Binary Open Clusters in the Milky Way: Photometric and Spectroscopic Analysis of NGC 5617 and Trumpler 22?
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Mon. Not. R. Astron. Soc. 000, 000–000 (0000) Printed 6 November 2018 (MN LATEX style file v2.2) Binary open clusters in the Milky Way: photometric and spectroscopic analysis of NGC 5617 and Trumpler 22? G.M. De Silva1;2y, G. Carraro3;8, V. D’Orazi4;5;7, V. Efremova1, H. Macpherson1;5, S. Martell6, L. Rizzo9 1Australian Astronomical Observatory, 105 Delhi Rd, NSW 2113, Australia 2Sydney Institute for Astronomy, School of Physics, The University of Sydney, NSW 2006, Australia 3ESO, Alonso de Cordova 3107, 19001, Santiago de Chile, Chile 4INAF - Osservatorio Astronomico di Padova, vicolo dell’Osservatorio 5, 35122, Padova, Italy 5Monash Center for Astrophysics,School of Physics and Astronomy, Monash University, VIC 3800, Australia 6School of Physics, University of New South Wales, Sydney, NSW 2052, Australia 7Department of Physics and Astronomy, Macquarie University, North Ryde, NSW 2109, Australia 8Dipartimento di Astronomia e Fisica, Universita di Padova, Vicolo dell’Osservatorio 3, I-35122, Padova, Italy 9Facultad de Ciencias Astron´omicasy Geof´ısicas(UNLP), Universidad de La Plata (CONICET, UNLP), Paseo del Bosque s/n, La Plata, Argentina 6 November 2018 ABSTRACT Using photometry and high resolution spectroscopy we investigate for the first time the phys- ical connection between the open clusters NGC 5617 and Trumpler 22. Based on new CCD photometry we report their spatial proximity and common age of ∼70 Myr. Based on high- resolution spectra collected using the HERMES and UCLES spectrographs on the Anglo- Australian telescope, we present radial velocities and abundances for Fe, Na, Mg, Al, Si, Ca, and Ni. The measured radial velocities are -38.63 ± 2.25 kms−1 for NGC 5617 and -38.46 ± 2.08 kms−1 for Trumpler 22. The mean metallicity of NGC 5617 was found to be [Fe/H] = −0.18 ± 0.02 and for Trumpler 22 was found to be [Fe/H] = −0.17 ± 0.04. The two clusters share similar abundances across the other elements, indicative of a common chemical enrich- ment history of these clusters. Together with common motions and ages we confirm that NGC 5617 and Trumpler 22 are a primordial binary cluster pair in the Milky Way. Key words: (Galaxy): open clusters and associations: general – (Galaxy): open clusters and associations: individual: NGC 5617 and Trumpler 22 1 INTRODUCTION tion into two bound clusters. The models of Bhatia (1990) suggest that binary cluster lifetimes range from a few 106 yrs to 4x107 yrs The current model of the global star formation hierarchy is that depending on factors such as cluster separation, tidal force of the arXiv:1507.03230v1 [astro-ph.SR] 12 Jul 2015 giant molecular clouds fragment into cloud cores, producing stel- parental galaxy and encounters with giant molecular clouds. The lar complexes, OB associations and open clusters which dissipate identification of primordial binary clusters is further complicated into individual stars (Efremov 1995). The exact mechanism that with possible other explanations for their origins. The mechanisms lead from the contraction of molecular clouds to star clusters is not of tidal capture and resonant trapping can place two unrelated clus- completely understood, and likely there are different paths that lead ters in pairs (Leon et al. 1999), and these tend to eventually merge to the formation of different systems of clusters. In the latter stages (de Oliveira et al. 2000a,b). Some candidate binary clusters could of this hierarchy, it has been suggested that star clusters could form be merely optical doubles due to super-positioning along the field in pairs or multiples (de la Fuente Marcos & de la Fuente Marcos of view. 2009). We refer to such clusters as primordial binary clusters. Primordial binary clusters are expected to be transient in na- In the Large and Small Magellanic Clouds (LMC and SMC), ture, where possible subsequent evolutionary paths include merg- at least 10% of the known open cluster systems may be in pairs ing of the pair, tidal disruption of one or both clusters, and separa- and perhaps more than 50% of these are primordial binary clusters (Bhatia & Hatzidimitriou 1988; Dieball & Grebel 2000; Dieball et al. 2002). Interaction between the LMC and the SMC may be ? Based on observations obtained at the Anglo-Australian Telescope, Sid- the conditions giving rise to the formation of primordial binary star ing Spring Observatory, Australia clusters (Fujimoto & Kumai 1997). Binary and multiple star clus- y E-mail: [email protected] ters have been seen in other violent environments including