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The Optical Gravitational Lensing Experiment. Age of Star Clusters

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The Optical Gravitational Lensing Experiment. Age of Star Clusters ACTA ASTRONOMICA Vol. 49 (1999) pp. 157±164 The Optical Gravitational Lensing Experiment. Age of Star Clusters from the SMC by G. P i e t r z y n s k i, and A. Udalski Warsaw University Observatory, Al. Ujazdowskie 4, 00-478 Warszawa, Poland e-mail: (pietrzyn,udalski)@sirius.astrouw.edu.pl Received May 4, 1999 ABSTRACT We present determination of age of clusters from 2.4 square degree region of the SMC bar. The photometric data were taken from the BVI maps of the SMC and catalog of clusters in this galaxy obtained during the OGLE-II microlensing survey. For 93 well populated SMC clusters their age is derived with the standard procedure of isochrone ®tting. The distribution of age of cluster from the SMC is presented. It indicates either non-uniform process of cluster formation or very effective disruption of clusters. Key words: Magellanic Clouds ± Galaxies: star clusters. 1. Introduction Studies of star clusters provide important information about parent galaxy and processesconnectedwith their formation and evolution. The rich systemof clusters from the SMC is especially well suited for such investigations. In particular, based on the age distribution of clusters one can look into the cluster formation history and obtain information on processes of cluster formation and disruption. Unfortunately the SMC was neglected photometrically for years. Until recently only a few papers presented precise photometric data obtained with modern obser- vational techniques for clusters from this galaxy. A few old clusters from the SMC were studied using the HST telescope (Mighell et al. 1998). Analysis of other SMC clusters can be found very sporadically in the literature. As a result precise age of the SMC clusters based on good photometric data has been derived for very limited group of objects. Based on observations obtained with the 1.3 m Warsaw telescope at the Las Campanas Observa- tory of the Carnegie Institution of Washington. 158 A. A. Information about the age of clusters from the SMC is based mostly on indirect methods like integrated photometry or calibration of brightness of the brightest star. However, the method of deriving the age from integrated photometric data may be affected by several sources of uncertainty (Girardi et al. 1995). In this paper we present new, homogeneous determination of age of 93 clusters from the center of the SMC. After presentation of the observational data, the pro- cedure of deriving the mean reddening and age is described followed by discussion of distribution of age of the SMC clusters. 2. Observational Data The observational data used in this paper were collected during the OGLE-II microlensing search with the 1.3-m Warsaw telescope located at the Las Campanas Observatory, Chile, which is operated by the Carnegie Institution of Washington. Eleven slightly overlapping ®elds located in the center of the SMC were observed. About 2.2 million stars were monitored since June 1997. The photometric data were calibrated to the standard BVI system based on a few hundred observations of standard stars from Landolt (1992) ®elds. The typical uncertainty of photometric calibration was about 0.01 mag. Astrometric position of every star was also derived with typical accuracy of 0.1 arcsec. Details on the OGLE-II project can be found in Udalski, Kubiak and Szymanski (1997). The comprehensive description of photometric and astrometric data is given by Udalski et al. (1998b). Using these data the catalog of star clusters was constructed by Pietrzynski et al. (1998). Altogether 238 objects were found. 72 of them are newly discovered clusters. The catalog of clusters in the SMC is very well suited for studies of properties of population of clusters from the SMC. In this paper we present age determination of large sample of objects from this catalog. 3. Age of the SMC Clusters 3.1. Interstellar Reddening Recently Paczynski and Stanek (1998) proposed a new method of distance determination based on the mean I-band brightness of the red clump stars. Red clump stars seem to form a very homogeneousgroup of objects. Their mean I-band magnitude does not depend on age for stars younger than 10 Gyr, and weakly depends on metallicity (Udalski 1998a,b). Large number of these stars allows to derive their mean I-band magnitude with high precision. Recent calibration based on Hipparcos measurements of a few hundred nearby red clump stars makes these objects the best calibrated standard candles. Red clump stars were used for modeling the galactic bar (Stanek et al. 1997) and determining the distances to the Magellanic Clouds (Udalski et al. 1998a). They can