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Spectroscopic Search for Multiple Stellar Populations in the Main Sequence of the Globular Cluster M3

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Spectroscopic Search for Multiple Stellar Populations in the Main Sequence of the Globular Cluster M3 Spectroscopic search for multiple stellar populations in the Main Sequence of the globular cluster M3 by Julio César Mendoza Cervantes A thesis submitted to the Instituto Nacional de Astrofísica, Óptica y Electrónica for the degree of Master of Science in the departament of Astrophysics. Supervisors: Dr. Miguel Chávez Dagostino Dr. Emanuele Bertone Santa María Tonantzintla, Puebla September, 2017 ©INAOE 2017 El autor otorga al INAOE el permiso de reproducir y distribuir copias parcial o totalmente de esta tesis. Abstract We present preliminary results on the investigation of the nature of multiple stellar populations in Main Sequence stars (this is, in stars in an evolutionary stage that still preserves their natal chemical composition because they have not yet experienced chemical mixing through deep convection processes) of the Globular Cluster M3 (NGC 5272) by means of a theoretical study using stellar model atmospheres, synthetic spectra and spectroscopic indices. The work is composed of two main parts that include: (i) The calculation of new Opacity Distribution Functions (ODFs) appropriate for the chemical mix observed in evolved stars of globular clusters, and their use in the creation of a new grid of theoretical spectra at very high resolution. (ii) The computation of a set of theoretical absorption line spectroscopic indices and their correlation with the three leading atmospheric parameters: effective temperature, surface gravity and global metallicity (Teff/log g/[Fe/H]). For the first part we implemented a series of numerical routines within the code DFSYNTHE to calculate the distribution of opacities in agreement with the chemical partitions used by Sbordone et al.(2011) which represent, to some extent, the prototypical elemental abundance combinations of globular clusters with multiple stellar populations. Namely, these partitions were an α-enhanced solar-scaled Reference abundance of [Fe/H] = -1.62 dex (Z = 0:001), compatible with the metallicity of M3 of -1.5 (Harris 1996, 2010 Edition) and a Helium fraction of Y = 0:246 and three other combinations that were labeled as CNONa1Y2, CNONa2Y2 and CNONa1Y4, and incorporate an abundance enhancement of N and Na and depletion of C and O, as well as, for the latter, an enhancement of Helium (Y = 0:400). The newly created ODFs were subsequently implemented as input in the ATLAS9 code to build a grid of 3,472 stellar atmosphere models in a range of effective temperature from 4,300 to 7,000 K, at steps of 100 K, surface gravity log g from 2.0 to 5.0, at steps of 0.1 dex, and the four different chemical compositions from Sbordone et al.(2011). Finally, the set of model atmospheres were used in the code SYNTHE to calculate synthetic spectra at a resolution of R = λ/∆λ = 500; 000 and subsequently broadened to a resolution of R = 2; 500 compatible with the multi-object spectroscopic mode of the instrument OSIRIS on the Gran Telescopio Canarias (GTC). The numerical codes used in this work were developed by R. L. Kurucz of the Harvard-Smithsonian Center for Astrophysics (CfA). The second part is devoted to the calculation of spectroscopic indices, either as defined in the literature as part of the Lick/IDS system (Trager et al. 1998) or those reported by Pancino et al.(2010), and additionally, a set of new 10 indices introduced in this thesis. The later set was assembled by identifying features that vary the most among the four different chemical compositions. We explored the effects of atmospheric parameters on the index values and identified the optimal spectral diagnostics that allow to trace the signatures of two stellar populations. We specifically found that the indices involving molecular bands (in particular CN and CH, but also NH, OH and NO) and hydrogen features are very promising for separating stellar populations in globular clusters. As an addition to the theoretical work, we present, in the last part, preliminary results from an observing program. Spectroscopic data of relatively low luminosity stars of M3 were obtained for the first time. Observations were carried out with the MOS mode of OSIRIS on the GTC. We used two grisms: R2500U and R2500V to observe a sample of 71 stars members of M3 that included 44 main sequence stars, 24 objects at the Turn-off and 3 stars of the red giant branch. The sample was selected by matching the catalogs from Ferraro et al.