내용
사진
사진 업로드
DSS Images Other Images
관련 글
Ca II Triplet Spectroscopy of Large Magellanic Cloud Red Giants. I. Abundances and Velocities for a Sample of Populous Clusters
Using the FORS2 instrument on the Very Large Telescope, we have obtainednear-infrared spectra for more than 200 stars in 28 populous LMCclusters. This cluster sample spans a large range of ages (~1-13 Gyr)and metallicities (-0.3>~[Fe/H]>~-2.0) and has good areal coverageof the LMC disk. The strong absorption lines of the Ca II triplet areused to derive cluster radial velocities and abundances. We determinemean cluster velocities to typically 1.6 km s-1 and meanmetallicities to 0.04 dex (random error). For eight of these clusters,we report the first spectroscopically determined metallicities based onindividual cluster stars, and six of these eight have no publishedradial velocity measurements. Combining our data with archival HubbleSpace Telescope WFPC2 photometry, we find that the newly measuredcluster, NGC 1718, is one of the most metal-poor ([Fe/H]~-0.80)intermediate-age (~2 Gyr) inner disk clusters in the LMC. Similar towhat was found by previous authors, this cluster sample has radialvelocities consistent with that of a single rotating disk system, withno indication that the newly reported clusters exhibit halo kinematics.In addition, our findings confirm previous results that show that theLMC lacks the metallicity gradient typically seen in nonbarred spiralgalaxies, suggesting that the bar is driving the mixing of stellarpopulations in the LMC. However, in contrast to previous work, we findthat the higher metallicity clusters (>~-1.0 dex) in our sample showa very tight distribution (mean [Fe/H]=-0.48, σ=0.09), with notail toward solar metallicities. The cluster distribution is similar towhat has been found for red giant stars in the bar, which indicates thatthe bar and the intermediate-age clusters have similar star formationhistories. This is in good agreement with recent theoretical models thatsuggest the bar and intermediate-age clusters formed as a result of aclose encounter with the SMC ~4 Gyr ago.
| A New Giant Branch Clump Structure in the Large Magellanic Cloud
We present Washington C and T1 CCD photometry of 21 fieldslocated in the northern part of the Large Magellanic Cloud (LMC), andspread over a region of more than 2.5 deg2 approximately 6deg from the bar. The surveyed areas were chosen on the basis of theirproximity to SL 388 and SL 509, whose fields showed the presence of asecondary giant clump, observationally detected by Bica et al. We alsoobserved NGC 2209, located ~14 deg away from SL 509. From the collecteddata, we found that most of the observed field CMDs do not show aseparate secondary clump, but rather reveal a continuous verticalstructure (VS), which is clearly seen for the first time. The VS alsoappears in the field of NGC 2209. Its position and size are nearly thesame throughout the surveyed regions: it lies below the red giant clump(RGC) and extends from the bottom of the RGC to ~0.45 mag fainter,spanning the bluest color range of the RGC. In two fields in addition tothe NGC 2209 field the RGC is slightly tilted, following approximatelythe reddening vector, while the VS maintains its verticality. We foundthat the number of stars in the VS box defined byΔ(C-T1)=1.45-1.55 mag andΔT1=18.75-19.15 mag has a strong spatial variation,reaching the highest VS star density just northeast of SL 509. Moreover,the more numerous the VS stars in a field, the larger the number of LMCgiants in the same zone. We also found that, in addition to SL 509, tworelatively massive star clusters, SL 515 and NGC 2209, separated by morethan 10 deg from each other, develop giant clumps with a considerablenumber of VS stars. This result demonstrates that VS stars belong to theLMC and are most likely the result of some kind of evolutionary processin the LMC, particularly in those LMC regions with a noticeable largegiant population. Our results are successfully predicted by the modelsof Girardi in the sense that a large proportion of 1-2 Gyr old starsmixed with older stars and with metallicities higher than [Fe/H]~=-0.7should result in a fainter and bluer secondary clump near the mass atwhich degenerate core He burning takes place. However, our resultsapparently suggest that in order to trigger the formation of VS stars,there should be other conditions in addition to the appropriate age,metallicity, and the necessary red giant star density. Indeed, starssatisfying the requisites mentioned above are commonly found throughoutthe LMC, but the VS phenomenon is only clearly seen in some isolatedregions. Finally, the fact that clump stars have an intrinsic luminositydispersion further constrains the use of the clump magnitude as areliable distance indicator.
| A Revised and Extended Catalog of Magellanic System Clusters, Associations, and Emission Nebulae. II. The Large Magellanic Cloud
A survey of extended objects in the Large Magellanic Cloud was carriedout on the ESO/SERC R and J Sky Survey Atlases, checking entries inprevious catalogs and searching for new objects. The census provided6659 objects including star clusters, emission-free associations, andobjects related to emission nebulae. Each of these classes containsthree subclasses with intermediate properties, which are used to infertotal populations. The survey includes cross identifications amongcatalogs, and we present 3246 new objects. We provide accuratepositions, classification, and homogeneous measurements of sizes andposition angles, as well as information on cluster pairs andhierarchical relation for superimposed objects. This unification andenlargement of catalogs is important for future searches of fainter andsmaller new objects. We discuss the angular and size distributions ofthe objects of the different classes. The angular distributions show twooff-centered systems with different inclinations, suggesting that theLMC disk is warped. The present catalog together with its previouscounterpart for the SMC and the inter-Cloud region provide a totalpopulation of 7847 extended objects in the Magellanic System. Theangular distribution of the ensemble reveals important clues on theinteraction between the LMC and SMC.
