysis reveals that there are two kinds of CH indiffuse molecular gas: CN-like CH and CH+-like CH. Evidenceis provided that CO is also associated with CN in diffuse molecularclouds. Different species are distributed according to gas density inthe diffuse molecular gas. Both calcium and potassium may be depletedonto grains in high-density gas, but with different dependencies onlocal gas density. Gas densities for components where CN was detectedwere inferred from a chemical model. Analysis of cloud structureindicates that our data are generally consistent with the large-scalestructure suggested by maps of CO millimeter-wave emission. On smallscales, the gas density is seen to vary by factors greater than 5.0 overscales of ~10,000 AU. The relationships between column densities of COand CH with that of H2 along a line of sight show similarslopes for the gas toward Cep OB2 and Cep OB3, but the CO/H2and CH/H2 ratios tend to differ, which we ascribe tovariation in average density along the line of sight.
| 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.
| Cepheus OB2: Disk Evolution and Accretion at 3-10 Myr
We present the results of MMT observations of young stars for our studyof protoplanetary disks at ages 1-10 Myr in two young clusters locatedin the Cepheus OB2 region: Trumpler 37 (embedded in the H II region IC1396) and NGC 7160. Using low-resolution optical spectra from theHectospec multifiber spectrograph, we have tripled the number of knownlow-mass cluster members, identifying ~130 new members in Tr 37 and ~30in NGC 7160. We use indicators of youth (Li absorption at 6707 Å)and accretion/chromospheric activity (Hα emission) to identify andclassify the low-mass cluster members. We derive spectral types for allthe low-mass candidates and calculate the individual extinctions and theaverage over the clusters. With the extended member samples, we estimatethe disk fraction in the clusters, finding that ~40% of the low-massstars in Tr 37 are actively accreting, whereas only 1 of the ~55low-mass stars in NGC 7160 shows indications of accretion. Opticalphotometry and theoretical isochrones are used to determine the age ofthe cluster members, confirming the estimates of ~4 Myr for Tr 37 and~10 Myr for NGC 7160. Accretion rates in Tr 37(~10-8Msolaryr-1 on average) arederived from U-band photometry. We find that only ~50% of the accretingstars have near-IR excesses (from 2MASS), which could be due to thegeometry of their disks or be an indication dust of settling/graingrowth. Finally, we study the high- and intermediate-mass members of theclusters. With the extended member list, we revise the spatialdistribution of stars with disks. Our results are crucial forinterpreting Spitzer Space Telescope studies of accretion disks at theages of planet formation (3-10 Myr).Observations reported here were obtained at the MMT Observatory, a jointfacility of the Smithsonian Institution and the University of Arizona.
| Far Ultraviolet Spectroscopic Explorer Measurements of Interstellar Fluorine
The source of fluorine is not well understood, although core-collapsesupernovae, Wolf-Rayet stars, and asymptotic giant branch stars havebeen suggested. A search for evidence of the ν-process during Type IIsupernovae is presented. Absorption from interstellar F I is seen inspectra of HD 208440 and HD 209339A acquired with the Far UltravioletSpectroscopic Explorer. In order to extract the column density for F Ifrom the line at 954 Å, absorption from H2 has to bemodeled and then removed. Our analysis indicates that for H2column densities less than about 3×1020cm-2, the amount of F I can be determined from λ954.For these two sight lines, there is no clear indication for enhanced Fabundances resulting from the ν-process in a region shaped by pastsupernovae.Based on observations made with the NASA/CNES/CSA Far UltravioletSpectroscopic Explorer (FUSE), which is operated for NASA by the JohnsHopkins University under NASA contract NAS 5-32985.
| An Examination of Main Sequence Stars within the Instability Strip for Open Clusters Over a Range of Ages
We are conducting a study of 14 open clusters which have an approximateage range of 7 to 9 Gyr. The observations for this project were takenwith the 1.8-m telescope at the Dominion Astrophysical Observatory usinga y filter and with the 0.4-m David Derrick telescope of the Orson PrattObservatory using a set of VRI filters. The exposure lengths wereoptimized for main sequence stars in the A-F range in order to bracketthe instability strip. We plan to correlate the cluster ages with 1) thenumber of pulsating variables in the instability strip, 2) thepercentage of variable stars in the instability strip, and 3) theamplitude of the pulsations. We will present preliminary results for 4of the 14 clusters: NGC 6811, NGC 6940, NGC 7142, and NGC 7160.We wish to thank the Dominion Astrophysical Observatory for access tothe 1.8-m telescope. We also acknowledge funding from an AAS SmallResearch Grant that helped make the telescope time at DAO possible.
