A Catalogue of previously unstudied Berkeley clusters Free

Abstract

The main astrophysical parameters of 24 previously unstudied open clusters of Berkeley catalogue are presented here. JHK near-infrared (Two Micron All Sky Survey) photometry and the proper motions astrometry of Naval Observatory Merged Astrometric Dataset (NOMAD) are used. The clusters' centres, angular diameters, ages, distances and colour excesses for these clusters are estimated for the first time.

1 INTRODUCTION

The Naval Observatory Merged Astrometric Dataset (NOMAD) contains astrometric and photometric data for over 1 billion stars derived from the Hipparcos, Tycho-2, UCAC2 and USNO-B1.0 catalogues for astrometry and optical photometry, supplemented by Two Micron All Sky Survey (2MASS) near-infrared photometry. For each unique star the ‘best’ astrometric and photometric data are chosen from the source catalogues and merged into a single dataset (cf. Zacharias et al. 2004). Using this data base; compared with the 2MASS of Cutri et al. (2003); we could determine the fundamental parameters of 24 (of 96) open clusters of Berkeley, which were never studied so far. The only information known about these clusters are the coordinates and the apparent diameters, which are taken from WEBDA1 and DIAS2 sites; listed here in Table 1. Note that Be 53, Be 77, Be 83, Be 88 and Be 92 are removed from our list because they are poor clusters and do not have prominent main sequences; or/and difficult for fitting.

This paper is a part of our continuation series in determining the main astrophysical properties of rarely or un-studied open clusters using modern data bases (cf. Tadross 2008 and references therein). For calibration, the main parameters of five previously studied clusters are re-estimated and compared with those available in the literature.

This paper is organized as follows. In Section 2, the sample selection and data reduction are described. In Section 3, the radial structure of clusters under investigation is presented based on the star count astrometry. The photometric results of the colour–magnitude diagrams (CMDs) fitting are given in Section 4. The results of the basic parameters of the five calibrated clusters are discussed in Section 5. The final conclusions are obtained in Section 6.

2 DATA REDUCTIONS

Data extraction have been performed using the known tool of VizieR for 2MASS3 and NOMAD4 data bases. The investigated clusters have been selected from WEBDA and DIAS catalogues under the following conditions:

• the clusters' data extracted at a preliminary radius of about 10 arcmin from their obtained centres;

• the clusters should have good blue images on the Digitized Sky Surveys (DSS)5 and clearly separated from the background field, see the example of Fig. 1;

• the foreground stars have been separated from the clusters stars using the proper motions of NOMAD dataset. Sometimes the proper motions decontamination removed too many stars, especially in the upper parts of CMDs, which affected the giant clumps on the right, or/and the binaries and blue stragglers on the left. Avoiding this situation, this decontamination is applied only on the lower parts of the CMDs;

• the clusters should have enough members with prominent sequences in their CMDs;

• a cut-off of photometric completeness limit at J < 16.5 mag is applied on the 2MASS data to avoid the over-sampling (cf. Bonatto, Bica & Girardi 2004);

• stars with observational uncertainties ε_J,H,K > 0.15 mag have been eliminated; and

• membership criteria is adopted for the location of the stars in the CMDs. According to Bonatto, Bica & Santos (2005), colour and magnitude filters have been applied to the J∼ (J−H) and K∼ (J−K) sequences where the stars located away from the main sequences are excluded, the maximum departure accepted here is about 0.15 mag; see Fig. 4.

3 ASTROMETRY

To derive better cluster centre, the cluster centre is taken at the maximum stellar density of the cluster's area. The location of the cluster centre is found by fitting a Gaussian to the profiles of star counts in right ascension α and declination δ, as shown in Fig. 2, (cf. Tadross 2008).

Within concentric shells in equal incremental steps from the cluster centre, the stellar density is derived out to the preliminary radius. The real radius (genuine border) of the cluster can be defined at that point, which reaches a stable background density and covers all the cluster area, see Fig. 3. At that radius, the JHK photometric data would be extracted and taken into account for estimating the clusters' properties. The redetermined centres' coordinates and diameters of our investigated clusters are listed in Table 3.

4 PHOTOMETRY

The CMDs comprising stars inside radii of 1, 2 and 3 arcmin from the adopted cluster centre. The simultaneous fitting of the solar metallicity isochrones of Bonatto et al. (2004) were attempted on the J∼ (J−H) and K∼ (J−K) diagrams for the inner stars, at which they should be less contaminated by the field stars. If the number of stars were not enough for an accepted fitting, the next larger area would be included and so on. In this respect, several fittings of different isochrones with different ages have been applied on the CMDs of each cluster. These isochrones are fitted to the lower envelope of the points matching the main sequence stars, turn-off point and red giant positions. The reddening and distance modulus are taken at the proper values within a ranging fitting error of about ±0.10 mag. Our estimated reddening values have been guided by the galactic absorption values, which are available in the literature, that is Schlegel, Finkbeiner & Davis (1998) and Dutra & Bica (2000). Although Schlegel reddening data are often overestimated at low galactic latitudes, it is still a useful source of data. Comparing our estimated reddening values with the reliable ones of Schlegel, we found that the differences are not exceeding 0.15 mag. In the same time, the clusters at low galactic latitudes are compared to the other ones of Dutra & Bica at the nearest coordinates, see Tables 2 and 3. R_V= 3.2, A_J/A_V= 0.276, A_K/A_V= 0.118 and E(J−H)/E(B−V) = 0.33 have been used for reddening and absorption transformations according to Dutra, Santiago & Bica (2002) and references therein. E(J−K)/E(J−H) ≈ 1.6 ± 0.15, which was derived from absorption rations in Schlegel et al. (1998).

5 CALIBRATED CLUSTERS

Be 13, Be 24, Be 35, Be 55 and Be 78 have been selected for calibrating our reduction's method. Be 24, Be 35 and Be 78 are found to be studied by Ortolani, Bica & Barbuy (2005), while Be 13 and Be 55 were studied by Maciejewski & Niedzielski (2007). The astrophysical parameters of these clusters have been redetermined in the present work (PW) and compared with those in the current literature (L), as shown in Table 2. Schlegel reddening values are listed at the end of Table 2; those for Be 13 and Be 24 are unreliable due to the low galactic latitudes. Fig. 5 shows the CMDs and the isochrone fits for the five clusters. The derived parameters are found very close to the published ones.

6 CONCLUSION

Following the above procedure, the basic astrophysical parameters, the cluster centre, angular diameter, age, reddening, distance modulus, distance from the sun, distance from the Galactic Centre, R_gc, and the projected distances on the Galactic plane from the Sun, X_⊙, Y_⊙ and the distance from Galactic plane, Z_⊙ have been obtained for 24 open clusters of Berkeley for the first time, as shown in Table 3. For comparison and calibration of our reduction, the main parameters of the known five open clusters of Berkeley have been re-estimated and compared with those in the literatures. The CMDs and the isochrone fits of the 24 investigated clusters can be seen in Figs 6 and 7.

This publication makes use of data products from the NOMAD and the 2MASS, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Centre/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. Catalogues from CDS/SIMBAD (Strasbourg) and DSS images from the Space Telescope Science Institute have been employed.

REFERENCES