| Integrated UBV Photometry of 624 Star Clusters and Associations in the Large Magellanic Cloud
We present a catalog of integrated UBV photometry of 504 star clustersand 120 stellar associations in the LMC, part of them still embedded inemitting gas. We study age groups in terms of equivalent SWB typesderived from the (U-B) X (B-V) diagram. The size of the spatialdistributions increases steadily with age (SWB types), whereas adifference of axial ratio exists between the groups younger than 30 Myrand those older, which implies a nearly face-on orientation for theformer and a tilt of ~45^deg^ for the latter groups. Asymmetries arepresent in the spatial distributions, which, together with thenoncoincidence of the centroids for different age groups, suggest thatthe LMC disk was severely perturbed in the past.
| Spectroscopy of giants in LMC clusters. II - Kinematics of the cluster sample
Velocities for 83 star clusters in the LMC are analyzed, based onindividual stellar velocities measured at the Calcium triplet. One-halfof the clusters are objects in the outer parts of the LMC which had noprevious velocity determinations. Published velocities for intermediateand old clusters are shown to have had systematic errors. These newvelocities with various rotation curve analyses of the LMC, and testaspects of the twisted disk model proposed by Freeman et al. (1983).When the transverse motion of the LMC is taken into account, a singlerotating disk solution fits the old and intermediate-aged clusters andother tracers (i.e., there is no need for an additional 'tilted disk'system).
| Spectroscopy of giants in LMC clusters. I - Velocities, abundances, and the age-metallicity relation
Velocities and equivalent widths are presented for a large sample of LMCclusters. The calcium abundance is found to be a sensitive abundanceindicator over a very wide range of (Fe/H) between 0.0 and -2.2. Theage-metallicity relation is constructed for the inner and outer parts ofthe LMC. This relationsip can be characterized by a simple one-zoneenrichment model. The abundances for the inner and outer clusters at anage of 2 Gyr are nearly identical, so that little radial abundancegradient is evident in the cluster system.
| The cluster system of the Large Magellanic Cloud
A new catalog of clusters in the Large Magellanic Cloud has beenconstructed from searches of the IIIa-J component of the ESO/SERCSouthern Sky Atlas. The catalog contains coordinate and diametermeasurements of 1762 clusters in a 25 deg x 25 deg area of sky centeredon the LMC, but excluding the very crowded 3.5 sq deg region around theBar. The distribution of these clusters appears as two superimposedelliptical systems. The higher density inner system extends over about 8deg; the lower density outer system can be represented by a 13 deg x 10deg disk inclined at 42 deg to the line of sight. There are suggestionsof two weak 'arms' in the latter.
| A catalog of LMC star clusters outside the Hodge-Wright atlas
The paper presents a catalog of 156 clusters outside the boundaries ofthe Hodge and Wright (1967) LMC atlas. The catalog contains coordinatesaccurate to 1-2 arcsec, offsets from the edge of the appropriate SRCJplates, cross references to previous identifications, and finding chartsof the brighter clusters. As defined by the clusters, the Hodge andWright atlas is found to represent the extent of the LMC to the west,and reasonably well to the east. To the north and the south, the clustersystem extends substantially beyond the boundaries of the atlas. Thesouthern clusters delineate a portion of the 'spiral arm' noted by deVaucouleurs (1955).
| Distribution of spectral types in the LMC clusters
The distribution of spectral types in 42 LMC globular star clusterscovering all evolutionary ages was determined using objective prismspectra taken with the 1.2-m U.K. Schmidt Telescope in Australia. Thederived spectral type distributions show that the clusters can bedivided into five age categories from about 10 to the 7th to more than10 to the 9th yr. Several clusters were found to contain carbon starswith C/M ratios ranging from 0.07 to 0.4. These ratios were comparedwith those found for the SMC clusters and the Milky Way. It was foundthat the stars of the LMC exhibit a smaller range of C/M ratios than inthe SMC, but larger than in the Galaxy, thus providing an additionaltest of the theoretical models predicting a correlation between the C/Mratio and metal content. It was also found that the majority of youngclusters were embedded in older fields, while the intrmediate clusterswere embeded in younger fields, and the remote old clusters wereembedded in a stellar content of similar age.
| A Catalogue of Clusters in The LMC
Not Available
| A catalogue of clusters in the outer parts of the Large Magellanic Cloud
Abstract image available at:http://adsabs.harvard.edu/abs/1963MNRAS.127...31L
|
새 글 등록
관련 링크
새 링크 등록
다음 그룹에 속해있음:
|
관측 및 측정 데이터
천체목록:
|
