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Cluster Structural Parameters and Proper Motions

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Cluster Structural Parameters and Proper Motions Astronomy Reports, Vol. 47, No. 4, 2003, pp. 263–275. Translated from Astronomicheski˘ı Zhurnal, Vol. 80, No. 4, 2003, pp. 291–303. Original Russian Text Copyright c 2003 by Kharchenko, Pakulyak, Piskunov. The Subsystem of Open Clusters in the Post-Hipparcos Era: Cluster Structural Parameters and Proper Motions N. V. Kharchenko1,L.K.Pakulyak1,andA.E.Piskunov2 1Main Astronomical Observatory, ul. Akademika Zabolotnogo 27, Kiev, 03680 Ukraine 2Institute of Astronomy, Russian Academy of Sciences, ul. Pyatnitskaya 48, Moscow, 109017 Russia Received August 23, 2002; in final form, October 10, 2002 Abstract—The wide neighborhoods of 401 open clusters are analyzed using the modern, high-precision, homogeneous ASCC-2.5 all-skycatalog. More than 28 000 possible cluster members (including about 12 500 most probable members) are identified using kinematic and photometric criteria. Star counts with the ASCC-2.5 and USNO-A2.0 catalogs are used to determine the angular and linear radii of the cluster cores and coronas, which exceed the previouslypublished values byfactors of two to three. The segregation (differing central concentration) of member stars bymagnitude is observed. The mean proper motions are determined directlyin the Hipparcos system for 401 clusters, for 183 of them for the first time. The heliocentric distances of 118 clusters are determined for the first time based on color–magnitude diagrams for their identified members. c 2003 MAIK “Nauka/Interperiodica”. 1. INTRODUCTION velocity, color–magnitude diagram, stellar content, and age. Open star clusters are typical Galactic disk- The last decade has witnessed sweeping qualita- population objects. At the same time, the spatial tive and quantitative developments in the creation of coordinates, velocities, and ages of open clusters can all-skycatalogs of positional, kinematic, and pho- be determined much more accuratelythan for other tometric stellar data. The results of the Hipparcos Population-I objects. The reason is that, usually, up space mission formed the basis for several catalogs to several hundred stars can be observed in each (Hipparcos, Tycho-1 [1], and Tycho-2 [2]), which cluster, and the spatial–kinematic parameters of all contain high-precision stellar data and establish an the cluster stars are either virtuallyidentical or obey inertial reference frame and two-color photometric certain relations (spectroscopic and photometric da- system over the entire sky. Many ground-based cata- ta). This makes it possible to select cluster members logs have alreadybeen reduced to these systems,in- among all stars seen in the direction of a cluster, cluding the currentlymost massive and deepest cat- making these objects an especiallyvaluable tool alog, containing more than 500 million stars down to for analyzing the structure and evolution of stellar a limiting magnitude of B 22m—the USNO-A2.0 systems such as individual clusters or the Galactic catalog [3] (hereafter, the USNO catalog). Recently, disk as a whole. Kharchenko [4] combined all the data from modern, The principal drawback of clusters as objects for high-precision, astrometric catalogs containing stars such studies is the inhomogeneity, insufficient ac- down to V =12− 14m into the “All-SkyCombined curacy, and incompleteness of stellar data. This is Catalog of Astrometric Data for 2.5 Million Stars” due to the fact that the available stellar data—proper (hereafter, the ASCC). motions and photometric parameters being the most About 1500 open clusters have been discovered in abundant among them—have been obtained in clus- the Galaxyso far; however, onlyafter the Hipparcos ter neighborhoods byvarious authors using di fferent mission had been successfullycompleted and its re- instruments and different instrumental systems with sults implemented in practice did it become possible different degrees of completeness and accuracy. Fur- to create catalogs of open-cluster parameters deter- thermore, the apparent surface distribution of cluster mined in systems that are uniform over the entire sky members can be distorted byinterstellar clouds along in terms of the observational material and/or methods the line of sight and/or inside young open clusters. used. All this makes the correct identification of cluster The most complete source of geometrical and as- member stars rather difficult, distorting the principal trophysical parameters of open clusters is the Lund parameters of the cluster: its spatial structure, space Catalog of Open Cluster data (LCOC), compiled by 1063-7729/03/4704-0263$24.00 c 2003 MAIK “Nauka/Interperiodica” 264 KHARCHENKO et al. Lynga˚ [5] and containing a total of 1150 objects. precision stellar data obtained in all-sky, homoge- These data are the result of compiling an extensive neous, astrometric and photometric systems, having bibliographyand contain, among other things, the in view the subsequent use of these parameters for distances and ages of 420 open clusters. The