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The detailed knowledge of young open clusters (age &lt

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Chapter 1 Introduction

1.1 Young Open Clusters

An open cluster is a group of stars that are bounded by gravitation, but more concentrated than associations, and looser than globular clusters. All the members of an open cluster are thought to be originated from the same cloud, due to their similar chemical composition. The members are formed in a relatively short period comparing to its age, most stars could be considered as the same age, so that the open clusters are good tool to study stellar structure and evolution (e.g. Pérez 1988, Pallavicini 2003, etc.). The detailed knowledge of young open clusters (age < 108 years) is especial important to understand the process of star formation and initial mass function (IMF) (e.g. Williams 1994).

The age of open clusters span a wider range, from a few million years to about 10 Gyr (e.g. Pandey 1986), so open clusters are also one of the most important tools to study the global properties of Galactic disk (e.g. Lynga 1987), such as the Galactic structure, dynamics, evolution (e.g. Vogt 1975) and chemical abundance gradient (e.g. Panagia 1980). The orbits of open clusters are more stable than that of the individual star, hence, the metallicity gradients of Galactic disk are better preserved in open cluster.

Furthermore, the young open clusters and OB associations are also the good tracer to understand the motion of spiral arms and the rotation curve of Galaxy (e.g. Hron 1987 and Babu 1991).

To be a tracer of Galactic arms, the young open clusters are superior to others. In general, the spiral arms locate at Galactic disk. The extinction confines the observable distance. However, the most massive, brightest stars of young open clusters are still on the main sequence. Even in a large distance and serious extinction, these bright stars are detectable. And the young open clusters in other spiral galaxies are also good tool to trace out the spiral arms and appearances of galaxies. (e.g. Schmidt-Kaler 1967 and Blanco 1990)

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Finally, studies of Galactic clusters are relevant in extragalactic astronomy. Detailed knowledge of young open clusters and OB associations will be helpful in understanding of starburst galaxies and the population of young galaxies in early universe. (e. g. Franceschini 1998 and Anders 2004)

New Catalog of Optically Visible Open Clusters and Candidates (Dias, et al. 2003) listed all 1,690 presently known open clusters in our Galaxy.

However, most of them are short in details, only 38% the distance, E(B-V) and age are determined. The data on kinematics, only 22% have mean proper motions listed, 14% have mean radial velocities, and 11% have information both. It shows that observations of open clusters are still lack and required.

This research is to study the young open clusters NGC 6910 and Berkeley 87 by combining multi-color CCD photometry and star counting method to derive its fundamental parameters, namely size, reddening, distance, age and mass.

1.2 NGC 6910

NGC 6910 is located in the field of RA (2000): 20h 23m 12s, Dec (2000):

+40°46′ 42″ (l= 78°.68, b = +2°.01) (Fig.1-1), the heavily obscured region in Cygnus (Delgado, 2000), and the field near the bright star ? Cyg, proximity to the compact OB association Cyg OB2. The field of NGC 6910 is the part of the complex mixing with clusters, associations, clouds, dusts and star formation region RSF 2 Cyg (Shevchenko 1989) ( Fig.1-2 and 1-3).

NGC 6910 is a very young cluster with age 7~13 Myrs (Dias et al 2002 Delgado and Alfaro 2000, Vansevicius 1989, Hagen 1970 and Markarjan 1951). Photoelectric, photographic and CCD photometry for the stars in NGC 6910 have been observed by many investigators, Becker and Stock (1948), Hoag, et al. (1965), Onegina (1956), Mathews (1963), Tifft (1958), Vansevicius (1989), Delgado and Alfaro (2000). The spectral classification of bright stars are also listed by many researchers, Morgan W.W., Code A.D., Whitford (1955), Hiltner (1956), Hoag and Applequist (1965), Walborn (1971), Sowell (1987), Tifft (1958) and Yoss (1961).

The available data of NGC 6910 are summerized in Table 1.

