The chemical evolution of the Milky Way: pushing APOGEE to higher precision and accuracy
(2023) ASTM32 20231Astrophysics
Department of Physics
- Abstract
- In the recent decade, a rapid increase in all-sky spectroscopic surveys has occurred, and with it, the wealth of observational data and information on our Galaxy. Consequently, numerous detections of new structures, such as new open clusters, accreted and in-situ components, are discovered. However, the accuracy and results of industrialised pipelines to derive stellar parameters and abundances have been shown to be problematic for different types of stars, and their accuracy and precision are debated.
In this thesis, spectra released by APOGEE are re-analysed and compared with results from the high-resolution spectrograph IGRINS available on GEMINI South. A new pipeline to derive elemental abundances for numerous stars is designed by... (More) - In the recent decade, a rapid increase in all-sky spectroscopic surveys has occurred, and with it, the wealth of observational data and information on our Galaxy. Consequently, numerous detections of new structures, such as new open clusters, accreted and in-situ components, are discovered. However, the accuracy and results of industrialised pipelines to derive stellar parameters and abundances have been shown to be problematic for different types of stars, and their accuracy and precision are debated.
In this thesis, spectra released by APOGEE are re-analysed and compared with results from the high-resolution spectrograph IGRINS available on GEMINI South. A new pipeline to derive elemental abundances for numerous stars is designed by analysing the effects of resolution and stellar parameters. Here the focus is on stars in the APOKASC Kepler sample, which have highly accurate asteroseismic log g and thereby eliminate log g uncertainties.
I calculate elemental abundances for more than 3000 giant stars using the developed automated pipeline. The abundance trends are generally similar to the abundances reported by APOGEE but include smaller differences. They differ primarily in the metal-poor end and, for some, at super-solar metallicities. For cerium, the computed abundances are of higher precision and follow abundances derived from optical spectra more closely than APOGEE abundances. For Ti and iron-peak elements V and Co, the lower resolution and the APOGEE stellar parameters cause problems for metal-poor stars, generally resulting in larger scatter and too low abundances. For α-elements, their respective abundance trends are similar as their spectral lines remain their strength at the metal-poor end. In addition, multiple stars enhanced in s-process elements are detected, for which additional s-process elements show promise to be derived from APOGEE spectra.
To conclude, with careful spectral analysis, the abundances derived shows a minor effect from the spectral resolution. However, the higher resolution has been shown to perform better for blended spectral lines. The major difference on abundances derived in this thesis are caused by the adopted stellar parameters. As a result spectral lines with high dependence on stellar parameters become problematic. It generates too low abundances and increases the scatter in the abundance trends. Finally, by re-analysing APOGEE spectra, the possibility of deriving abundances for neodymium looks promising. (Less) - Popular Abstract
- Understanding the formation, evolution, and properties of galaxies, including our own Milky Way, is a subject that has puzzled researchers for a long time. To answer these questions, astronomers resort to studying the dynamics of stars, their chemical composition, and utilise computer simulations to reproduce observations. This is not limited to our own Milky Way but is done for external galaxies as well. The study of the kinematics and chemical composition of stars to answer these questions is often referred to as galactic archaeology.
Just as fossils on Earth are used to study Earth's history, the light from stars are used to understand the properties, formation, and evolution of stellar populations. This is because the light a star... (More) - Understanding the formation, evolution, and properties of galaxies, including our own Milky Way, is a subject that has puzzled researchers for a long time. To answer these questions, astronomers resort to studying the dynamics of stars, their chemical composition, and utilise computer simulations to reproduce observations. This is not limited to our own Milky Way but is done for external galaxies as well. The study of the kinematics and chemical composition of stars to answer these questions is often referred to as galactic archaeology.
Just as fossils on Earth are used to study Earth's history, the light from stars are used to understand the properties, formation, and evolution of stellar populations. This is because the light a star emits will travel through its atmosphere, and when it does, signatures will be left as an imprint on the light’s energy distribution. When this light is recorded by telescopes here on Earth, this imprint can be used to reconstruct the star’s atmospheric composition.