| Comparison of the Luminosity Functions of Open Clusters Based on USNO-A1 Data
The luminosity and mass functions of a group of Galactic open clustersare constructed by applying a statistical method to photometric datafrom the USNO-A1 catalog. Despite some limitations, this catalog can beused for statistical analyses in Galactic astronomy. Pairwisecomparisons of the derived cluster luminosity functions are performedfor five age intervals. The differences between the luminosity functionsof the open clusters are not statistically significant in most cases. Itis concluded that the luminosity functions are approximately universalthroughout a large volume in the solar neighborhood. Combined luminosityand mass functions are constructed for six age intervals. The slope ofthe mass spectrum may vary somewhat from cluster to cluster, and themean slope may be somewhat higher than the Salpetervalue.
| Low-Mass Stars and Accretion at the Ages of Planet Formation in the Cepheus OB2 Region
We present the first identification of low-mass (spectral types K-M)stars in the young clusters Tr 37 and NGC 7160, members of the CepOB2association. This is part of a program to follow the evolution ofprotoplanetary accretion disks through the ages thought to be crucial tounderstanding disk dissipation and planet formation (~3-10 Myr).Combining optical photometry and optical spectroscopy, we haveidentified ~40 members in Tr 37 and ~15 in NGC 7160, using severalindependent tests for determining the membership (optical colors,optical variability, Hα emission, and Li λ6707 absorption).We confirm previous age estimates of 1-5 Myr for Tr 37 and 10 Myr forNGC 7160. We find active accretion in some of the stars in Tr 37, withaverage accretion rates of ~10-8 Msolaryr-1, derived from their U-band excesses. These resultsexpand the existing samples of accreting stars and are consistent withthe models of viscous accretion disk evolution. No signs of activeaccretion have been detected so far in the older cluster NGC 7160,suggesting that disk accretion ends before the age of 10 Myr. Theseresults are consistent with those from other populations and are a clearsign of disk evolution within the CepOB2 region. We also investigate thespatial asymmetries in Tr 37 and the possible presence of youngerpopulations triggered by Tr 37 itself, and we outline an efficientmethod to detect and study the rest of the clusters members and theircharacteristics.This research has made use of the WIYN Observatory facilities. The WIYNObservatory is a joint facility of the University of Wisconsin-Madison,Indiana University, Yale University, and the National Optical AstronomyObservatory.
| Small Magellanic Cloud-Type Interstellar Dust in the Milky Way
It is well known that the sight line toward HD 204827 in the clusterTrumpler 37 shows a UV extinction curve that does not follow the averageGalactic extinction relation. However, when a dust component, foregroundto the cluster, is removed, the residual extinction curve is identicalto that found in the SMC within the uncertainties. The curve is verysteep and has little or no 2175 Å bump. The position of HD 204827in the sky is projected onto the edge of the Cepheus IRAS bubble. Inaddition, HD 204827 has an IRAS bow shock, indicating that it may beembedded in dust swept up by the supernova that created the IRAS bubble.Shocks due to the supernova may have led to substantial processing ofthis dust. The HD 204827 cloud is dense and rich in carbon molecules.The 3.4 μm feature indicating a C-H grain mantle is present in thedust toward HD 204827. The environment of the HD 204827 cloud dust maybe similar to the dust associated with HD 62542, which lies on the edgeof a stellar wind bubble and is also dense and rich in molecules. Thissight line may be a Rosetta Stone if its environment can be related tothose in the SMC having similar dust.
| Basic physical properties of the close binary V497 Cep in the open cluster NGC 7160
New light and radial-velocity curves of V497 Cep , a binary in the opencluster NGC 7160, were obtained and the linear ephemeris of the systemwas refined to HJD (Min I) = (2 446 299.1596 +/- 0.0064) +(1.2028287d +/- 0.0000015d ) x E. The first light and radial-velocitycurve solutions allowed us to derive the basic physical properties ofthis astrophysically important binary. It was found that the observedlight variation of V497 Cep consists of a strong ellipticity effect anda small contribution from grazing eclipses. A comparison of masses andradii of V497 Cep with theoretical evolutionary tracks indicates thatboth binary components are very close to the zero-age main sequence. Acomparison of disentangled line profiles of the He I 6678 line withsynthetic, rotationally broadened line profiles indicates that therotation of both stars is synchronized with the orbital revolution asexpected. This finding increases the credibility of our solutions. Wefind E(B-V)=0fm39 . The distance to the cluster NGC 7160 was found to beabout 760 (+/- 100) pc which agrees well with other available estimates.Tables 1 and 2 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/405/1087
| On the Galactic Disk Metallicity Distribution from Open Clusters. I. New Catalogs and Abundance Gradient
We have compiled two new open cluster catalogs. In the first one, thereare 119 objects with ages, distances, and metallicities available, whilein the second one, 144 objects have both absolute proper motion andradial velocity data, of which 45 clusters also have metallicity dataavailable. Taking advantage of the large number of objects included inour sample, we present an iron radial gradient of about -0.063+/-0.008dex kpc-1 from the first sample, which is quite consistentwith the most recent determination of the oxygen gradient from nebulaeand young stars, about -0.07 dex kpc-1. By dividing clustersinto age groups, we show that the iron gradient was steeper in the past,which is consistent with the recent result from Galactic planetarynebulae data, and also consistent with inside-out galactic diskformation scenarios. Based on the cluster sample, we also discuss themetallicity distribution, cluster kinematics, and space distribution. Adisk age-metallicity relation could be implied by those properties,although we cannot give conclusive result from the age- metallicitydiagram based on the current sample. More observations are needed formetal-poor clusters. From the second catalog, we have calculated thevelocity components in cylindrical coordinates with respect to theGalactic standard of rest for 144 open clusters. The velocitydispersions of the older clusters are larger than those of youngclusters, but they are all much smaller than that of the Galactic thickdisk stars.
