目录
图像
上传图像
DSS Images Other Images
相关文章
Equilibrium Star Cluster Formation
We argue that rich star clusters take at least several local dynamicaltimes to form and so are quasi-equilibrium structures during theirassembly. Observations supporting this conclusion include morphologiesof star-forming clumps, momentum flux of protostellar outflows fromforming clusters, age spreads of stars in the Orion Nebula cluster (ONC)and other clusters, and the age of a dynamical ejection event from theONC. We show that these long formation timescales are consistent withthe expected star formation rate in turbulent gas, as recently evaluatedby Krumholz & McKee. Finally, we discuss the implications of thesetimescales for star formation efficiencies, the disruption of gas bystellar feedback, mass segregation of stars, and the longevity ofturbulence in molecular clumps.
| Deepsky delights.
Not Available
| Search for pulsating pre-main-sequence stars in NGC6383
A search for pulsating pre-main-sequence (PMS) stars was performed inthe young open cluster NGC 6383 using CCD time-series photometry inJohnson B and V filters. With an age of only ~1.7Myr all cluster memberslater than spectral type A0 have not reached the ZAMS yet, hence beingideal candidates for investigating PMS pulsation among A- and F-typestars. In total 286 stars have been analysed using classical Fouriertechniques. From about a dozen stars within the boundaries of theclassical instability strip, two stars were found to pulsate: NGC 6383#170, with five frequencies simultaneously, and NGC 6383 #198, with asingle frequency. In addition, NGC 6383 #152 is a suspected PMS variablestar, but our data remain inconclusive. Linear, non-adiabatic modelsassuming PMS evolutionary phase and purely radial pulsation werecalculated for the two new PMS pulsators. NGC 6383 #170 appears topulsate radially in third and fifth overtones, while the other threefrequencies seem to be of non-radial nature. NGC 6383 #198 pulsatesmonoperiodically, most probably in the third radial overtone. Magnitudesand B-V colours were available in the literature for only one third ofall stars and we used them for calibrating the remaining.
| The Evolutionary Status of Be Stars: Results from a Photometric Study of Southern Open Clusters
Be stars are a class of rapidly rotating B stars with circumstellardisks that cause Balmer and other line emission. There are threepossible reasons for the rapid rotation of Be stars: they may have beenborn as rapid rotators, spun up by binary mass transfer, or spun upduring the main-sequence (MS) evolution of B stars. To test the variousformation scenarios, we have conducted a photometric survey of 55 openclusters in the southern sky. Of these, five clusters are probably notphysically associated groups and our results for two other clusters arenot reliable, but we identify 52 definite Be stars and an additional 129Be candidates in the remaining clusters. We use our results to examinethe age and evolutionary dependence of the Be phenomenon. We find anoverall increase in the fraction of Be stars with age until 100 Myr, andBe stars are most common among the brightest, most massive B-type starsabove the zero-age main sequence (ZAMS). We show that a spin-up phase atthe terminal-age main sequence (TAMS) cannot produce the observeddistribution of Be stars, but up to 73% of the Be stars detected mayhave been spun-up by binary mass transfer. Most of the remaining Bestars were likely rapid rotators at birth. Previous studies havesuggested that low metallicity and high cluster density may also favorBe star formation. Our results indicate a possible increase in thefraction of Be stars with increasing cluster distance from the Galacticcenter (in environments of decreasing metallicity). However, the trendis not significant and could be ruled out due to the intrinsic scatterin our data. We also find no relationship between the fraction of Bestars and cluster density.
| 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.
