Baltic Astronomy, vol. 25, 439–445, 2016

SEARCH FOR AND INVESTIGATION OF DOUBLE OPEN CLUSTERS

L. N. Yalyalieva1, E. V. Glushkova1,2 and A. K. Dambis2 1 Faculty of Physics, M. V. Lomonosov Moscow State University, Leninskie Gory 1, Bld. 2, Moscow 119991, Russia; [email protected], [email protected] 2 Sternberg Astronomical Institute, M. V. Lomonosov Moscow State University, Universitetsky pr. 13, Moscow 119991, Russia

Received: 2016 November 22; accepted: 2016 December 14

Abstract. The physical and kinematical parameters of three pairs of open clusters, NGC 6755 and Czernik 39, NGC 1857 and Czernik 20, Berkeley 70 and SAI 47, are determined using the data from several photometric catalogs in order to confirm or disprove the binarity of these pairs. Key words: open clusters and associations: individual (NGC 6755, Czernik 39, NGC 1857, Czernik 20, Berkeley 70, SAI 47)

1. INTRODUCTION Most of the stars are known to form in clusters rather than in regions isolated from other objects. Star-forming regions make up a hierarchical structure: giant molecular clouds produce stellar complexes, OB-associations, and star clusters, which may afterwards disrupt under the action of gravitational and tidal forces. Mechanisms governing the transitions between these stages are not yet entirely understood. The study of open clusters may provide an invaluable contribution to our understanding of the evolution of star-forming regions. Furthermore, the population of open star clusters (OSC) in our Galaxy has some features that dis- tinguish it from cluster populations in other stellar systems. One of such features is the much smaller fraction of binary clusters in the Milky Way compared, e.g., to the Large Magellanic Cloud, where about 10% of all OSC are the members of pairs. This fact can be explained by several factors: different conditions of clus- ter formation in our Galaxy; short lifetime of open clusters in the form of binary objects; incompleteness of open-cluster studies, which prevents the discovery of their eventual binarity, etc. Understanding the causes of this feature may bring about a more comprehensive knowledge of the evolution of our entire Galaxy and its distinction from its nearest neighborhood. That is why we undertook a search for paired clusters using the data from the open cluster catalogs of Dias et al. (2002), Glushkova et al. (2010), and Kharchenko et al. (2013). At the first stage we investigated three OSC pairs: NGC 6755 and Czernik 39, NGC 1857 and Czernik 20, Berkeley 70 and SAI 47. In the process, we refined the physical and kinematic properties of the clusters involved. 440 L. N. Yalyalieva, E. V. Glushkova, A. K. Dambis

2. NGC 6755 AND CZERNIK 39

We determined the distance moduli, color excesses, and ages of the OSCs by fitting their main sequences to solar-metallicity isochrones by Girardi et al. (2002) on the color–magnitude diagrams (CMDs) based on the data of the 2MASS (Skrutskie et al. 2006) and IPHAS (Drew et al. 2005; Barentsen et al. 2014) photometric surveys. When fitting the isochrones we took into account the radial distribution of star number density in the cluster field (Koposov et al. 2008). We determined the absolute proper motions of stars in the cluster fields based on their positions reported in the UCAC4 (Zacharias et al. 2013), IPHAS, URAT1 (Zacharias et al. 2015), 2MASS, and WISE (Wright et al. 2010) catalogs combined with individual DSS Schmidt plate positions reconstructed from the data in the USNO-B1.0 catalog (Monet et al. 2003) as described by Dambis (2009). Note that we reduced the positions from all the above catalogs, except UCAC4 and URAT, to the frame defined by the UCAC4 catalog by performing a linear tangent-plane adjustment; for this we used UCAC4 stars fainter than r = 13 mag within 30 arcmin of the cluster center as reference objects (the positions reported in UCAC4 and URAT are already on the UCAC4 frame by definition or according to the catalog description, respectively). We computed the average proper motions of the clusters using the method proposed by Sanders (1971). Table 1 summarizes the results obtained for NGC 6755 and Czernik 39.

Table 1. Physical parameters of NGC 6755 and Czernik 39.

−1 −1 Open cluster log t EB−V Distance, pc µRA, mas yr µDec, mas yr NGC 6755 8.65 ± 0.05 0.76 1960 ± 40 –0.21 ± 0.16 –3.55 ± 0.16 Czernik 39 8.80 ± 0.05 0.82 1790 ± 30 –0.62 ± 0.18 –3.98 ± 0.18

As is evident from Table 1, NGC 6755 and Czernik 39 are located at practically the same heliocentric distance, they have close proper motions and approximately the same age. The projected distance between the centers of the clusters is about 4 pc, and their apparent core diameters are equal to 1.7 and 1.5 pc, respectively.

3. NGC 1857 AND CZERNIK 20

3.1. Determination of the parameters of NGC 1857 We determined the physical parameters of the open cluster NGC 1857 using the new method developed by Dambis et al. (2017). We first determined the Er−i color excess from the IPHAS catalog data. To this end, we plotted the (hindex, r−i) diagram, where hindex = 0.755 r + 0.245 i − Hα is the only extinction-independent Q-index (up to an arbitrary multiplicative constant) that can be constructed from the observed colors of the IPHAS survey. In this diagram, the reddening lines are parallel to the horizontal axis because, as we pointed out above, hindex does not depend on extinction: the linear combination 0.755 r + 0.245 i interpolates between r- and i-band magnitudes and sort of simulates a very broad band Hα magnitude, and hence 0.755 r + 0.245 i − Hα serves as an extinction-independent measure of the Hα line strength (with the reverse sign), similar to the β index of the Str¨omgrensystem. Hydrogen line strengths are known to be the highest in Search for and investigation of double open clusters 441

