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The Evolution of the Milky Way: New Insights from Open Clusters

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The Evolution of the Milky Way: New Insights from Open Clusters Mon. Not. R. Astron. Soc. 000, 1–?? (2016) Printed 12 September 2016 (MN LATEX style file v2.2) The evolution of the Milky Way: New insights from open clusters Arumalla B. S. Reddy1⋆, David L. Lambert1 and Sunetra Giridhar2 1W.J. McDonald Observatory and Department of Astronomy, The University of Texas at Austin, Austin, TX 78712, USA 2Indian Institute of Astrophysics, Bangalore 560034, India Accepted 2016 September 07. Received 2016 September 07; in original form 2016 May 12 ABSTRACT We have collected high-dispersion echelle spectra of red giant members in the twelve open clusters (OCs) and derived stellar parameters and chemical abundances for 26 species by either line equivalent widths or synthetic spectrum analyses. We confirm the lack of an age−metallicity relation for OCs but argue that such a lack of trend for OCs arise from the limited coverage in metallicity compared to that of field stars which span a wide range in metallicity and age. We confirm that the radial metallicity gradient of OCs is steeper (flatter) for Rgc < 12 kpc (>12 kpc). We demonstrate that the sample of clusters constituting a steep radial metallicity −1 gradient of slope −0.052±0.011 dex kpc at Rgc < 12 kpc are younger than 1.5 Gyr and located close to the Galactic midplane (| z| < 0.5 kpc) with kinematics typical of the thin disc. Whereas the clusters describing a shallow slope of −0.015±0.007 −1 dex kpc at Rgc > 12 kpc are relatively old, thick disc members with a striking spread in age and height above the midplane (0.5 < | z| < 2.5 kpc). Our investigation reveals that the OCs and field stars yield consistent radial metallicity gradients if the comparison is limited to samples drawn from the similar vertical heights. We argue via the computation of Galactic orbits that all the outer disc clusters were actually born inward of 12 kpc but the orbital eccentricity has taken them to present locations very far from their birthplaces. Key words: Galaxy: abundances – Galaxy: disc – Galaxy: kinematics and dynamics – (Galaxy:) open clusters and associations: general – stars: abundances 1 INTRODUCTION much of the dynamical information is lost. However, the chemical content of the disc stars may have preserved the The formation and evolution of galaxies is one of the major dissipative history, the key to unraveling the formation his- puzzles of astrophysics. Large-scale cosmological simulations tory of the Milky Way (De Silva et al. 2009; Freeman & utilizing the currently favoured Lambda Cold Dark Matter Bland-Hawthorn 2002). Since the surface density of gas and arXiv:1609.02619v1 [astro-ph.GA] 8 Sep 2016 (ΛCDM) paradigm of cosmology predicts the large-scale dis- star formation rate (SFR) in the Galactic disc have varied, tribution of galaxies in space as observed today, but we have as seen in many galaxies, the measured chemical abundances not yet met with equal success on small scales due to incom- are a function of position as well. Therefore, measurement of plete knowledge of how the baryons are distributed (Vogels- the chemical abundance distribution in the disc (i.e. the ra- berger et al. 2014). As these baryons mostly reside in stellar dial abundance gradient) and the gradient’s temporal vari- discs in galaxies, understanding the formation and evolution ation over the disc’s lifetime present strong observational of galactic discs is the heart of the galaxy formation theory constraints on the theoretical models of Galactic chemical (Feltzing 2015 and references therein). evolution (GCE; Magrini et al. 2009; Minchev et al. 2013; In this regard, the Milky Way Galaxy provides an ex- Kubryk et al. 2015). cellent testing ground for models of galaxy formation and evolution thanks to the ability to resolve individual stars of Abundance gradients in the Milky Way can be mea- its stellar populations and analyse them in exquisite detail. sured using a wide variety of tracers including the young As the disc formed dissipatively and evolved dynamically, population of H ii regions, OB stars, and Cepheid variables, and planetary nebulae of different ages, cool unevolved stars and red giants in the field (see for example, Magrini et al. ⋆ E-mail: [email protected] 2010; Cheng et al. 2012; Hayden et al. 2014) and in open c 2016 RAS 2 A. B. S. Reddy, D. L. Lambert and S. Giridhar clusters (OCs; Friel et al. 2002, 2010; Magrini et al. 2010; of NGC 2632 was estimated previously for the elements Na, Pancino et al. 2010; Yong et al. 2012; Frinchaboy et al. Al, Si, Ca, Ti, Cr, Fe and Ni by Pace, Pasquini & Fran¸cois 2013). The cooler main sequence stars or red