| Control of star formation by supersonic turbulence
Understanding the formation of stars in galaxies is central to much ofmodern astrophysics. However, a quantitative prediction of the starformation rate and the initial distribution of stellar masses remainselusive. For several decades it has been thought that the star formationprocess is primarily controlled by the interplay between gravity andmagnetostatic support, modulated by neutral-ion drift (known asambipolar diffusion in astrophysics). Recently, however, bothobservational and numerical work has begun to suggest that supersonicturbulent flows rather than static magnetic fields control starformation. To some extent, this represents a return to ideas popularbefore the importance of magnetic fields to the interstellar gas wasfully appreciated. This review gives a historical overview of thesuccesses and problems of both the classical dynamical theory and thestandard theory of magnetostatic support, from both observational andtheoretical perspectives. The outline of a new theory relying on controlby driven supersonic turbulence is then presented. Numerical modelsdemonstrate that, although supersonic turbulence can provide globalsupport, it nevertheless produces density enhancements that allow localcollapse. Inefficient, isolated star formation is a hallmark ofturbulent support, while efficient, clustered star formation occurs inits absence. The consequences of this theory are then explored for bothlocal star formation and galactic-scale star formation. It suggests thatindividual star-forming cores are likely not quasistatic objects, butdynamically collapsing. Accretion onto these objects varies depending onthe properties of the surrounding turbulent flow; numerical models agreewith observations showing decreasing rates. The initial massdistribution of stars may also be determined by the turbulent flow.Molecular clouds appear to be transient objects forming and dissolvingin the larger-scale turbulent flow, or else quickly collapsing intoregions of violent star formation. Global star formation in galaxiesappears to be controlled by the same balance between gravity andturbulence as small-scale star formation, although modulated by coolingand differential rotation. The dominant driving mechanism instar-forming regions of galaxies appears to be supernovae, whileelsewhere coupling of rotation to the gas through magnetic fields orgravity may be important.
| Ambipolar-Diffusion Timescale, Star Formation Timescale, and the Ages of Molecular Clouds: Is There a Discrepancy?
We reexamine critically the estimates of the duration of differentphases of star formation and the lifetimes of molecular clouds based onthe ages of T Tauri stars, age spreads of stars in clusters, andstatistics of prestellar cores. We show that all available observationaldata are consistent with lifetimes of molecular clouds comparable to~107 yr, as well as with the predictions of the theory ofself-initiated, ambipolar-diffusion-controlled star formation. Weconclude that there exists no observational support for either ``young''molecular clouds or ``rapid'' star formation.
| tex2html_wrap_inlineuvbyb#beta; Photoelectric Photometry of the Open Cluster NGC823
Absolute uvby beta photoelectric photometry of the brightest stars inthe direction of the Open Cluster NGC823 has been secured and ispresented. From the data analysis, reddening, distance, temperature andgravity are determined for the B, A, F stars and, from these, reddeningand a mean distance modulus ( 12.475+/-0.414) to the cluster aredetermined. Also, their membership in the cluster has been establishedand a numerical value of the age is determined through direct comparisonwith theoretical models. A brief analysis of the short period variablestars in the direction of NGC823 is made. We have concluded that theymight not be pre-MS delta Scuti stars.
| Supernova remnants and γ-ray sources
Electronic Article Available from Elsevier Science.
| 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.
| 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.
| Urban Astronomy: Observing the Messier Objects from the City
Not Available
| Proper motions of open clusters based on the TYCHO2 Catalogue. II. Clusters farther than 1 kpc
We determined the mean absolute proper motion of 94 open clusterssituated farther than 1 kpc from the Sun. The results are derived fromthe stellar proper motion data given in the Tycho2 Catalogue. The meanproper motion of the clusters and membership probability of individualstars were obtained from the proper motion data by applying thestatistical method proposed by Sanders (\cite{Sanders1971}). Themeasurements made use of a large number of stars, usually several tens,for each cluster. The total number of stars investigated in the fieldsof the clusters is 4864 of which 2021 were considered members. For 55clusters, this is the first determination of the proper motion. Based onobservations of the ESA Hipparcos satellite. Tables 1 to 95 are 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/388/168
| A study of spatial structure of galactic open star clusters
In order to study the relation between the core and corona in galacticstar clusters, the spatial structure of 38 rich open star clusters hasbeen studied using radial density profiles derived from the photometricdata of the Digital Sky Survey. The shape of the radial density profileindicates that the corona, most probably, is the outer region around thecluster. It can exist from the very beginning of the cluster formationand dynamical evolution is not the reason for its occurrence. The studydoes not find any relation between cluster size and age but indicatesthat the clusters with galacto-centric distances >9.5 kpc have largersizes. Further, we find that the average value of the core radius is1.3+/- 0.7 pc and that of annular width of the corona is 5.6+/- 1.9 pc,while average values of densities of cluster members in the core andcorona are 15.4+/- 9.9 star/pc2 and 1.6+/- 0.99star/pc2 respectively. Average field star contaminations inthe core and corona are ~ 35% and 80% respectively. In spite of smallerdensities in the coronal region, it contains ~ 75% of the clustermembers due to its larger area in comparison to the core region. Thisclearly demonstrates the importance of the coronal region in studiesdealing with the entire stellar contents of open star clusters as wellas their dynamical evolution. In contrast to the cluster cores, thestructure of coronal regions differs significantly from one cluster toother.
