COSMIC-LAB: Unexpected Results from High-Resolution Spectra of AGB Stars in Globular Clusters
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Alma Mater Studiorum Università degli Studi di Bologna DIPARTIMENTO DI FISICA E ASTRONOMIA Dottorato di ricerca in Astronomia Ciclo XXVIII COSMIC-LAB: Unexpected Results from High-resolution Spectra of AGB Stars in Globular Clusters Dottorando: Relatore: Emilio Lapenna Chiar.mo Prof. Francesco R. Ferraro Co-Relatori: Dr. Alessio Mucciarelli Dr. Livia Origlia Chiar.ma Prof. Barbara Lanzoni Coordinatore: Chiar.mo Prof. Lauro Moscardini Esame finale anno 2015 Settore Concorsuale: 02/C1 – Astronomia, Astrofisica, Fisica della Terra e dei Pianeti Settore Scientifico-Disciplinare: FIS/05 – Astronomia e Astrofisica Contents Introduction 1 1 Elemental Abundances and Chemical Evolution 5 1.1 α and other light elements . 6 1.2 Iron-peakelements .............................. 11 1.3 Neutron-capture elements . 13 2 Globular Clusters: Evolutionary Sequences and Chemical Composition 15 2.1 Evolutionary sequences . 15 2.2 Overallchemistry ............................... 18 2.2.1 Metallicity distribution . 19 2.2.2 Pre-enrichment ............................ 23 2.2.3 Star-to-star variations and signatures of self-enrichment . 24 2.2.4 Openquestions ............................ 31 3 Chemical abundances of AGB stars in globular clusters 35 3.1 NLTEeffectsinAGBstars ... ...... ..... ...... ..... 36 3.2 ThelackofSGAGBstarsinGCs . 37 3.3 AGB overconcentration in the core of 47Tuc . ..... 38 3.3.1 Blue straggler stars in GCs . 39 3.4 AfewwordsontheNLTEeffect . 40 4 Non Local Thermodynamic Equilibrium Effects on Asymptotic Giant Branch Stars in 47Tucanae 43 4.1 Observations ................................. 43 4.2 Analysis.................................... 45 4.2.1 Radial velocities . 45 i CONTENTS 4.2.2 Chemicalanalysis ... ...... ..... ...... ..... 45 4.3 Ironabundance ................................ 47 4.3.1 Sanitychecks............................. 48 4.4 Discussion................................... 53 4.4.1 A possible signature of NLTE effects? . 53 4.4.2 Impact on traditional chemical analyses . 55 4.4.3 Searching for evolved BSSs among AGB stars: a new diagnostic? . 57 4.5 Summary ................................... 59 5 The Origin of the Spurious Iron Spread in the Globular Cluster NGC3201 63 5.1 Observations ................................. 63 5.2 Analysis.................................... 64 5.2.1 Analysis with spectroscopic gravities . 65 5.2.2 Analysis with photometric gravities . 66 5.2.3 Uncertanties ............................. 68 5.3 Discussion................................... 68 5.4 Conclusions.................................. 72 6 A Chemical Trompe-l’œil: No Iron Spread in the Globular Cluster M22 77 6.1 Observations ................................. 77 6.2 Ironabundance ................................ 78 6.2.1 Method (1): spectroscopic Teff and log g .............. 81 6.2.2 Method (2): spectroscopic Teff and photometric log g ....... 82 6.2.3 Method (3): photometric Teff and log g ............... 84 6.3 Asanitycheck:NGC6752 ... ...... ..... ...... ..... 86 6.4 NoironspreadinM22 ............................ 86 6.5 The s-process elements abundance . 90 6.6 Discussion: re-thinking M22 . 91 7 Chemical Analysis of Asymptotic Giant Branch Stars in M62 101 7.1 Observations and spectral analysis . 101 7.1.1 Radial velocities . 102 7.1.2 Atmospheric parameters and stellar masses . 104 7.1.3 Chemical abundances . 104 ii CONTENTS 7.2 Results..................................... 106 7.2.1 Iron and titanium . 106 7.2.2 Oxygen, sodium, magnesium and aluminum . 111 7.3 Discussion and conclusions . 116 8 Weighing Stars: the Identification of an Evolved Blue Straggler Star in the Globular Cluster 47Tucanae 125 8.1 Observations and membership . 126 8.2 Chemicalanalysis............................... 128 8.2.1 Uncertanties ............................. 131 8.3 Discussion................................... 133 8.4 Summaryandconclusions . 136 Conclusions 141 Appendix 145 A Radial Velocities from VLT-KMOS Spectra of Giant Stars in the Globular Cluster NGC6388 145 A.1 Introduction .................................. 146 A.2 Observations and data reduction . 148 A.3 Kinematicanalysis .............................. 152 A.3.1 Accuracy of the wavelength calibration . 153 A.3.2 Radial velocity measurements . 157 A.3.3 Line-of-sight rotation and velocity dispersion profiles . 159 A.4 Discussion and conclusions . 159 B Lines and Continuum Sky Emission in the Near Infrared: Observational Con- straints from Deep High Spectral Resolution Spectra with GIANO-TNG 165 B.1 Introduction .................................. 166 B.2 Observations and spectral analysis . 167 B.3 The sky lines and continuum emission . 170 B.3.1 OH lines and the hot-OH component . 170 B.3.2 O2 and unidentified lines . 