giants which (2008), and for 17 elements in the range from O to Ba by are members of OCs provide not only abundance estimates Yang, Chen & Zhao (2015). The chemical abundances of for elements sampling all the major processes of stellar nu- none of the elements but for iron was explored previously cleosynthesis – but a collection of stars whose age, distance, for red giants in NGC 2287 and NGC 6633 by Santos et al. kinematics and metallicity can be measured with greater (2012) and Santos et al. (2009), respectively. For NGC 2099 certainty than for field stars. Moreover, it is possible from and NGC 2632, we either provide a more extensive anal- an OC’s space motion and a model of the Galactic gravita- ysis of stars previously analysed or the first analysis of a tional potential to study the dynamics and estimate a clus- member star while NGC 2287 and NGC 6633 are analysed ter’s birthplace (Wu et al. 2009; Vande Putte et al. 2010). comprehensively for the first time for many elements. In recent years, modern high-resolution spectrographs We have presented in our previous papers (Reddy et al. and large reflectors have provided high-quality spectra of 2012, 2013, 2015) abundance measurements for 16 OCs with stars in OCs for secure measures of abundances, and ex- ages from 130 Myr to 4.3 Gyr and Rgcs from 8.3 and 11.3 tended abundance estimates to distant OCs in the direc- kpc. We combine our sample of twenty-eight OCs (12 from tion of Galactic anti-centre (Carraro et al. 2007; Yong et this study and 16 from our previous papers) with a sample al. 2005, 2012; Sestito et al. 2006, 2008). However, not all of 51 OCs drawn from the literature for which we have re- the investigators have arrived at similar measures of the ra- measured the chemical content using their published EWs, dial metallicity gradient which may be partly related to the our models, linelists and reference solar abundances to es- different methods adopted (see for example, Magrini et al. tablish a common abundance scale. Useful data on chemical 2009; Heiter et al. 2014). Our current understanding sug- composition of the literature sample is presented previously gests a gradient of about −0.06 dex kpc−1 (Friel et al. 2002; in Table 13 from Reddy et al. (2015). Pancino et al. 2010) to −0.20 dex kpc−1 (Frinchaboy et al. This paper is organized as follows: In Section 2 we de- 2013) in the radial range 5 to 10 kpc with a nearly flat trend scribe observations, data reduction and radial velocity mea- (−0.02 dex kpc−1) beyond 13 kpc (Carraro et al. 2007; Ses- surements. Section 3 is devoted to the abundance analysis tito et al. 2008; Pancino et al. 2010; Yong et al. 2012; Frinch- and Section 4 to discussing the age−abundance relations. aboy et al. 2013; Cantat-Gaudin et al. 2016). We present in Section 5 the radial abundance distribution It is especially puzzling that the flattening of the metal- of OCs and field stars and discuss the results in comparison licity distribution of OCs at large radii is not shown by the with theoretical chemodynamical models of GCE. In Section Galactic field stars (Cheng et al. 2012; Hayden et al. 2014) 6 we compute the birthplace of OCs with other relevant or- including Cepheids (Luck & Lambert 2011; Genovali et al. bital parameters and focus on discussing the origin of older 2014) where field stars suggest a constant steep decline of OCs in the outer Galactic disc. Finally, Section 7 provides metallicity out to Rgc of 18 kpc. Another fascinating re- the conclusions. sult, in common for both the field stars and OCs at large radii, is the evidence for enhanced [α/Fe] ratios in the outer Galactic disc (Yong et al. 2005; Bensby et al. 2011; Luck & Lambert 2011). The fact that the α-enriched clusters older 2 OBSERVATIONS AND DATA REDUCTION than 4−5 Gyr, reminiscent of rapid star formation history, The sample of red giants in each of the target clusters was have populated the Galactic disc beyond 12 kpc, where the extracted from the WEBDA1 database and the astrometric star formation is slow due to low surface density of gas, is and photometric measurements are cross-checked with the a puzzle in conflict with the inside-out formation scenarios SIMBAD2 astronomical database. of the Milky Way (Bovy et al. 2012; Roˇskar et al. 2013). High-resolution and high signal-to-noise (S/N) ratio op- A significant flattening of the metallicity gradient observed tical spectra of 32 red giants across the twelve OCs were in the outskirts of other spiral galaxies (Scarano & L´epine acquired during 2011 November and 2013 July, October, 2013) calls for a thorough understanding of many processes November and December with the Robert G. Tull coud´e regulating the evolution of Milky Way. cross-dispersed echelle spectrograph (Tull et al.
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