0 Fig. 1. The (hindex, r − i) diagram for stars within 2 of the center of NGC 1857. The solid line shows the isochrone shifted by 0.43 mag along the abscissa. stars of spectral type A2 (McBride et al. 2008) and hence hindex has a minimum at the corresponding color index. According to Padova isochrones, the intrinsic color index (r − i)0 of this minimum is practically independent of both metallicity and age over a wide range of these parameters, allowing the Er−i color excess of the cluster to be estimated by shifting the observed (hindex, r − i) diagram along the sole horizontal axis (Fig. 1). Given the inferred color excess, the distance modulus and age of the cluster can then be determined from some color-magnitude diagram, e.g., (r, r − i) (Fig. 2). 3.2. Determination of the interstellar extinction law in the field of NGC 1857

Given the unambiguously determined color excess Er−i toward the NGC 1857 cluster, we could investigate the interstellar extinction law in the cluster field. It is known that the extinction law can differ from one direction to another and for objects located at different distances along the same line of sight. Hence to more accurately determine the distance to an object, we must know the extinction law in the field studied. To this end, we used star magnitudes in the following filters: r (IPHAS); J, H, K (UKIDSS); J, H, KS (2MASS); w1, w2 (WISE) (Fig. 3). We fitted our inferred wavelength dependence of color excess to a function of the −α form Er−λ = a+bλ and found a = 1.68 ± 0.05, b = –0.64 ± 0.09, α = 2.03 ± 0.24, implying a total-to-selective extinction ratio of Ar/Er−i = 3.87 ± 0.12. The corresponding total-to-selective extinction ratio for normal extinction law (Cardelli et al. 1989) is Ar/Er−i = 3.98. 442 L. N. Yalyalieva, E. V. Glushkova, A. K. Dambis

Fig. 2. The (r, r − i) CMD for stars within 1.50 of the center of NGC 1857. The solid line shows the isochrone for the age log t = 8.65.

3.3. Determination of the parameters of Czernik 20

We determined the distance modulus, color excess, and age of the cluster Czernik 20 using the methods described in Section 2 and the photometric data of the UKIDSS (Lawrence et al. 2007) and IPHAS surveys (Fig. 4). 3.4. Comparison of the inferred parameters of NGC 1857 and Czernik 20

We determined the average proper motions of the star clusters NGC 1857 and Czernik 20 using the catalogs and techniques described in Subsection 3.2. The results for both clusters are summarized in Table 2.

Table 2. Physical parameters of NGC 1857 and Czernik 20.

−1 −1 Open cluster log t EB−V Distance, pc µRA, mas yr µDec, mas yr NGC 1857 8.65 ± 0.05 0.43 2900 ± 140 –0.66 ± 0.14 –2.36 ± 0.14 Czernik 20 9.08 ± 0.05 0.43 2790 ± 120 +0.66 ± 0.07 –1.93 ± 0.07 Search for and investigation of double open clusters 443

Fig. 3. Wavelength dependence of the color excess E(r − λ) for NGC 1857. The −α asterisks and solid line show the inferred values and the corresponding Er−λ = a + bλ fit, respectively.

As is evident from Table 2, the clusters NGC 1857 and Czernik 20 are located at approximately the same distance and have similar ages and proper motions. The projected distance between the centers of the clusters is 13 pc.

4. BERKELEY 70 AND SAI 47

We determined the ages, distances, and color excesses of the clusters in this pair using the method described in Subsection 3.2 and the photometric data from the IPHAS and UKIDSS surveys. The average proper motions of the clusters are determined using the same catalogs and techniques as in the case of NGC 6755 and Czernik 20. The results for Berkeley 70 and SAI 47 are summarized in Table 3.

Table 3. Physical parameters of Berkeley 70 and SAI 47.

−1 −1 Open cluster log t EB−V Distance, pc µRA, mas yr µDec, mas yr Berkeley 70 9.55 ± 0.05 0.34 5380 ± 10 –0.73 ± 0.69 +2.37 ± 0.69 SAI 47 8.60 ± 0.05 0.42 3680 ± 70 –0.03 ± 1.97 +2.55 ± 1.97

As is evident from the table, Berkeley 70 is located almost 2 kpc farther from the Sun than SAI 47. In addition, Berkeley 70 is appreciably older than SAI 47.

5. CONCLUSIONS

Of the three pairs of open clusters investigated in this paper, one pair, NGC 6755 and Czernik 39, proved to be a binary open star cluster, because its constituent clusters have similar physical parameters. Another pair, Berkeley 70 444 L. N. Yalyalieva, E. V. Glushkova, A. K. Dambis

Fig. 4. The (r, r − i) diagram for stars within 20 of the center of Czernik 20. The solid line shows the isochrone for the age log t = 9.1. and SAI 47, proved to consist of open clusters randomly projected onto neigh- boring sky areas. Further investigations, e.g., radial-velocity measurements, are needed to definitively confirm or disprove the binarity of the third pair, NGC 1857 and Czernik 20.

ACKNOWLEDGMENTS. The determination of fundamental open-cluster parameters was supported by the Russian Scientific Foundation through grant No. 1422-00041, and the data mining and determination of the parameters of the local interstellar extinction law in the field of NGC 1857, by the joint grant from the Russian Foundation for Basic Research and the Department of Science and Tech- nology of India through projects Nos. RFBR 15-5245121 and INT/RUS/RFBR/P- 219.

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