(1997) and Gaia (Gaia-Collaboration et al. 2016) in a region of 1 × 7.5 arcmin2 placed as close as possible to center of M3 but avoiding the most crowded part of the cluster. We present preliminary results of these observations. A mi familia Agradecimientos Al Instituto Nacional de Astrofísica, Óptica y Electrónica, INAOE, por la beca de excelencia académica otorgada durante el periodo enero - mayo de 2017. Al Dr. Emanuele Bertone y al Dr. Miguel Chávez por el apoyo y las facilidades brindadas para la realización de esta Tesis. A mis amigos Sandy, Andrea y Edgar. vi Contents Contents vii 1 Introduction1 1.1 Globular Clusters.............................1 1.1.1 General properties of Globular Clusters.............1 1.2 Globular Clusters as single stellar population systems...........3 1.3 Globular Cluster with multiple stellar populations.............4 1.3.1 Spectroscopic evidence of multiple stellar population......5 1.3.2 Photometric evidence of multiple stellar population.......9 1.4 The chemical composition and the origin of multiple stellar populations in GCs................................... 11 1.5 Chemical partitions of multiple stellar populations in Galactic GCs... 13 1.6 Description of this dissertation...................... 16 2 The Globular Cluster M3 19 2.1 General Properties............................ 19 2.2 Multiple population in M3........................ 19 3 Model atmospheres and synthetic spectra 25 3.1 The library of Kurucz’s codes....................... 26 3.2 ODF computation............................. 28 3.3 ATLAS9 model computation....................... 28 3.4 SYNTHE theoretical spectra....................... 36 4 Spectroscopic indices 39 4.1 Lick/IDS system............................. 39 4.2 CN and CH indices............................ 41 4.3 New indices................................ 42 4.3.1 Index error and sensitivity.................... 44 5 Results 49 5.1 Global properties of the spectroscopic indices............... 49 5.2 Suitable indices for identifying multiple population in M3........ 58 vii 5.2.1 The Main Sequence........................ 63 5.2.2 The Red Giant Branch...................... 69 5.2.3 The Horizontal Branch...................... 72 6 GTC Observations of M3 75 6.1 GTC and the MOS of OSIRIS...................... 75 6.2 Definition of the stellar sample...................... 76 6.3 Preliminary observation data....................... 86 7 Conclusions 89 A Abundances of the four chemical compositions 91 B Theoretical indices calculated for the case of M3 95 Index of Figures 109 Index of Tables 115 References 117 viii Chapter 1 Introduction 1.1 Globular Clusters Globular Clusters (GCs) are nearly spherical systems of stars bounded by gravity, with a high concentration of stars near their centers, that orbit the core of a galaxy as satellites. In spiral galaxies as the Milky Way, these systems are found in the halo and the bulge, and are usually composed hundreds of thousands of old stars of low of metallicity. GCs do not contain significant amounts of gas and therefore lack signs of current star formation. Due to their old age, GCs are considered to carry on the imprints of the formation of the Milky Way and therefore may preserve valuable information about the initial conditions with which the Galaxy was formed and of its early evolutionary processes. The study of globular clusters has played a key role in the understanding of the overall properties, including clues of fundamental questions such as the age of the Universe and its early history. 1.1.1 General properties of Globular Clusters Although GCs have a circular appearance, most of them have a certain grade of ellipticity, up to 0.27 as in the case of M19 (NGC 6273) that is the most elongated GC of the Milky Way. Other GCs with a high grade of ellipticity are NGC 6144 and NGC 7492 with 0.25 and 0.24, respectively, while many others have zero ellipticity, such as NGC 6093 (M80), NGC 6121 (M4), NGC 6254 (M10), NGC 6569, NGC 6723 and NGC 6838 (M71) (Harris 1996, 2010 Edition). 1 1Hereafter this reference corresponds to the 2010 of the edition of the catalogue. 1 As mentioned above, in spiral galaxies, the GCs are mostly found in the halo. In the Milky Way, the halo has an approximately spherical shape and is extended to a distance of ∼ 60 kpc, comparable to the distance to the Magellanic Clouds, with a decrease in density with distance from the galactic center. Unlike the galactic disk, the most outstanding properties of the halo are its low metallicity, the lack of gas and its old age of 10-12 Gyr (Jofré & Weiss 2011), characteristic of Population II stars according to Baade(1944). Stars in GCs are in general metal-poor: in the Milky Way, the stellar metallicity [Fe/H] 2 varies from -2.40 to 0.0 dex. Figure 1.1 shows the metallicity distribution of 152 GCs out of the 157 systems reported in Harris(1996). Figure 1.1: Distribution of metallicity [Fe/H] of 152 globular clusters of the Milky Way. The data were extracted from the catalog of Harris(1996). 2 This abundance ratio is defined as the logarithm of the ratio between the number of iron nFe and hydrogen nH atoms per unit of volume in the stellar photosphere compared with that of the Sun and is expressed as: nFe nFe [Fe/H] = log10 − log10 (1.1) nH ? nH 2 As it can be seen in Figure 1.1, two populations of GCs can be distinguished: a metal-poor one with a metallicity distribution that peaks at [Fe/H]∼ −1:5, and metal-rich systems with [Fe/H]∼ −0:5.
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