Investigating Lithium Enrichment in Infrared Excess K-G Giants Open Access

Abstract

The observance of anomalously high lithium abundances in K-G type giant stars challenges conventional models of stellar evolution, prompting a deeper investigation into the underlying processes responsible for this phenomenon. In this study, we present a comprehensive analysis of a sample of K-G giants exhibiting peculiar lithium enrichments, leveraging optical spectra obtained from the FAST and REOSC spectrographs. Through examination of equivalent widths of the lithium absorption line at 6707.8 Angstroms, we quantify lithium abundances and explore potential correlations with other stellar parameters like infrared excess, metallicity, and projected rotational velocity. Our analysis encompasses a diverse range of evolutionary stages, including giants in the HB red clump, RGB helium flash tip/AGB, subgiant, and RGB luminosity bump phases. This study represents a significant step towards unraveling the complex interplay between stellar evolution, nucleosynthesis processes, and lithium enrichment in K-G giant stars, paving the way for a deeper understanding of the stellar evolution of giants and the short-lived mechanisms of lithium enrichment they may involve.

Table of Contents

CHAPTER 1.................................................................................................................................. 1

INTRODUCTION......................................................................................................................... 1

1.1      Evaluating Current Models of Stellar Evolution.............................................. 1

1.1.1   General Processes of Red Giant Evolution by Stellar Mass Profile........................... 3

1.1.2   Lithium Enrichment in Evolved Intermediate-Mass Giants Contradict...................... 5

Current Models of Stellar Evolution....................................................................................... 5

1.2      Identification of Lithium Enriched Disk Detective IR Excess Objects.............. 6

1.3      Existing Hypotheses for Origins of Lithium Enrichment in Giants.................... 8

1.3.1   In situ Lithium Enrichment Mechanisms..................................................................... 8

1.3.2   Auto-enrichment Lithium Generating Mechanisms.................................................. 11

1.4      Constructing a Novel Approach to the Lithium Enrichment Discourse......... 15

CHAPTER 2: METHODS.......................................................................................................... 17

2.1      Constraining Stellar Parameters of Our Set of IR Excess K-G Giants.......... 17

2.1.1   Obtaining Stellar Effective Temperature via HR Diagram....................................... 17

2.1.2   Estimating Surface Gravity via Absolute Magnitude and......................................... 20

Stellar Evolutionary Tracks.................................................................................................. 20

2.2      Accounting for Radial Velocity............................................................................ 22

2.3      Deriving Lithium Abundance................................................................................... 25

2.3.1   Spectral Line Equivalent Width Evaluation.............................................................. 25

2.3.2   Lithium Abundance Evaluation................................................................................. 27

2.4      Deriving Metallicity................................................................................................ 30

2.4.1   Generating Synthetic Spectra Using pyMOOG......................................................... 31

2.4.2   Developing the Chi-Squared Minimization Scheme.................................................. 32

2.5      Deriving Projected Rotational Velocity via Iron Lines’.................................. 34

Full-Width Half Max Values............................................................................................. 34

2.5.1   Determining Significant Iron Lines........................................................................... 35

2.5.2   FWHM and Project Rotational Velocity Calculation............................................... 36

CHAPTER 3: RESULTS & DISCUSSION.............................................................................. 38

3.1 Analysis of Our K-G Giants’ Spectra-Derived Properties....................................... 38

3.1.1 Lithium Abundance Evaluation.................................................................................... 38

3.1.2   Metallicity Evaluation.............................................................................................. 38

3.1.3   Rotational Velocity Evaluation................................................................................. 39

3.2      Investigating Correlations Between Spectra Properties and Lithium Abundance     41

3.2.1       Correlations Between Lithium Abundance and Calculated Stellar Parameters 42

3.2.2 Correlations Between IR Excess and Calculated Stellar Parameters......................... 44

3.4      Comparisons to “On Infrared Excesses Associated with Li-Rich K-Giants” (Rebull et al. 2015)       45

3.5      Classifying Lithium Enrichment Origin and Process Hypotheses for Our Lithium-Rich IR Excess K-G Giants      46

3.5.1 HB Red Clump Lithium-Enrichment Processes........................................................... 47

3.5.2 RGB Luminosity Bump Lithium Enrichment Processes............................................... 48

3.5.3 AGB Bump Lithium Enrichment................................................................................... 48

CHAPTER 4: CONCLUSION................................................................................................... 49

CHAPTER 5: FUTURE WORK & APPLICATIONS............................................................ 51

BIBLIOGRAPHY....................................................................................................................... 53

APPENDIX A.............................................................................................................................. 55

Doppler Shift Calculation and Graphing Protocol....................................................... 55

APPENDIX B............................................................................................................................... 59

Evaluation of Lithium Abundance via FGK LTE Gaia-ESO Curve of Growth........... 59

APPENDIX C.............................................................................................................................. 61

pyMOOG Metallicity Evaluation Using Chi-Squared Minimization Scheme............. 61

APPENDIX D.............................................................................................................................. 66

Neutral Iron Lines’ Identification, FWHM Calculation, and Projected Rotational Velocity Specutils Protocol        66

About this Honors Thesis

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School	Emory College
Department	Chemistry
Degree	B.S.
Submission	Honors Thesis
Language	English
Research Field	Physics, Astronomy and Astrophysics Chemistry, Physical
Keyword	Astrochemistry Stellar Evolution Spectral Analysis Python
Committee Chair / Thesis Advisor	Dr. Alissa Bans, Emory University
Committee Members	Dr. Erin Bonning, Emory University Dr. Merida Batiste, Emory University Dr. Raphael Ribeiro, Emory University

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