A Photometric Study of Open Clusters Candidate Kronberger 81

1. A Photometric Study of Open Clusters Candidate Kronberger 81 Jubee Sohn and Myung Gyoon Lee Department of Physics & Astronomy, Seoul National University Abstract We investigate the properties of the open cluster candidate Kronberger 81 based on BVI imaging data from the Maidanak Observatory. Kronberger 81 was presented as one of open cluster candidates bys Kronberger et al (2006), but they only make a rough estimate of cluster’s properties. We perform the photometry of this cluster using DAOPHOT. Using this result, we obtain the radial number density profile and Color-Magnitude Diagram(CMDs) of cluster and its control field. Kronberger 81 seems old open cluster. We use isochrone fitting method to find physical parameters with PADOVA isochrone. We conclude that age of Kronberger 81 is 3.2±0.8Gyrs, reddening E(B-V)=1.6±0.1mag and distance of d=3.6±0.4kpc. 1. Introduction There are several studies to search open cluster candidates; Kronberger et al., 2006, Bica et al.,2003c, Alessi et al.,2003, etc. However, many of them are not studied yet. Kronberger 81 is included in the open cluster candidate catalog and (Kronberger et al., 2006) studied it based on 2MASS data. In this study, we use optical images and infrared catalog to analysis properties of Kronberger 81. Figure 4 display the V vs. V-I diagram of cluster and its control filed. Both region selected with same criterion. Kronberger et al.(2006) show the J vs. J-H diagram of Kronberger 81, using 2MASS catalog. However, they didn’t adopt isochrone fitting method. Therefore we check our optical band fitting result with 2MASS catalog. Figure 5 display the J vs. J-H CMD of Kronberger 81. This figure shows very strong red giant branch, even though red clump, which consistent result with optical band CMDs. PADOVA isochrone with age log age 9.5 also well fitted on this diagram. Therefore, we could verify that Kronberger 81 is old open cluster. Figure 4. The V vs. V-I diagram of Kronberger 81. Line, shape of dots and error bar have same properties with Figure 3. In this figure, red giant branch looks remarkable. 2. Data We observed BVI CCD images of Kronberger 81 using the Site CCD at the Maidanak observatory 1.5m telescope. Stellar photometry was carried out using DAOPHOT. Standard calibration has done with six Landolt standard stars. Figure 1 is I band images of Kronberger 81. We use right area of the cluster as a control field. Figure 3. The V vs. B-V diagram of Kronberger 81. Overlaid red line is the PADOVA isochrones with log age 9.5 and metallicity Z=0.010. The red crosses are cluster’s core members which have distance less than 0.88’. (B-V) color error also display at left panel. Figure 1 The I band image of Kronberger 81. A red symbol and a circle means cluster center and region. 3. Results In this study, center of cluster is determined the peak of surface number density map. We also estimate the size of Kronberger 81, using radial number density profile (Figure 2). The radius of cluster is 1.75’. Figure 3 display the V vs. B-V diagram of Kronberger 81 and its control field. The control field is right region of cluster with same size. In figure 3, Kronberger 81 shows shortage of main sequence and strong giant branch, compare with control field. Main sequence stars are probably too faint to detect, so we could not find enough number of those stars. Strong feature of red giant branch indicate that Kronberger 81 is old open cluster. Therefore we perform the isochrone fitting with the PADOVA isochrones (Marigo et al. 2008) for log age 9.5 and metallicity Z=0.010. We obtain the age t=3.2±0.8 Gyrs, distance modulus (m-M)0=12.8±0.2 and the reddening E(B-V)=1.6±0.1 mag. In B filter image, Kronberger 81 looks faint, so we could not obtain many stellar sources in V vs. B-V diagram. In V vs. V-I diagram, however, cluster looks more rich, especially main sequence and giant branch features. Figure 3. The J vs. J-H diagram of Kronberger 81. PADOVA isochrones with log age 9.5 and metallicity Z=0.010. is overlaid The red crosses are cluster’s core members which have distance less than 0.85’. Figure 2. The radial number density profile of Kronberger 81. Density decreases until 1.75’. 4. Conclusion We conclude that Kronberger 81 is old cluster with very faint main sequence and strong red giant branch. We use PADOVA isochrone fitting method to derive the properties of this cluster. To check this result, same isochrone overlaid on infrared filter CMD and it fits well on the red giant branch. Table 1 shows the brief summary of cluster’s properties. Table 1. Parameters of Kronberger 81 5. Reference Kronberger et al. 2006, A&A, 447, 921Dias et al. 2002, A&A, 389, 871Girardi et al., 2004, A&A, 422, 205