in the c 0000 RAS 2 De Silva et al. Antennae galaxies (Fall et al. 2005; Whitmore et al. 2005) and in Table 1. UBVI photometric observations of Trumpler 22 and Landolt stan- the young starburst galaxy M51 (Larsen 2000; Bastian et al. 2005), dard stars on June 3, 2011. presumably initiated by galaxygalaxy interactions. The merger of large binary cluster systems have been suggested to lead to the for- Field Filter Exposures (s) airmass (X) mation of massive globular clusters, such as Omega Cen (Minniti et al. 2004) and NGC 1851 (Carretta et al. 2010). Trumpler 22 U 30, 2x180, 900 1.07−1.28 In the Milky Way 10% of the Galactic open clusters have been B 10, 2x120,600 1.07 −1.27 proposed to be in binary or multiple systems (e.g. de la Fuente Mar- V 5, 2x60, 300 1.07−1.27 cos & de la Fuente Marcos 2010; Subramaniam et al. 1995). The I 5, 2x60, 300 1.07−1.26 PG1323 U 4x240 1.17−2.05 most well established binary cluster in the Galaxy is the pair h and B 4x120 1.18−2.00 χ Persei (Uribe et al. 2002; Dufton et al. 1990; Marco & Bern- V 4x60 1.18−2.05 abeu 2001). None of the other proposed binary candidates have I 4x60 1.20−2.10 been verified with spectroscopic studies. Kopchev & Petrov (2008) PG16333 U 4x240 1.17−1.87 studied the binary candidate pair NGC 7031/NGC 7086 and based B 4x120 1.18−1.95 on photometry, found a discrepancy in age, which may be due to V 4x60 1.18−1.92 large age uncertainties, and encouraged radial velocity follow-up. I 4x60 1.20−1.88 Vazquez´ et al. (2010) searched for candidate binary clusters in the Mark A U 4x240 1.03−1.57 3rd Galactic Quadrant with a negative outcome and concluded bi- B 4x120 1.03−1.55 nary clusters would form preferentially in environments closer to V 4x60 1.03−1.54 I 4x60 1.03−1.56 the environment of the LMC, suggesting further investigations in denser and more violent regions of the Milky Way, such as the in- ner Galaxy. 2 PHOTOMETRY The apparent lack of confirmed binary or multiple open clus- ters in the Galaxy leads to several questions. Did the Milky Way The photometry used in this paper comes from two sources. In not undergo a violent past to form such structures? If it did, perhaps the case of NGC 5617, modern CCD photometry in the UBVI the timescales have been too short to preserve any binary clusters passbands is available from Carraro (2011). In this study, esti- today, as the transient nature of binary clusters meant they are no mates of the cluster fundamental parameters are derived. The age longer in existence as a binary system. Or have we not looked hard is constrained to 70±10 Myr, while the distance from the Sun is enough for binary clusters in our own Galaxy? 2.1±0.2 kpc , and the reddening E(B-V)=0.45±0.05. According to this study, the reddening law in the line of sight is normal (RV =3.1). A possible tracer of cluster origins is the motion and chemi- cal abundance composition of their stars. Presumably if two clus- ters formed at about the same time and site, then they should have For Trumpler 22, only old photographic data exist (Haug similar ages, motions and chemical properties, assuming that the 1978), and for this reason we exploit here a new data-set. Photom- larger gas cloud was sufficiently mixed, with no major contamina- etry in UBVI was acquired at Las Campanas Observatory (LCO) tion events (e.g. supernovae) taking place during the cluster forma- on the nights from June 03, 2011 and are published here for the tion timescales. As the chemical abundance patterns in low-mass first time. We used the SITe#3 CCD detector onboard the Swope stars are preserved during their lives, they reflect the conditions of 1.0m telescope1. With a pixel scale of 0.435 arcsec/pixel, this CCD their birth site. Hence a pair of primordial binaries should share allows to cover 14.8 × 22.8 arcmin on sky. We stress that this a common age, radial velocity and chemical composition. Binary setup (telescope/instrument) is the same that Carraro (2011) used clusters caused by tidal capture may show common motion, but the for NGC 5617. The night was photometric with seeing ranging chemical properties and ages are likely to be different. For any op- from 0.8 to 1.5 arcsec. We obtained multiple exposures per filter, tical doubles we expect both the motion and chemical composition and observed three times along the night the standard star field to be different between the two clusters. PG 1323, Mark A, and and PG 1633 , to cover a wide airmass range, and to secure proper photometric calibration (see Table 1). In this paper we present the first spectroscopic study of stars After removing problematic stars, and stars having only a few in the binary open cluster pair Trumpler 22 & NGC 5617.