| NGC 2571: An intermediate-age open cluster with a White Dwarf candidate
CCD UBVI imaging photometry was carried out in the field of the opencluster NGC 2571. From the analysis of our data we state the cluster isat a distance of 1380 +/- 130 pc and its age is 50 +/- 10 x106 yr. The cluster mass function has a slope larger than atypical Salpeter's law. There are two notorious features in NGC 2571:the cluster contains a high proportion of stars located below thereference line that are serious candidates to be metallic line stars(probably Am-Fm), and shows also a sharp gap along its main sequencethat cannot be explained by a random process nor by a biased rejectionof cluster members. A striking blue object was detected in the clusterfield that could be a white dwarf candidate. Based on observationscollected at the European Southern Observatory ESO at La Silla, Chile.
| Young star clusters in the Large Magellanic Cloud: NGC 1805 and 1818
We present colour-magnitude diagrams for two rich(~104Msolar) Large Magellanic Cloud star clusterswith ages ~107yr, constructed from optical and near-infrareddata obtained with the Hubble Space Telescope. These data are part of anHST project to study LMC clusters with a range of ages. In this paper weinvestigate the massive star content of the young clusters, anddetermine the cluster ages and metallicities, paying particularattention to Be-star and blue-straggler populations and evidence of agespreads. We compare our data with detailed stellar-populationsimulations to investigate the turn-off structure of ~25Myr stellarsystems, highlighting the complexity of the blue-straggler phenomenon.
| Estimation of Errors in the Distances to Intrinsically Reddened Stars
Not Available
| Star Formation in a Crossing Time
Observations suggest that star formation occurs in only one or twocrossing times for a range of scales spanning a factor of ~1000. Theseobservations include (1) measurements of embedded cluster ages incomparison with the cloud core dynamical times, (2) measurements of theage difference versus separation for clusters in the Large MagellanicClouds in comparison with the crossing time versus size correlation formolecular clouds, (3) the hierarchical structure of embedded youngclusters, and (4) the high fraction of dense clouds that contain starformation. Such a short overall timescale for star formation impliesthat sources of turbulent energy or internal feedback are not requiredto explain or extend cloud lifetimes and that star and protostarinteractions cannot be important for the stellar initial mass function.Stars appear in a cloud as if they freeze out of the gas, preserving theturbulent-driven gas structure in their birth locations. The Galaxy-widestar formation rate avoids the Zuckerman-Evans catastrophe, which haslong been a concern for molecular clouds that evolve this quickly,because the multifractal structure of interstellar gas ensures that onlya small fraction of the mass is able to form stars. Star formation onlarge scales operates more slowly than on small scales, but in mostcases the whole process is over in only a few dynamical times.
