Kavli Affiliate: Alexander P. Ji
| First 5 Authors: Alexander P. Ji, Alexander P. Ji, , ,
| Summary:
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 currently 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). Here we present the 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 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.
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