CAREER: The Most Ancient Stars in the Milky Way

The first stars in the Universe—the so-called Population III stars—were made up of only the products of Big Bang nucleosynthesis: hydrogen, helium, and a sprinkling of lithium. They are theoretically predicted to have formed more than 13.4 billion years in the past. They have never been observed. Many properties of Pop III stars are highly uncertain, and resolving these uncertainties is necessary to understand the origin of the elements, the properties of the first galaxies, the reionization of the Universe, and the seeds of supermassive black holes. Two graduate students will be supported by this program to perform the large-scale data analysis, and one undergraduate student from an underrepresented minority will participate in the research each summer. In addition, the team will start a long-term, sustainable, bimonthly program of portable planetarium visits to Baltimore City Public Schools elementary/middle schools. Even with the James Webb Space Telescope it is thought to be impossible to directly study individual Pop III stars, so they must be studied indirectly through the unique elemental signature they inject into the interstellar medium during their supernovae. The ratios of the elements created depend on the Pop III initial mass function as well as their binarity, rotation, and fragmentation rates. Those elemental abundances consequently constrain the otherwise unobservable properties of Pop III stars. The project will conduct a comprehensive spectral analysis of high-resolution optical spectra to calculate the photospheric stellar parameters, detailed abundances, isochrone-derived ages, and orbits of 341 stars in the inner and outer regions of the Milky Way galaxy. The data will be used to explore the Pop III initial mass function and its role in the origin of the elements, the properties of the first galaxies, the reionization of the Universe, and the origin of supermassive black holes. The chemodynamic analysis of the phase-space of an unbiased solar neighborhood sample will explore the first stage of Milky Way formation and reveal the galactic origin of metal-poor stars. Results from this project will determine whether metal-poor stars formed in the Milky Way or in accreted dwarf galaxies. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.