172 B.3.3 The sky continuum emission . 173 iii CONTENTS B.4 Discussion and conclusions . 176 iv Introduction Globular clusters (GCs) are among the most interesting stellar systems and have been tar- geted by several studies from the beginning of the astrophysical research. These systems are thought to be the first stellar aggregates formed in the very early epochs of the Galaxy formation. For decades, the GCs have been thought to be the best example of simple stellar popula- tion (SSP). This terminology was suggested to highlight the overall property of a population in which stars show similar characteristics in terms of age and metallicity. In fact, a SSP is assumed to be originated in a single star formation episode from a cloud with an homo- geneous chemical composition. In this way, all the stars share the same age, since they were born at the same time, and the same metallicity, since they were formed from the same material. Moreover, since GCs typically are older than 10 Gyr, they can be considered ∼ as living “relics” of the first stellar aggregates formed and they can be used as tracers of the chemical enrichment history of the host galaxies. Today, the huge progress that has been accomplished thanks to the rising number of pho- tometric and spectroscopic facilities demonstrated, however, that the SSP concept should be revised. In fact, important chemical anomalies have been detected in GCs, the main ones consisting in the so-called “anticorrelations” among light-elements and in peculiar be- haviours observed for iron-peak and neutron-capture elements. This observational evidence suggests that GCs are complex systems, which during their lifetimes undergo a series of processes able to deeply affect the global characteristics. In this sense, GCs can be considered, at least to a first approximation, as “closed” systems, which mainly suffered auto-enrichment processes with a negligible interaction with the In- ter Galactic Medium. In this way, the chemical imprint of different polluters takes a fun- damental role in defining the chemical characteristics of the descendants. The study of the chemical composition of stars in GCs is thus fundamental not only to test their chemical models and to unveil the nature of the polluters, but also to better constrain the nucleosyn- 1 Introduction thesis of the elements. In this context, my thesis is focused on AGB stars, which have an important role in defin- ing the characteristics of GCs for several reasons: (1) the AGB stars dominate the integrated light of stellar populations of intermediate ages (t < 2 Gyr), (2) they are an important nu- cleosynthesis site for the chemical element formed through proton- and neutron-capture chains and (3) they are thought to be the main polluters in the self-enrichment processes during GC lifetimes. However, due to their short evolutive timescales, AGB stars are nu- merically the smallest population in GCs, and nowadays only a few studies have investigated their chemical characteristics. Moreover, a few recent results (Ivans et al., 2001; Beccari et al., 2006; Campbell et al., 2013) have awoken the interest for these stars and highlighted the need of new and comprehensive characterizations. This thesis is aimed at chemically clarifying this poorly studied evolutionary phase. The thesis presents the analysis of a large sample of high-resolution spectra of AGB stars in GCs acquired at the Very Large Telescope (ESO) and at the MPG-2.2m telescope (ESO). The results are quite unexpected and they are contributing to a new understanding of GC chemistry. The work is part of the project Cosmic-Lab, a five year research program funded by the European Research council. The work is organized as follows: Chapter 1 presents an introduction to the nucleosyn- thesis sites and channels from which the main chemical elements form. Chapter 2 is focused on the main properties of GCs, their stellar populations and the main formation scenarios suggested so far. Chapter 3 is devoted to illustrate the known properties of AGB, in light of a few recent results from the literature. Chapter 4 reports on the spectroscopic analysis of a sample of 24 AGB stars belonging to the GC 47Tucanae ([Fe/H] = 0.7 dex) which has − shown evidence of important non-local thermodynamic equilibrium (NLTE) effects. Chap- ter 5 and 6 present the discussion of the iron content of two GCs (namely NGC3201 and M22), demonstrating that (at odds with previous claims) they show no intrinsic metallic- ity spreads. Chapter 7 presents the high-resolution spectroscopic analysis of a sample of 19 giant stars in M62 which has revealed that the same NLTE mechanism discovered in 47Tucanae affects also the titanium lines. Chapter 8 describes how the ionization balance between chemical abundances derived from neutral and ionized elements can be used as a powerful