My research focuses on the intertwined goals of galactic archaeology and nuclear astrophysics: 1) deepening our understanding of the origin of the elements, which originated from the very first stars, 2) constraining the birth mass distribution (so-called initial mass function, IMF) of the first stars to scrutinize how they contribute to the Galactic chemical enrichment, and 3) unraveling the chemodynamical assembly history of the Galaxy. However, the first stars are thought to be massive, short-lived, and enriched the surrounding pristine gas shortly after their birth. Thus, I achieve these goals by indirect studies of their direct descendants, most metal-poor (metal-deficient) stars, by scrutinizing the unique and peculiar nucleosynthetic signatures imprinted on the atmospheres of these ancient, most metal-poor stars via spectroscopy and analyzing their kinematics.
Galactic Archaeology/Nuclear Astrophysics
Background
Astrophysicists did not recognize until the early 20th century that stars in our Galaxy have different chemical compositions from the Sun. The likely origin and evolution of metals were not known until the 1950s. Since then, the advances in astrophysics and nuclear physics led to the conclusion that the first generation of stars formed out of pristine gas (H, He) 200-300 million years after the Big Bang. The first metals (heavier than H, He, and Li ) were synthesized during the evolution of the very first stars and supernovae explosions at the end of their life and ejected into the surrounding metal-free gas. Multiple generations of star formation, evolution, and element production have since then enriched the Universe to its present composition, while concurrently galaxies have grown by accreting numerous smaller galaxies.
Many stellar surveys including large-scale surveys such as the SDSS over the last four decades for the rare metal-deficient stars in the Milky Way (MW) halo have led to the discovery of many thousands of such stars. These metal-deficient stars have shed light on a deeper understanding of not only the origin of elements but also the formation and evolution of the stars and galaxies in the very early Universe. Among these, the sub-class known as CEMP-no stars are of particular interest. They are characterized by a distinctive abundance pattern, with carbon abundance that is strongly enhanced and heavy neutron-capture elements (such as Ba) that are under-abundant compared to Fe. Many of them, in particular, the extremely metal-poor (EMP; [Fe/H] < −3.0) CEMP-no stars (Beers & Christlieb 2005) also exhibit enhancements of light elements such as N, O, Na, and Mg. Also, the frequency of CEMP-no stars increases with decreasing metallicity; almost all of the ultra metal-poor (UMP; [Fe/H] < −4.0) stars are CEMP-no stars (Lee et al. 2013, Placco et al. 2014, Yoon et al. 2018a). CEMP-no stars are known to be predominantly members of the outer-halo population, characterized by retrograde, energetic orbits based on studies of their kinematics (e.g., Carollo et al. 2010, 2014 Lee et al. 2019). This outer-halo membership has suggested that CEMP-no stars had been accreted from low-mass dark matter-dominated mini-halos (their birthplaces) into the MW halo, while this notion has been challenged (e.g., Schönrich et al. 2011, 2014, Hansen et al. 2019). Thus, scrutinizing the EMP/UMP CEMP-no stars from both the MW halo and its satellite galaxies enables understanding of the nature of the first stars, Galactic chemical evolution (GCE), and the history of Galactic assembly.
Decoding the nature of the first stars
Obtaining a better characterization of the first-star nucleosynthesis is critical for understanding the nature of the first stars, which requires detailed chemical abundance analyses of various elements in order to determine the likely astrophysical progenitors (parent stars) among various suggested possibilities, including "faint supernovae" and "spinstars (rapidly rotating massive stars)." The relative abundances of C, N, and O are among the most important factors to differentiate between these models, which is linked to my doctoral research about rapid rotational effect on chemical composition. Recently, I recognized that the CEMP-no stars can be sub-divided into at least two groups (Group II CEMP-no and Group III CEMP-no), based on their distinct morphology in the A(C)−[Fe/H] space, indicating the likely existence of multiple pathways for their formation. I also found that this distinct CEMP-no group morphology is identified in the absolute abundance spaces of A(Na)−A(C), A(Mg)−A(C), and A(Ba)−A(C) as well (Yoon et al. 2016, 2019). Thus a full detailed abundance analysis is crucial for the characterizing the nucleosynthesis happened during the evolution of the first stars. This research has been featured in multiple media outlets including Forbes.