dis- analyzing individual open clusters and the Galactic tances, color excesses, and ages of 340 open clusters, disk as a system of such clusters. We address some recentlyexpanded to include a total of 425 open clus- of these tasks in this paper. Section 2 describes the ters, can be found in [6–8]. sources of the observational data, and Section 3, The studyof the nearest clusters was among the the open-cluster sample used and the technique em- primarytasks of the Hipparcos [1] mission. Robichon ployed. Section 4 summarizes the underlying princi- et al. [10], Baumgardt et al. [11], and Loktin and ples of identifying probable cluster members based on Beshenov [12] determined the trigonometric paral- proper-motion data and BV spaceborne photometry laxes of 50, 205, and 45 open clusters based on pre- and the results of applying these principles. Sec- selected lists of 550, 715, and 638 members, respec- tion 5 analyzes the structural parameters of the open tively. Robichon et al. [10] and Baumgardt et al. [11] clusters obtained, and Section 6, specific features obtained the proper motions of the open clusters in of the distributions of stars of various masses in- the Hipparcos reference frame. Dias et al. [13, 14] side these open clusters. Section 7 reports the open- determined these parameters for 205 clusters us- cluster proper motions in the Hipparcos frame deter- ing data for about 5900 Tycho-2 [2] stars identi- mined in this paper. fied as cluster members using the Sanders statistical method. As a result, open-cluster proper motions 2. CATALOGS OF STELLAR DATA were first determined directlyin an inertial reference frame. Glushkova et al. [15] determined the proper Our principal tools for determining the parameters motions of 331 open clusters using 2980 photomet- of the open clusters were the ASCC and USNO star ricallyidenti fied cluster members selected from the catalogs. “Catalog of About Four Million Stars” [16] created The ASCC, which is based on large, modern, at the Sternberg Astronomical Institute and then re- high-precision catalogs and covers the magnitude duced the resulting proper motions to the Hipparcos interval V =12− 14m, is most complete in terms reference frame. In all these studies, the authors have of its list of stars and the composition of the stel- adopted previouslypublished structural parameters lar data reduced to homogeneous, all-sky, standard for the open clusters (the central coordinates and systems. The input data for the ASCC include both sizes) without redetermining them. catalogs based directlyon data from the Hippar- The size of a cluster is one of its main parame- cos and Tycho [1] projects (the files hip_main.dat ters, which is related to the dynamical states of the and h_dm_com.dat, with multiple-star components, cluster and the Galactic disk and plays a key role in and tyc_main.dat), and catalogs based on earlier da- determining whether particular stars belong to the ta from the Astrographic Catalog: ACT RC [19], cluster or not. Often, however, onlythe size of the TRC [20], and Tycho-2 [2]. The list of Hipparcos central region of the cluster has been considered, stars is incomplete, especiallyin crowded fields, where although manystudies (see Kholopov’s [17] mono- some of the clusters are located. The same draw- graph for references) have found that clusters pos- back is also characteristic of catalogs based on much sess extended coronas whose identification is made more complete observations performed in the Tycho difficult bysuperposed rich stellar fields. It is the project, due to the specifics of the TUC 1/2 input size of the core—even if the cluster was known to star lists, which were based primarilyon the Guide possess a corona—that Lynga˚ tended to include in Star Catalog [21]. Therefore, an appreciable fraction the LCOC, and manyauthors mistook these sizes for of stars in such areas of the skyis provided bythe those of the entire clusters. Researchers have rarely ground-based CMC11 [22] and PPM [23–25] cata- redetermined cluster sizes, and veryfew of them have logs, which we also used in compiling the ASCC. done this on a massive scale using a homogeneous The ASCC includes 2 501 968 stars with J2000 technique. Danilov and Seleznev [18] (hereafter DS) equatorial coordinates for the observing epoch of created the most homogeneous catalog of structural 1991.25 and compiled proper motions in the Hip- characteristics of 103 Northern clusters based on star parcos system. The trigonometric parallaxes are counts down to B 16m performed on wide-angle computed as weighted averages over the catalogs [1]. plates obtained with a single instrument—the SBG With few exceptions, the magnitudes are adopted camera of the Kourovka Observatory. from the Tycho catalogs and reduced to the Johnson Our aim is to determine the structural and kine- B,V system using a photometric reference system matic parameters of all open clusters whose mem- defined by20 000 stars never suspected to be variable. bers can be found in large modern catalogs of high- The final catalog gives the standard errors for all these ASTRONOMY REPORTS Vol.
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