| Absolute proper motions of open clusters. I. Observational data
Mean proper motions and parallaxes of 205 open clusters were determinedfrom their member stars found in the Hipparcos Catalogue. 360 clusterswere searched for possible members, excluding nearby clusters withdistances D < 200 pc. Members were selected using ground basedinformation (photometry, radial velocity, proper motion, distance fromthe cluster centre) and information provided by Hipparcos (propermotion, parallax). Altogether 630 certain and 100 possible members werefound. A comparison of the Hipparcos parallaxes with photometricdistances of open clusters shows good agreement. The Hipparcos dataconfirm or reject the membership of several Cepheids in the studiedclusters. Tables 1 and 2 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| The Galactic metallicity gradient
We have previously published intermediate to high resolutionspectroscopic observations of approximately 80 early B-typemain-sequence stars situated in 19 Galactic open clusters/associationswith Galactocentric distances distributed over 6<= Rg<=18 kpc. This current study collates and re-analyses theseequivalent-width datasets using LTE and non-LTE model atmospheretechniques, in order to determine the stellar atmospheric parameters andabundance estimates for C, N, O, Mg, Al and Si. The latter should berepresentative of the present-day Galactic interstellar medium. Ourextensive observational dataset permits the identification ofsub-samples of stars with similar atmospheric parameters and ofhomogeneous subsets of lines. As such, this investigation represents themost extensive and systematic study of its kind to date. We concludethat the distribution of light elements (C, O, Mg & Si) in theGalactic disk can be represented by a linear, radial gradient of-0.07+/-0.01 dex kpc-1. Our results for nitrogen and oxygenviz. (-0.09+/-0.01 dex kpc-1 and -0.067+/-0.008 dexkpc-1) are in excellent agreement with that found from thestudy of H II regions. We have also examined our datasets for evidenceof an abrupt discontinuity in the metallicity of the Galactic disk neara Galactocentric distance of 10 kpc (see Twarog et al. \cite{twa97}).However, there is no evidence to suggest that our data would be betterfitted with a two-zone model. Moreover, we observe a N/O gradient of-0.04+/-0.02 dex kpc-1 which is consistent with that foundfor other spiral galaxies (Vila-Costas & Edmunds \cite{vil93}).
| The central part of the young open cluster NGC 6383
The spectral and extinction properties of 14 pre-main sequencecandidates in the central part of the very young open cluster NGC 6383were investigated. None of these stars shows evidence for anomalouscircumstellar extinction. However, six out of 14 programme stars do showan infrared excess, indicative of the presence of circumstellar dust,heated up by the central star. One of these stars (number 4), also showsHα in emission and shows some indications for the presence ofcircumstellar gas in its spectrum, and might therefore be a newlyidentified Herbig Ae star. Based on observations collected at theEuropean Southern Observatory, La Silla, Chile.
| On the evolutionary status of Be stars
We present a study of the incidence of Be stars in open clusters as afunction of the cluster age, using whenever possible ages determinedthrough Strömgren uvby photometry. For the first time in studies ofthis kind we have considered separately classical and Herbig Be stars.The main results can be summarized as follows: Clusters associated toemitting nebulosities and undergoing stellar formation are rich inemission line objects, which most likely are all pre main-sequencestars. No bona fide classical Be star has yet been identified amongthem. Clusters younger than 10 Myr and without associated nebulosity arealmost completely lacking Be stars, although they have a completeunevolved B main sequence. Classical Be stars appear at an age of 10Myr, and reach the maximum abundance in the age interval 13-25 Myr. Weinterpret our results in the sense that the Be phenomenon is anevolutionary effect which appears in the second half of the mainsequence lifetime of a B star. We propose that it can be related to mainstructural changes happening at this evolutionary phase, which also leadto the recently discovered non-monotonic helium abundance enhancement.The semiconvection or turbulent diffusion responsible of the surfacehelium enrichment, coupled with the high rotational velocity, cangenerate magnetic fields via the dynamo effect and thereby originate theBe phenomenon. Observational tests to this hypothesis are proposed.
| Mass segregation in very young open clusters.
The study of the very young open cluster NGC 6231 clearly shows thepresence of a mass segregation for the most massive stars. Theseobservations, combined with those concerning other young objects andvery recent numerical simulations, strongly support the hypothesis of aninitial origin for the mass segregation of the most massive stars. Theseresults led to the conclusion that massive stars form near the center ofclusters. They are strong constraints for scenarii of star and stellarcluster formation.