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Fig.1-1: The positions of NGC 6910 and Berkeley 87 (plotted with Guide 7.0)

Table. 1. The fundamental parameters of NGC 6910

R.A. (J2000) 20h 23m 12s (Dias et al. 2002) Dec. (J2000) +40° 46' 42? (Dias et al. 2002)

l 78.68° (Dias et al. 2002)

b 2.01° (Dias et al. 2002)

Diameter 10' (Markarjan 1951)

7' (Dias et al. 2002)

Distance 1738 pc (Delgado and Alfaro 2000) 1139 pc (BDA: Mermilliod 1995) 1530 pc (Vansevicius 1989) 1650 pc (Lynga 1987) 1507 pc (Hagen 1970)

E(B-V) 0.971 (Dias et al 2002)

1.02±0.13 (Delgado and Alfaro 2000)

1.05 (Hagen 1970)

1.05 (Hardorp 1960)

0.93 (Barkhatova 1958)

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Extinction (Av) 3.65 mag (Vansevicius 1989) log(Age) 7.127 (Dias et al. 2002)

6.81 (Delgado and Alfaro 2000) 6.85 (Vansevicius 1989)

10 (Hagen 1970)

7 (Markarjan 1951)

Fig. 1-2: The map of NGC 6910 and Berkeley 87 region. The clusters are superimposed on the IRAS 60 micron image to present the distribution of dust.

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Fig. 1-3: The objects from SIMBAD are marked on the same field of IRAS image as Fig.1-2.

1.3 Berkeley 87

The location of Berkeley 87 is RA(2000):20h21m42s, Dec(2000):

+37°22'00?, l=75.71°, b=+0.31°) (Fig.1-1), and it is a sparse young cluster.

The same as NGC 6910, it also locates near the bright star ? Cyg, a quite heavily obscured region. It lies at the edge of the large molecular cloud complexs in the Cyg X region (Cong 1977), and associated with a group of compact HII regions ON2 complex (Pitault 1981). The local arm stretches out in the line of sight, many structures such as star formation regions, clusters, associations, clouds and dusts all overlap (Fig.1-2 and 1-3). It contains some rare object, such as WR star, Be supergiant, are valuable for studying the intrinsic properties and evolutionary status of these rare type stars.

Table. 2. The fundamental parameters of Berkeley 87

R.A.(J2000) 20h 21m 42s (Dias et al. 2002) Dec.(J2000) +37° 22' 00? (Dias et al. 2002)

l 75.71° (Dias et al. 2002)

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b 0.31° (Dias et al. 2002) Diameter 10' (Dias et al. 2002)

12' (Lynga 1987)

10' (Alter, et al. 1970) Distance 633 pc (Dias et al. 2001)

836 pc (BDA: Mermilliod 1995) E(B-V) 1.369var. mag (BDA: Mermilliod 1995)

1.35 mag (Ahumada et al. 1995) Age(log) 7.152 yrs (BDA: Mermilliod 1995)

6.3 yrs (Ahumada et al. 1995)

The photometry of photoelectric, photographic and CCD for the field of Berkeley 87 have been carried out by Sharpless (1966), Hiltner (1956) and Turner (1982), respectively. The spectral type classification are listed by Lee T.A.(1970) and Massey P., DeGioia-Eastwood K., Waterhouse E. (2001)

In chapter 2, the observations and data reductions are described. The photometry calibration is presented in chapter 3. Color-Magnitude Diagrams (CMD) and Two-Color Diagrams (TCD) are discussed in chapter 4. The stars spatial distributions of the cluster and King model are presented in chapter 5. Finally, the results and discussion are summarized in chapter 6.

數據

Fig. 1-2: The map of NGC 6910 and Berkeley 87  region. The clusters are  superimposed  on  the  IRAS  60  micron  image  to  present  the  distribution of dust
Fig. 1-3: The objects from SIMBAD are marked on the same field of IRAS  image as Fig.1-2

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