also be used as a reference for interstellar extinction determination. For example, Stanek (1996) presented the Vol. 49 159 extinction map of the Baade's Window in the Galactic bulge determined with this method. SMC0001 SMC0005 SMC0158 40 40 N N N 60 30 30 40 20 20 10 20 10 0 0 0 18 18.5 19 19.5 18 18.5 19 19.5 18 18.5 19 19.5 I mag I mag I mag Fig. 1. Histograms of brightness of red clump stars with ®tted function given by Eq. (1) for regions located around three clusters. In order to derive the mean reddening toward a given cluster we identi®ed red clump stars located in the cluster neighborhood excluding cluster stars. Their mean I-band magnitude was obtained in a similar manneras in Udalski et al. (1998a). All I V I objects having I-band brightness in the range of 17 5 19 5 mag and V I color index in the range of 0 7 1 1 mag were selected and histograms of the I-band brightness of these stars in 0.05 mag bins were constructed. Then, we ®t the following function: # " max 2 N (I I ) max max 2 RC p n(I ) = a + b(I I ) + c(I I ) + ) ( exp 2 1 RC 2 2 RC 2 max N I where RC RC and are the number of red clump stars, their standard deviation of brightness and maximum brightness, respectively. Fig. 1 shows ex- emplary histograms with ®tted function given by Eq. (1) obtained for regions located around three clusters. It is clearly seen that the well pronounced red clump allows for reliable ®t and thus precise determination of its mean I-band brightness. Assuming that extinction free brightness of the red clump stars in the = SMC is equal to I 18 34 mag (Udalski 1998b) and standard extinction curve A = E (B V ) E (V I ) = E (B V ) ( I 1 96 , 1 28 , Schlegel, Finkbeinerand (V I ) E (B V ) Davis1998)wederivedextinctioninthe I-bandaswellas E and reddenings. The results are given in Table 1. 3.2. Age Determination Clear distinction between cluster and ®eld stars is dif®cult in the dense regions in the center of the SMC. Because the color-magnitude diagrams (CMD) of ®elds around clusters very often resemble CMDs of clusters we decided to perform statistical subtraction of ®eld stars from the cluster CMD. A procedure described by Mateo and Hodge (1986) was adopted. 160 A. A. Table1 Age of the SMC clusters E (B V ) t Name 2000 2000 Radius log log t OGLE-CL- [ 00 ] 0 h m s SMC0002 0 37 33.06 73 36 42 6 47 0.06 8.4 0.1 0 h m s SMC0003 0 37 42.24 73 54 29 5 42 0.10 9 ± 0 h m s SMC0008 0 40 30.54 73 24 10 4 43 0.07 8.0 0.1 0 h m s SMC0009 0 40 44.11 73 23 00 2 36 0.07 8.0 0.1 0 h m s SMC0011 0 41 06.16 73 21 07 1 36 0.08 7.9 0.1 0 h m s SMC0012 0 41 23.78 72 53 27 1 61 0.06 9 ± 0 h m s SMC0013 0 42 22.37 73 44 03 1 23 0.05 7.3 0.3 0 h m s SMC0015 0 42 54.13 73 17 37 0 30 0.10 8.1 0.1 0 h m s SMC0016 0 42 58.46 73 10 07 2 42 0.08 8.3 0.1 0 h m s SMC0017 0 43 32.74 73 26 25 4 26 0.10 7.9 0.1 0 h m s SMC0018 0 43 37.57 73 26 37 9 26 0.10 7.9 0.1 0 h m s SMC0019 0 43 37.59 72 57 30 9 12 0.08 8.6 0.1 0 h m s SMC0020 0 43 37.89 72 58 48 3 9 0.08 8.6 0.1 0 h m s SMC0025 0 45 13.88 73 13 09 2 15 0.12 8.0 0.1 0 h m s SMC0032 0 45 54.33 73 30 24 2 30 0.10 8.0 0.1 0 h m s SMC0033 0 46 12.26 73 23 34 0 18 0.15 7.2 0.2 0 h m s SMC0038 0 47 06.15 73 15 24 9 21 0.12 8.1 0.2 0 h m s SMC0039 0 47 11.61 73 28 38 1 49 0.11 8.0 0.1 0 h m s SMC0043 0 47 52.38 73 13 20 3 22 0.09 8.5 0.1 0 h m s SMC0045 0 48 00.68 73 29 10 3 35 0.07 8.4 0.1 0 h m s SMC0047 0 48 28.14 72 59 00 3 36 0.12 7.8 0.1 0 h m s SMC0049 0 48 37.47 73 24 53 2 38 0.06 7.0 0.2 0 h m s SMC0050 0 48 59.02 73 09 03 8 14 0.12 8.3 0.1 0 h m s SMC0054 0 49 17.60 73 22 19 8 27 0.10 8.0 0.1 0 h m s SMC0058 0 49 45.43 72 51 58 0 36 0.17 8.3 0.1 0 h m s SMC0059 0 50 16.06 73 01 59 6 25 0.10 7.8 0.1 0 h m s SMC0060 0 50 21.95 73 23 16 5 36 0.08 8.4 0.1 0 h m s SMC0061 0 50 00.26 73 15 17 7 21 0.12 7.4 0.2 0 h m s SMC0064 0 50 39.55 72 57 54 8 36 0.10 8.1 0.1 0 h m s SMC0066 0 50 55.39 73 12 11 0 17 0.09 7.8 0.1 0 h m s SMC0067 0 50 55.54 72 43 39 7 42 0.08 8.2 0.1 0 h m s SMC0068 0 50 56.26 73 17 21 1 55 0.09 7.7 0.2 0 h m s SMC0069 0 51 14.13 73 09 41 5 36 0.08 7.6 0.1 0 h m s SMC0070 0 51 26.15 73 16 59 8 14 0.09 7.8 0.1 0 h m s SMC0071 0 51 31.78 73 00 38 3 32 0.07 7.5 0.1 0 h m s SMC0072 0 51 41.69 73 13 46 8 28 0.08 7.6 0.2 0 h m s SMC0073 0 51 44.03 72 50 25 1 42 0.11 8.2 0.2 0 h m s SMC0074 0 51 52.91 72 57 13 9 42 0.09 8.1 0.1 0 h m s SMC0075 0 51 54.32 73 05 52 9 15 0.07 8.4 0.1 0 h m s SMC0076 0 52 12.47 72 31 51 2 29 0.07 7.4 0.3 0 h m s SMC0077 0 52 13.34 73 00 12 2 18 0.08 7.9 0.2 0 h m s SMC0078 0 52 16.56 73 01 04 0 36 0.08 7.9 0.1 0 h m s SMC0081 0 52 33.65 72 40 53 6 31 0.12 7.4 0.3 0 h m s SMC0082 0 52 42.12 72 55 31 6 36 0.10 7.8 0.3 0 h m s SMC0083 0 52 44.27 72 58 47 8 29 0.09 7.8 0.2 0 h m s SMC0087 0 52 48.99 73 24 43 3 22 0.10 8.7 0.1 0 h m s SMC0089 0 53 05.28 72 37 27 8 118 0.09 7.3 0.4 Vol.
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