| Spectroscopic binary orbits from ultraviolet radial velocities. Paper 30: HD 164402
Not Available
| Statistical parallaxes and kinematical parameters of classical Cepheids and young star clusters
The statistical-parallax method is applied for the first time to spacevelocities of 270 classical Cepheids with proper motions adopted fromHIPPARCOS (1997) and TRC (Hog et al. 1998) catalogs and distances basedon the period-luminosity relation by Berdnikov et al. (1996). Thedistance scale of short-period Cepheids (with periods less than 9 days)is shown to require an average correction of 15-20%, whereas statisticalparallaxes of Cepheids with periods > 9 days are found to agree wellwith photometric distances. It is shown that the luminosities ofshort-period Cepheids must have been underestimated partly due to thecontamination of this subsample by a substantial (20 to 40%) fraction offirst-overtone pulsators. The statistical-parallax technique is alsoapplied for the first time to 117 open clusters younger than 100 millionyears and with proper motions reduced to the HIPPARCOS reference system.It is concluded that a 0.12-0.15 mag increase of the distance scales ofopen clusters and Cepheids would be sufficient to reconcile thestatistical-parallax results inferred for these two types of objects.Such approach leads to an LMC distance modulus of less than 18.40 mag,which agrees, within the errors, with the short distance scale for RRLyrae variables and is at variance with the conclusions by Feast andCatchpole (1998) and Feast et al. (1998), who argue that the LMCdistance modulus should be increased to 18.70 mag. The distance scalebased on the Cepheid period-luminosity relation by Berdnikov and Efremov(1985) seems to be a good compromise. Extragalactic distances, whichrely on long-period Cepheids, seem to require no substantial correction.In addition to statistical parallaxes, kinematical parameters have beeninferred for the combined sample consisting of Cepheids andopen-clusters: solar-motion components (U0 ,V0,W0) = (9, 12, 7) km/s (+/- 1 km/s); velocity-ellipsoid axes(σU; σV; σW) = (15.0,10.3, 8.5) km/s (+/- 1 km/s); the angular velocity of rotation of thesubsystem, ω0 = 28.7 +/- 1 km/s/kpc, the Oort constantA = 17.4 +/- 1.5 km/s, and the second derivative of angular velocity,⋰ω0= 1.15 +/- 0.2 km/s/kpc3.
| Corridors in the regions of young open stellar clusters.
Not Available
| UBVRI and H-alpha photometry of the young open cluster NGC 6231
UBVRI and H-alpha CCD photometry has been obtained for the young opencluster NGC 6231. From the R - H-alpha color, a measure of the H-alphaemission, we find 12 pre-main-sequence (PMS) members and seven possiblePMS members in the cluster. Applying recent stellar evolutionary tracksand PMS evolutionary tracks, we derive the age, age spread, and initialmass function of the cluster. The initial mass function of NGC 6231 is aGaussian with a turnover at log m about 0.4 (m about 2.5 solar masses)and shows an abrupt decrease in the number of low-mass stars. Possiblereasons for the suppression of low-mass star formation and starformation processes are discussed.
| Evolution of mass segregation in open clusters: some observational evidences
On the basis of the best available member list and duplicityinformation, we have studied the radial structure of Praesepe and of thevery young open cluster NGC 6231. We have found mass segregation amongthe cluster members and between binaries and single stars, which isexplained by the greater average mass of the multiple systems. However,the degree of mass segregation for stars between 1.5 and 2.3 M_sun isless pronounced in Praesepe than in the Pleiades. Furthermore, masssegregation is already present in the very young open cluster NGC 6231although this cluster is likely still not dynamically relaxed. Wediscuss the implications of these results and propose a qualitativescenario for the evolution of mass segregation in open clusters. InPraesepe the mass function of single stars and primaries appears to besignificantly different, like in the Pleiades. We observe an absence ofellipticity of the outer part of Praesepe. Tables 2 and 3 are availableonly in electronic form from the Strasbourg ftp server at 130.79.128.5
| Absolute proper motions of 181 young open clusters.
Not Available
| The Galactic Oxygen Abundance Gradient
We have previously published medium- to high-resolution spectroscopicobservations of early B-type main-sequence objects, lying in 18 young,stellar clusters and associations or the general disk population, overgalactocentric distances 6 <~ Rg <~ 18 kpc. Using a homogeneoussubset of stars with similar atmospheric parameters, we have reanalyzedthe O II line spectra using non-LTE line formation calculations. Wederive an oxygen abundance gradient of -0.07 +/- 0.01 dex kpc-1, whichshould be representative of the present-day interstellar medium withinthe Galaxy. This extensive and self-consistent data set represents aconsiderable advance on previous attempts to measure the Galactic oxygenabundance gradient using early-type stars and produces a significantgradient for the first time. The outer disk is well sampled, which givesthe result statistical significance, and the discrepancies with previoussimilar studies are discussed. We compare this new result to thosederived using Galactic H II regions and planetary nebulae and concludethat the different methods now appear to produce consistent agreement.
|
提交文章
相关链接
提交链接
下列团体成员
|
观测天体数据
目录:
|
