The Oldest Extremely Metal-poor Stars

Speaker: 
Henrique Reggiani (JHU)
Date: 
Friday, February 14, 2020 - 12:15pm to 1:00pm

The chemical abundances of the oldest stars in a galaxy can be used to investigate the earliest stages of its formation and chemical evolution.  It is tempting to assert that the most metal-poor stars in a large galaxy with a complex accretion history like the Milky Way are the direct descendants of the first stars.  This is not necessarily the case though.  Other properties beyond metallicity are therefore necessary to separate the old from the genuinely ancient metal-poor stars.  The bulge is the fastest forming and oldest component of the Milky Way, and numerous groups have used simulations to predict that the oldest stars at a given metallicity are found on bulge-like orbits.  On the other hand, for galaxies like the Magellanic Clouds that likely formed in isolation, metal-poor star formation continued for an extended period.  We have used the mid-infrared metal-poor star selection of Schlaufman & Casey (2014) to discover and study with Magellan/MIKE both the most metal-poor stars known in the inner bulge and the most metal-poor stars known in the Magellanic Clouds.  The detailed abundances of the metal-poor stars in the Milky Way's inner bulge reveal a distinct abundance signature that is not present in halo or dwarf galaxy stars at a similar metallicity.  We propose that this distinct abundance signature is a product of the high star-formation rate realized in the core of the proto-Milky Way. The seven stars in our LMC sample are highly r-process enhanced. The probability that we see similar r-process enhancement in the halo of the Milky Way is about five in one billion.  This indicates that the origins of the heaviest elements in the LMC and Milky Way were qualitatively differentIt is tempting to assert that the most metal-poor stars in the Galaxy are the direct descendants of the first stars. This is not necessarily the case though, as metal-poor stars form over a range of redshift. Other properties beyond metallicity are therefore necessary to separate the old from the genuinely ancient metal-poor stars. It has recently been proposed that the location of a metal-poor star within the Galaxy provides some extra information about its age. The bulge is the oldest component of the Milky Way, and numerous groups have used simulations to predict that the oldest stars at a given metallicity are found on bulge-like orbits. These tightly bound metal-poor stars have been impossible to find in the past, as most metal-poor stars have been discovered using short-wavelength data. These classical techniques fail in the bulge due to strong reddening and extinction. We have used the mid-infrared metal-poor star selection of Schlaufman & Casey (2014) on Spitzer/GLIMPSE data to overcome these problems and discover the first three extremely metal-poor stars in the inner bulge. I'll present the results of a detailed abundance and orbital analysis of three stars using Magellan/MIKE spectroscopy and Gaia DR2 astrometry. I'll conclude by outlining the constraints these data provide for Pop III stars and the earliest stage of the Milky Way's formation.

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