| Proper Motions of Open Star Clusters and the Rotation Rate of the Galaxy
The mean proper motions of 167 Galactic open clusters withradial-velocity measurements are computed from the data of the Tycho-2catalog using kinematic and photometric cluster membership criteria. Theresulting catalog is compared to the results of other studies. The newproper motions are used to infer the Galactic rotation rate at the solarcircle, which is found to be ω0=+24.6±0.8 km s-1 kpc-1.Analysis of the dependence of the dispersion of ω0 estimates onheliocentric velocity showed that even the proper motions of clusterswith distances r>3 kpc contain enough useful information to be usedin kinematic studies demonstrating that the determination of propermotions is quite justified even for very distant clusters.
| Gamma-ray line emission from OB associations and young open clusters. II. The Cygnus region
Gamma-ray and microwave observations of the Cygnus region reveal anintense signal of 1.809 Me line emission, attributed to radioactivedecay of 26, that is closely correlated with 53 GHz free-freeemission, originating from the ionised interstellar medium. We modelledboth emissions using a multi-wavelength evolutionary synthesis code formassive star associations that we applied to the known massive starpopulations in Cygnus. For all OB associations and young open clustersin the field, we determined the population age, distance, and richnessas well as the uncertainties in all these quantities from publishedphotometric and spectroscopic data. We propagate the populationuncertainties in model uncertainties by means of a Bayesian method. Theyoung globular cluster Cyg OB2 turns out to be the dominant26 nucleosynthesis and ionisation source in Cygnus. Our modelreproduces the ionising luminosity of the Cygnus region very well, yetit underestimates 26 production by about a factor of 2. Weattribute this underestimation to shortcomings of currentnucleosynthesis models, and suggest the inclusion of stellar rotationas possible mechanism to enhance 26 production. We alsomodelled 60Fe nucleosynthesis in the Cygnus region, yet thesmall number of recent supernova events suggests only little60Fe production. Consequently, a detection of the 1.137 Meand 1.332 Me decay lines of 60Fe from Cygnus by the upcomingINTEGRAL observatory is not expected. Appendices A and B, and Tables 1,2, and 5 are only available in electronic form athttp://www.edpsciences.org
| The outflow activity of the protostars in S140 IRS
The S140/L1204 cloud contains a deeply embedded region of star formationand a powerful molecular outflow. In this paper, we present images ofthe S140 region obtained in the light of the 2.12 mu m molecularhydrogen emission line and adjacent continuum. Our images reveal severalknots of H2 line emission originating from shocked materialclose to IRS 1 as well as further out. Strong H2 shockemission is found north-east of IRS1 (at position angle of ~20°-30°), as well as to the south-west of IRS1 (at positionangles around ~ 190°-220°), clearly demonstrating the presenceof outflow activity in the north-east/south-west direction. We also findpatches of H2 emission several arcminutes away from IRS1 at aposition angles of ~ 150° and 340degr , i.e. in directionsconsistent with the previously known north-west/south-east molecularoutflow. Our results therefore provide evidence for the existence of twodistinct bipolar outflow systems originating simultaneously from IRS 1.We also discuss general aspects of the star formation process in theS140 region. An inferred high ratio of stellar to gas mass suggests thatthe outflows have dispersed most of the cloud mass. Based onobservations obtained at the German-Spanish Astronomical Centre, CalarAlto, operated by the Max-Planck-Instiute for Astronomy, Heidelberg,jointly with the Spanish National Commission for Astronomy.
| Proper motions of open clusters within 1 kpc based on the TYCHO2 Catalogue
We present mean absolute proper motions of 112 open clusters, determinedusing the data from the Tycho2 Catalogue. For 28 clusters, this is thefirst determination of proper motion. The measurements made use of alarge number of stars (usually several tens) for each cluster. The totalnumber of stars studied in the fields of the 164 open clusters is 5016,of which 4006 were considered members. The mean proper motions of theclusters and membership probability of individual stars were obtainedfrom the proper motion data by applying the statistical method proposedby Sanders (\cite{Sanders71}). Based on observations of the ESAHipparcos satellite. Tables 1, 2 and 5 to 117 are only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/376/441
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