The star’s atmosphere resembles the composition of the gas cloud it was made from. And by studying stellar populations of varying ages, we can reconstruct how the chemical composition of our Galaxy has evolved. In the recent decade, there has been a rapid development in the number of all-sky surveys focusing on studying stellar populations and their chemical compositions. From these studies, researchers have found evidence of multiple stellar populations in the Galactic disk, often referred to as the thin- and the thick disk. However, a clear separation of the two is yet to be confirmed. Further, astronomers have linked these pieces of evidence to how it could be related to a delayed gas infall in our Galaxy, potentially a merger in the past between our and an external galaxy.
This work focuses on one of the spectroscopic all-sky surveys named Apache Point Observatory Galactic Evolution Experiment (APOGEE), which aimed to explore previously unknown regions in our Galaxy. With the rapid increase in spectroscopic surveys, new methods have been developed to analyse this previously unprecedented volume of observational data. These methods of deriving properties of stars, such as their temperature and chemical composition, have come with reports of calibration issues, providing uncertain results for different types of stars. However, these uncertainties must to be under control to correctly reconstruct formation scenarios of the Galaxy and the evolution of the stellar populations.
In this thesis, data from APOGEE has been re-analysed with a selection of its observations to improve its accuracy and identify existing problems. This analysis has found that there are some issues present in the analysis of specific elements. We find disagreements for specific elemental abundances, especially for stars that are poor in metals. Additionally, we find that there is more to uncover from the observed data in APOGEE than what has been shown to date. (Less)
Please use this url to cite or link to this publication:
http://lup.lub.lu.se/student-papers/record/9125531
- author
- Fridén, Erik LU
- supervisor
-
- Nils Ryde LU
- Govind Nandakumar LU
- organization
- course
- ASTM32 20231
- year
- 2023
- type
- H2 - Master's Degree (Two Years)
- subject
- report number
- 2023-EXA210
- language
- English
- id
- 9125531
- date added to LUP
- 2023-06-16 15:31:38
- date last changed
- 2023-08-30 15:09:40
@misc{9125531,
abstract = {{In the recent decade, a rapid increase in all-sky spectroscopic surveys has occurred, and with it, the wealth of observational data and information on our Galaxy. Consequently, numerous detections of new structures, such as new open clusters, accreted and in-situ components, are discovered. However, the accuracy and results of industrialised pipelines to derive stellar parameters and abundances have been shown to be problematic for different types of stars, and their accuracy and precision are debated.
In this thesis, spectra released by APOGEE are re-analysed and compared with results from the high-resolution spectrograph IGRINS available on GEMINI South. A new pipeline to derive elemental abundances for numerous stars is designed by analysing the effects of resolution and stellar parameters. Here the focus is on stars in the APOKASC Kepler sample, which have highly accurate asteroseismic log g and thereby eliminate log g uncertainties.
I calculate elemental abundances for more than 3000 giant stars using the developed automated pipeline. The abundance trends are generally similar to the abundances reported by APOGEE but include smaller differences. They differ primarily in the metal-poor end and, for some, at super-solar metallicities. For cerium, the computed abundances are of higher precision and follow abundances derived from optical spectra more closely than APOGEE abundances. For Ti and iron-peak elements V and Co, the lower resolution and the APOGEE stellar parameters cause problems for metal-poor stars, generally resulting in larger scatter and too low abundances. For α-elements, their respective abundance trends are similar as their spectral lines remain their strength at the metal-poor end. In addition, multiple stars enhanced in s-process elements are detected, for which additional s-process elements show promise to be derived from APOGEE spectra.
To conclude, with careful spectral analysis, the abundances derived shows a minor effect from the spectral resolution. However, the higher resolution has been shown to perform better for blended spectral lines. The major difference on abundances derived in this thesis are caused by the adopted stellar parameters. As a result spectral lines with high dependence on stellar parameters become problematic. It generates too low abundances and increases the scatter in the abundance trends. Finally, by re-analysing APOGEE spectra, the possibility of deriving abundances for neodymium looks promising.}},
author = {{Fridén, Erik}},
language = {{eng}},
note = {{Student Paper}},
title = {{The chemical evolution of the Milky Way: pushing APOGEE to higher precision and accuracy}},
year = {{2023}},
}