Except for a few elements, the surface chemical composition of a star doesn't change during its evolution, allowing us to study the chemical enrichment history of the Universe with long-lived low-mass stars. I study the history of the Milky Way and nucleosynthesis processes by measuring chemical abundance of stars from spectroscopy and combining the chemical information with stellar kinematics and asteroseismology. Contact me if you are interested in the following topics!
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Building blocks of the Milky Way
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Testing our understanding of chemical enrichments
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Nucleosynthesis in the early Universe (e.g., first stars, Big Bang)
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The very early phase of the Milky Way formation
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Asteroseismology: A unique way to study the age of red giants
Chemical characterization of kinematic substructures
The Gaia mission has enabled the discovery of kinematic substructures, over-densities found in spaces of stellar kinematics. They are candidates for remnants of dwarf galaxies that have accreted onto and merged with the Milky Way. I have been part of our attempts to search for kinematic substructures in a data-driven way (Yuan et al. 2020; Lovdal et al. 2022; Ruiz-Lara et al. 2022; Dodd et al. 2022). By characterizing their chemical abundance, we are able to further constrain their progenitor galaxies' properties and nucleosynthesis processes.
Properties of the progenitor galaxies of kinematic substructures
Chemical abundances tell us about the star formation environment. For example, [Mg/Fe] is an indicator of type Ia supernovae contribution in the system, which can then tell us about the star formation efficiency and timescale. By measuring chemical abundances very precisely, we showed that the progenitor galaxies of Sequoia and Helmi streams likely had lower masses than the progenitor of the most prominent kinematic substructure, Gaia-Enceladus.
Constraining nucleosynthesis with a kinematic substructure
Matsuno et al. (2022a, b)
Kinematic substructures are the closest dwarf galaxies. They experienced star formation histories different from the Milky Way, allowing us to study nucleosynthesis processes in different environments. We showed that Gaia-Enceladus stars contain a large amount of elements produced by the rapid neutron capture process (r-process). This supports the idea that the majority of r-process elements are produced by nucleosynthesis events that can occur after a delay time since star formation, such as neutron star mergers.
Metal-poor stars
The Universe began with hydrogen, helium, and a tiny amount of Li. All the other elements ("metal" in Astronomy) have been synthesized later as a result of star formation. Metal-poor stars formed when the Universe was not chemically enriched, and hence they are records of the early Universe. Their chemical abundances provide a way to investigate the early stage of the galaxy formation and nucleosynthesis processes.
Very metal-poor stars in the Gaia DR3 GSP-Spec catalog
While metal-poor stars provide rich information, they are rare; hence they need to be searched from large datasets. Gaia mission obtained stellar spectra around the Ca triplet absorption lines for an unprecedented number of stars. This Ca absorption feature has been used to search for very metal-poor stars. We combined photometric information with the results of the analysis of the spectra by the Gaia team, and we provide improved metallicities, from which one can efficiently select low-metallicity star
Lithium problems in the early Universe
Observed Li abundances of metal-poor stars are lower than the prediction from Big Bang nucleosynthesis calculations adopting standard cosmology with precisely constrained cosmological parameters from CMB observations. This discrepancy led to the idea of Li depletion in the atmospheres of the observed metal-poor stars. Extremely metal-poor stars ([Fe/H]<-3.5) may hold the key to understanding the mechanism. We have shown that there seems to be a metallicity-driven mechanism.
Matsuno et al. (2017b)
Combining asteroseismology and chemical abundance
Asteroseismology offers a way to precisely measure mass of stars. Its application to red giants enables measurements of their age, which is otherwise very challenging. I have worked on combining asteroseismology and high-resolution spectroscopy to study stellar populations in the Milky Way.
Asteroseismology for halo stars
Text in preparation.
Detailed abundance of ''young α-rich stars''
Text in preparation.