Cake Talk - Alexander Ji

Abstract:

The first stars formed out of pristine gas, causing them to be so massive that none are expected to have survived until today.  If their direct descendants were sufficiently low-mass stars, they could exist today and would be recognizable by having the lowest metallicity (abundance of elements heavier than helium). The lowest metallicity star previously known is a star in the thick disk of the Milky Way with total metallicity Z < 1.4 x 10^-6 (log Z/Zsun < -4.0). While other stars with lower iron abundance have been discovered, they have high carbon abundances and thus higher total metallicities (log Z/Zsun > -3. I present the independent discovery and detailed chemical analysis of the most metal-poor star yet found: the red giant star SDSS J0715-7334 with ultra-low abundances of both iron and carbon ([Fe/H] = -4.3, [C/Fe] < -0.2) resulting in total metallicity Z < 7.8 x 10^-7 (log Z/Zsun < -4.3). This star potentially has the most pristine composition of any object known in the universe. The star's orbit indicates that it originates from the halo of the Large Magellanic Cloud. Its detailed chemical composition implies a supernova progenitor with initial mass of 30 solar masses. Current models of low-mass star formation can explain the existence of SDSS J0715-7334 only if dust cooling was already able to operate at the time of its formation. SDSS J0715-7334 is over ten times more metal-poor than the most metal-poor high-redshift galaxies found by the James Webb Space Telescope, some of which have been claimed to be potentially metal-free. Substantially deeper observations of high-redshift galaxies would be needed to prove that they are truly pristine galaxies made of metal-free stars and not metal-enriched galaxies composed of second-generation stars like SDSS J0715-7334.