However, several of the light elements (in particular, N and O) are not readily measurable, even for the known CEMP-no stars, with high-resolution spectroscopy alone. I plan to obtain C, N, O abundances from molecular bands via midium-resolution spectrographs, the LBT/MODS and LUCI (near-IR). By combining these abundances with existing elemental abundances from high-resolution spectroscopy, I will conduct a full abundance analyses for the known CEMP-no stars, and better characterize the possible astrophysical nucleosynthesis pathways responsible for their origin.
Hunting for the most metal-poor stars
The slow discovery rate of UMP stars (~25 stars in 25 years as of 2016) has hampered progress in understanding the nature of the first stars. The conventional sub-classification scheme for CEMP-no stars requires a measured Ba abundance, which can only be made with high-resolution spectroscopy -- a time-intensive effort usually carried out with large telescopes. Thus, my colleagues and I have recently devised a method to identify CEMP-no stars with a success rate similar to the conventional Ba method that only requires an absolute carbon abundance measurement (A(C)) that is readily obtained with medium-resolution spectroscopy on smaller telescopes (Yoon et al. 2016). I have applied this method to the medium-resolution spectroscopic data from the AEGIS survey in the Southern Hemisphere and identified ~700 CEMP-no stars, confirming that they are predominantly members of the outer-halo population Yoon et al. (2018a). This method has also been applied to various samples such as spectroscopic data from SDSS, creating the first map of the distribution of [C/Fe] in the MW halo (Lee et al. 2017), and a medium-resolution spectroscopic study of the MW dwarf spheroidal galaxy (dSph) Sculptor (Chiti et al. 2018).
Developing analysis tools for very cool (<4500K) CEMP stars
Using the fact that the increasing fraction of CEMP stars with decreasing metallicity and my discovering method for UMP stars, I have been actively observing and analyzing the very cool CEMP stars using the Large Binocular Telescope(LBT)/MODS (optical) and the Gemini Telescopes/GMOS. The preliminary work has been published in Yoon et al. (2018b). I have also led a group of graduate students for a project of developing a stellar parameter pipeline specific to these types of stars. By applying this method to analyze a carbon giant in the dwarf spheroidal galaxy Canes Venatici I, we confirm that this star is a Group III CEMP star (Yoon et al. 2020).
Formation (assembly) and chemical evolution history of the Galactic halo
All of the chemical elements can play important roles in our understanding of GCE, since the production history of each element could be associated with different nucleosynthesis pathways (different astrophysical processes, sites, timescales, and/or masses of the stars). In particular, using the AEGIS large low-resolution spectroscopic data, I have studied the cosmic evolution of iron and carbon in the MW and implications of their spatial distribution on the formation and evolution history of the MW halo. Indeed, CEMP-no stars in the AEGIS survey show that they are the dominant class in the outer halo, supporting their accretion origin. I also recognized that CEMP stars among dwarfs and/or main sequence turn-off stars should not be included together with giant stars for frequency calculation becasue their intrinsically warmer temperature make the detection of carbon enhancement harder. Thus only considering CEMP giants, I not only calculated the true frequency of CEMP-no stars from this survey but also that of CEMP-s stars, a dominant subclass of CEMP stars whose origin of carbon enhancement is associated with binary mass-transfer rather than intrinsic property. The frequency indeed strongly increases with decreasing metallicity, doubling that from the SDSS CEMP stars. This true frequency is crucial for constraining the pristine IMF( birth mass distribution of the first stars), a long-sought question of near-field cosmology. The detailed analysis can be found in Yoon et al. (2018a).
Further, I have found strong evidence that the halo CEMP-no stars were indeed accreted from their likely birthplaces (disrupted first galaxies), by showing that the similar CEMP-no groups exist among the CEMP stars in satellite dwarf galaxies.The detailed analysis is reported in Yoon et al. (2019).
Rapid Rotation and Stellar Evolution
Summary
Unlike late-type (cool and low-mass) stars such as Sun, early-type stars such as Vega (Teff > 10,000K and Mass > 2 solar masses) rotate rapidly, faster than 50% breakup of a star. However, the role of rotation has been greatly underestimated, and thus, there have been numerous discrepancies between the models and the observations. Since rotation obscures the interpretation of the observed data, my main work was to numerically simulate rapidly rotating stars (so-called gravity-darkened Roche models, von Zeipel 1924) and predict both interferometric and spectroscopic observables for comparison with the observed data. Then I decoupled rotational effects from the physical properties such as temperature, radius, composition, mass, and in turn age by measuring accurate and precise true rotational velocity of Vega, a rapid rotator, by using both high angular resolution interferometry and high-resolution spectroscopy.