THE ORIGIN AND CHEMICAL EVOLUTION OF THE EXOTIC GLOBULAR CLUSTER NGC3201

Abstract

GCs are of paramount importance in the investigation of a wide variety of fundamental astrophysical phenomena. They are the perfect laboratories for studying a wide variety of fundamental problems in stellar and galactic astrophysics. Our knowledge of stellar evolution is grounded in the detailed comparison of GC color-magnitude diagrams with model isochrones. They are the oldest objects known whose ages are relatively easily and accurately measured, setting a rm lower limit to the age of the Universe. Only a few years ago GCs, with the lone exception of ! Cen, were considered Simple Stellar Populations, with all stars in a cluster believed to share the same age and chemical composition. This simplicity has been shattered by the discovery of increasing complexity in the latter parameter for a growing number of GCs, spawning the new and exciting eld of multiple populations (MPs). Both photometric and spectroscopic techniques are employed to trace and understand this phenomenon. The development of larger telescopes, more sensitive detectors and high resolution multiobject spectrographs has allowed a large sample of giants in each of a growing number of GCs to be studied with high resolution spectroscopy, leading to an everincreasing database of elemental abundances. The emerging picture, although quite complicated in detail, does show several salient features. In particular, light elements (C, N, O, Na, Mg, Al) are associated with the MP phenomenon. The most typical and best-studied characteristic is the Na-O anticorrelation. Indeed, the ubiquity of this anticorrelation in their sample of 19 GCs 13 Chapter 1. Introduction 14 studied prompted Carretta et al. (2009a,b) to introduce a new, chemical de nition of a GC as those objects which exhibit such a feature. This spread in the light elements must be due to self-enrichment that happens within a GC in the early stages of its formation, when a second generation of stars was born from gas polluted by ejecta of evolved stars of the rst generation (Caloi & D0Antona 2011). Several kinds of polluters for the light elements have been proposed: intermediate mass AGB stars (D0Antona et al. 2002), fast rotating massive MS stars (Decressin et al. 2007) and massive binaries (de Mink et al. 2010). In addition, a few of the most massive clusters such as ! Cen (Johnson et al. 2008;Marino et al. 2011a), M54 (Carretta et al. 2010b) and M22 (Marino et al. 2011b) are now known to show signi cant spreads in Fe as well. Indeed, the existence of such a metallicity spread has also been claimed in NGC3201 (Gonzalez & Wallerstein 1998; Simmerer et al. 2013), but remains controversial. Such clusters must also have been able to retain SNeII and/or SNeIa ejecta (Marcolini et al. 2009), as well as material ejected from polluters at lower velocity which led to the Na-O anticorrelation. Ejecta from evolved stars (cycled through a temperature of T>107 K where hot H-burning occurs) is returned to the interstellar medium of the GC and mixes with the remaining primordial gas left over from the rst star formation epoch. Given a su ciently deep potential well to retain this ejecta, which is the critical parameter, a second generation of star formation can occur once the appropriate conditions arise (e.g. Cottrel & Da Costa 1981; Carretta et al. 2010c). NGC3201 is unique among the Galactic GCs kinematically. It has the most extreme radial velocity (495 km s 1). Gonzalez & Wallerstein (1998) calculated an orbital velocity of 250 km s 1 around the Galactic center, but in a retrograde sense. This has been taken as strong evidence of a possible extraGalactic formation with subsequent capture by the MW (Rodgers & Paltoglou 1984; van den Bergh 1993). Examining the color-magnitude diagram (CMD) of NGC3201 (e.g. Layden & Sarajedini 2003) clearly shows that it has a very populated and extended horizontal branch (HB). According to D0Antona et al. (2002), the extension of the HB in a GC is proportional to the amount of helium variation due to self-pollution among its stars. Therefore, the large extension of the HB in NGC3201 suggests that it should display a large spread in the light elements. Chapter 1. Introduction 15 Kravtsov et al. (2009) and Carretta et al. (2010d) found radial inhomogeneities in the stellar populations of NGC3201, Kravtsov found radial variations in the CMD, that could not be explained by reddening. Carretta et al. (2010d) compared their spectroscopic sample of 100 stars with Na and O abundances with photometric data and found that the giant stars of the second generation have a tendency to be more concentrated than stars of the rst generation, as expected in the self-pollution scenario. Several chemical analyses have been carried out on NGC 3201 stars using highresolution spectroscopy. Unfortunately, these are not su cient to give a de nitive picture of the chemical evolution of this interesting GC. Gonzalez & Wallerstein (1998) measured the abundances of several chemical elements in eighteen stars but with large errors, based on CTIO 4m data. Carretta et al. (2009a) observed a large sample of some 100 stars but only measured the abundances of light elements (O , Na, Mg, Al and Si), as well as Fe. Simmerer et al. (2013) observed a sample of 24 stars but only measured Fe. They found an intrinsic spread in iron. We discuss this point in detail in chapter 4. Our sample (only eight stars), although much smaller than either the Gonzalez, Carretta or Simmerer samples, was observed with high resolution as well as excellent signal to noise (see chapter 2), which allows us to measure the abundances of more elements (29 total) with much smaller errors than in the previous studies.GCs are of paramount importance in the investigation of a wide variety of fundamentalastrophysical phenomena. They are the perfect laboratories for studying a widevariety of fundamental problems in stellar and galactic astrophysics. Our knowledgeof stell

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