Accurate Abundances of Giant Stars in the Local disk: A manual analysis of IR APOGEE spectra

Abstract

Purpose: In order to study the Galactic Chemical Evolution, elemental abundances are needed. The purpose of this work is to investigate the chemical evolution of the Milky Way by determining several elemental abundances of giants in the local disk. Large surveys are up and coming, and several surveys are already working, and with these, the aim is high quality abundance measurements of several elements of several thousands of stars. Apache Point Observatory Galactic Evolution Experiment (APOGEE) analyses abundances using a fast pipeline, industrialized to encompass measurements of all stars with the wealth of data. This pilot-study aims to investigate how accurate and precise the abundances can be measured and gain as much scientific information from the spectra by reanalyzing a sub-sample of APOGEE abundance measurements manually.
Method: The data used is a subset of IR spectra from the APOGEE survey, of giant stars that have also been analyzed accurately using high resolution optical spectra, which thus can be used as a benchmark. The stellar sample consists of 291 spectra in the local disk giants with a resolution of 22 500 and signal-to-noise of above 100. Giant stars enable us to probe deeper into the Milky Way. The stellar parameters of these stars have been determined in the optical in order to be as independent from APOGEE as possible. The abundances are determined by synthesizing a spectrum compared to the observed spectrum with focus on the line of interest using Spectroscopy Made Easy (SME), and the giants are assigned to the thin and thick disk using optical measurement.
Results: Abundance elements of light elements (Na, Al, K), ↵-elements (Mg, Si, S, Ca, Ti), iron peak elements, (V, Cr, Co, Ni), and neutron-capture elements (Cu and Ce) are determined. The traditional method of spectral synthesis will allow for each line of interest of each element to be investigated and allow for manual inspection of each analysis, thus maximizing the accuracy and precision without calibration. Using available distance measurements of the sample, ages are determined, showing higher ↵-abundance in older stars.
Conclusions: A manual analysis gives more accurate and precise abundance measurements, showing it is beneficial to measure APOGEE spectra with this method.

Popular Abstract

13.7 billion years ago, the simplest elements were formed; hydrogen and helium. Stars are formed of these elements, fueled by hydrogen. The temperature inside stars allows for a process called nucleosynthesis, where hydrogen is burned into helium, releasing energy as heat. This process occurs during the lifetime of a star, until the star burns all the hydrogen inside.
The strongest tool of an astronomer is light. Light can be split into its constituents, known as spectroscopy. In the 1850s, Gustav Kircho↵ explained that the dark lines in our Sun’s spectrum, also known as the Fraunhofer lines, are related to the material present in the atmosphere. Elements on the surface of a star absorb specific colors, creating darker lines in the spectrum of light.
Elements are formed in di↵erent environments. The elements in the periodic table are arranged according to the number of protons in the nucleus of an atom. In the core of stars, elements can be formed up until iron, the 26th element in the periodic table. With similar formation processes in stars, the periodic table can be split in four groups. The elements lighter than iron are divided into the odd-elements, which are odd-numbered, and ↵-elements, which are even-numbered. In the periodic table, the elements around iron are referred to as iron-peak elements, named due to their abundance in our Solar System. Elements heavier than iron are formed through a process called neutron-capture, where the nucleus of an atom grows by capturing the free neutrons, which is also a building block in a nucleus. Which elements are formed inside stars are also dependent on the mass of the star, because the mass determines the temperature inside stars.
The mass of a star determines the fate of a star. More massive stars have shorter lives, as they burn their hydrogen faster. The short-lived, massive stars form heavier elements, enriching our Galaxy through a spectacular explosion, known as a supernova. The mate- rial ejected by stars are building materials for the next generation of stars. Yet, the more abundant type of stars - much like our Sun - can live as long as the age of our Universe today. These long-lived, lower-mass stars enrich our Galaxy later, allowing us to analyze

how the dust, that forms stars, has changed since the beginning of the Milky Way.
The cycle of stars forming from dust created by stars means that younger stars contain more heavier elements. As a star contains the information of their birth environment, observing several stars of di↵erent ages can tell us how their birthplace - the dust in our home Galaxy - has formed and evolved.
In this thesis, I have analyzed the composition of 291 giant stars in the neighborhood of our Sun to investigate how our Galaxy has evolved. These stars have been observed with Apache Point Observatory Galactic Evolution Experiment, APOGEE, measuring gi- ant stars in the infrared wavelength. Giant stars are very bright, making it possible for us to see those that are more distant in our Galaxy. The dust, expelled by the death of stars, is transparent to infrared light. 14 elements of the four element-groups are measured in these 291 stars in this work. The abundances of the di↵erent groups show that they are formed in similar ways inside stars. These abundances also show that the lower mass stars are enriching our Galaxy later due to their long lifetimes. I show that a careful analysis of abundance measurements give better results, and this gives a better insight to how our Milky Way has evolved.

• author: Christensen, Ivalu Barlach
• supervisor: Nils Ryde; Henrik Jönsson
• organization: Lund Observatory - Undergoing reorganization; Department of Astronomy and Theoretical Physics - Undergoing reorganization
• course: ASTM31 20201
• year: 2020
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• keywords: Galactic Chemical Evolution, Astrophysics, Astronomy, IR Spectroscopy, Milky Way, Abundance
• publication/series: Lund Observatory Examensarbeten
• report number: 2020-EXA160
• language: English
• id: 9013674
• date added to LUP: 2020-06-10 16:30:59
• date last changed: 2020-06-10 16:31:21