1.1 Summary of the evolution of single low- and intermediate-mass stars . . . 3

Contents

1 Introduction 3

1.1 Summary of the evolution of single low- and intermediate-mass stars . . . 3

1.1.1 Pre-main sequence and main sequence . . . . 3

1.1.2 Evolution to the AGB: red giant branch and first dredge-up . . . . 5

1.2 The Asymptotic Giant Branch (AGB) . . . . 6

1.2.1 Early-AGB (E-AGB) . . . . 6

1.2.2 TP-AGB: The origin of thermal pulses . . . . 7

1.2.3 The third dredge-up (TDU) . . . . 8

1.2.4 s-process nucleosynthesis . . . . 11

1.2.5 Challenges in understanding AGB nucleosynthesis . . . . 17

1.2.6 Observational characteristics of AGB stars . . . . 21

1.2.7 Termination of the AGB phase . . . . 22

1.3 Evolution beyond the AGB: post-AGB, planetary nebulae and white dwarf 22 1.4 Binary evolution of low- and intermediate-mass stars . . . . 23

1.4.1 Binary interaction mechanisms . . . . 24

1.4.2 The zoo of chemically-peculiar objects . . . . 25

1.5 S-type stars . . . . 27

1.5.1 Summary of the evolution from M-type stars to carbon stars: Im- portance of S-type stars . . . . 27

1.5.2 General introduction to S stars . . . . 29

1.5.3 Tc dichotomy . . . . 30

1.5.4 Intrinsic and extrinsic S stars . . . . 30

1.5.5 Different methods to distinguish intrinsic and extrinsic S stars . . 31

1.5.6 Atmospheric parameters and abundances of S stars . . . . 33

1.5.7 S stars: Unsolved issues and challenges . . . . 34

1.6 Motivation and Outline . . . . 35

ii Contents

2 Stellar parameter and abundance determinations of S stars 37

2.1 Challenges in determining the parameters of S stars . . . . 37

2.2 Grid of MARCS model atmospheres of S stars . . . . 38

2.3 S4U: S StarS SED Fitting Utility . . . . 40

2.3.1 Introduction to the tool . . . . 40

2.3.2 Method . . . . 41

2.3.3 Discussion . . . . 41

2.3.4 Conclusion . . . . 42

2.4 Atmospheric parameters derived from spectral fitting . . . . 46

2.4.1 Limitations of the spectral fitting routine . . . . 48

2.5 Constraining log g with Gaia DR2 parallaxes . . . . 48

2.6 Abundance determination . . . . 50

2.7 Deriving abundances of S stars: General description . . . . 51

2.7.1 C, N, O . . . . 51

2.7.2 Carbon isotopic ratios . . . . 52

2.7.3 Metallicity . . . . 53

2.7.4 Heavy elements . . . . 53

3 S stars and s-process in the Gaia era 55 3.1 Introduction . . . . 56

3.2 Observations and target selection . . . . 61

3.2.1 S stars from the Gaia catalogues . . . . 61

3.2.2 High-resolution spectra . . . . 64

3.2.3 Classification based on Tc lines . . . . 64

3.3 Atmospheric parameter determination . . . . 67

3.3.1 Uncertainties on the atmospheric parameters . . . . 67

3.4 Abundance determination . . . . 71

3.4.1 C, N, O . . . . 71

3.4.2 Metallicity [Fe/H] . . . . 74

3.4.3 s-process elements . . . . 74

3.4.4 Uncertainties on the abundances . . . . 76

3.4.5 Abundance profiles of the three intrinsic S stars NQ Pup, UY Cen, and V915 Aql . . . . 79

3.5 Comparison of abundances with nucleosynthesis predictions . . . . 82

3.5.1 Stellar models and nucleosynthesis calculations . . . . 82

3.5.2 The s-process abundance distribution . . . . 82

3.5.3 Comparison between the modelled and measured carbon and s-

process enrichments . . . . 86

Contents iii

3.5.4 The Zr – Nb pair . . . . 87

3.6 The HR diagram of S stars . . . . 87

3.7 Infrared excess of the intrinsic S stars . . . . 89

3.8 Conclusions . . . . 89

4 Occurrence of third dredge-up in low-mass S stars 99 4.1 Introduction . . . 100

4.2 Observations . . . 103

4.3 Technetium detection . . . 103

4.4 Stellar parameter determination . . . 105

4.5 Abundance determination and uncertainties . . . 107

4.6 Comparison with STAREVOL nucleosynthesis predictions . . . 112

4.7 Robustness of the derived stellar masses . . . 116

4.7.1 Sensitivity to the adopted atmospheric-parameter set . . . 116

4.7.2 Sensitivity to the RGB mass-loss rate . . . 116

4.8 Conclusion . . . 116

5 Constraining the luminosity of the TDU via Tc-rich S stars 119 5.1 Introduction . . . 120

5.2 Observational sample . . . 121

5.3 Confirmation of Tc-rich nature . . . 124

5.4 Stellar parameter determination . . . 124

5.5 Abundance determination . . . 129

5.5.1 C, N, O . . . 129

5.5.2 [Fe/H] . . . 129

5.5.3 Light s-process elements (Sr, Y, Zr, Nb) . . . 129

5.5.4 Heavy s-process elements (Ba, Ce, Nd) . . . 131

5.5.5 Other heavy elements (Pr, Sm, Eu) . . . 131

5.5.6 Uncertainties on the abundances . . . 131

5.6 HR diagram of Tc-rich S stars . . . 141

5.6.1 Mass and metallicity distribution of intrinsic and extrinsic S stars 144 5.7 Discussion on the abundances . . . 144

5.7.1 [C/Fe] vs total [s/Fe] . . . 144

5.7.2 [hs/ls] vs [Fe/H] . . . 147

5.7.3 Comparison with STAREVOL predictions . . . 148

5.7.4 o

Ori and BD+79 156: The ‘trinsic’ S stars . . . 149

5.8 Conclusions . . . 150

iv Contents

6 Conclusions and future prospects 157

6.1 Summary and conclusions . . . 157

6.2 Future prospects . . . 159

6.2.1 HR diagram of Tc-rich M-type stars . . . 159

6.2.2 Tc and Nb of intrinsic S stars as chronometer . . . 162

6.2.3 Analysis of intrinsic S stars in the near-infrared (NIR) regime . . 162

6.2.4 Expansion of the S stars sample with Gaia Data Release 3 . . . . 162

6.2.5 Investigating the low-luminosity carbon stars . . . 163

A Annex 165 A.1 [s/Fe] in the MARCS grid . . . 165

A.2 Atomic Linelists . . . 165

Bibliography 173