The Most Metal-poor Stars in the Inner Bulge. (arXiv:2007.12728v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Reggiani_H/0/1/0/all/0/1">Henrique Reggiani</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Schlaufman_K/0/1/0/all/0/1">Kevin C. Schlaufman</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Casey_A/0/1/0/all/0/1">Andrew R. Casey</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ji_A/0/1/0/all/0/1">Alexander P. Ji</a>
The bulge is the oldest component of the Milky Way. Since numerous
simulations of Milky Way formation have predicted that the oldest stars at a
given metallicity are found on tightly bound orbits, the Galaxy’s oldest stars
are likely metal-poor stars in the inner bulge with small apocenters (i.e.,
$R_{mathrm{apo}}lesssim4$ kpc). In the past, stars with these properties have
been impossible to find due to extreme reddening and extinction along the line
of sight to the inner bulge. We have used the mid-infrared metal-poor star
selection of Schlaufman & Casey (2014) on Spitzer/GLIMPSE data to overcome
these problems and target candidate inner bulge metal-poor giants for
moderate-resolution spectroscopy with AAT/AAOmega. We used those data to select
three confirmed metal-poor giants ($[mathrm{Fe/H}]=-3.15,-2.56,-2.03$) for
follow-up high-resolution Magellan/MIKE spectroscopy. A comprehensive orbit
analysis using Gaia DR2 astrometry and our measured radial velocities confirms
that these stars are tightly bound inner bulge stars. We determine the
elemental abundances of each star and find high titanium and iron-peak
abundances relative to iron in our most metal-poor star. We propose that the
distinct abundance signature we detect is a product of nucleosynthesis in the
Chandrasekhar-mass thermonuclear supernova of a CO white dwarf accreting from a
helium star with a delay time of about 10 Myr. Even though chemical evolution
is expected to occur quickly in the bulge, the intense star formation in the
core of the nascent Milky Way was apparently able to produce at least one
Chandrasekhar-mass thermonuclear supernova progenitor before chemical evolution
advanced beyond $[mathrm{Fe/H}]sim-3$.
The bulge is the oldest component of the Milky Way. Since numerous
simulations of Milky Way formation have predicted that the oldest stars at a
given metallicity are found on tightly bound orbits, the Galaxy’s oldest stars
are likely metal-poor stars in the inner bulge with small apocenters (i.e.,
$R_{mathrm{apo}}lesssim4$ kpc). In the past, stars with these properties have
been impossible to find due to extreme reddening and extinction along the line
of sight to the inner bulge. We have used the mid-infrared metal-poor star
selection of Schlaufman & Casey (2014) on Spitzer/GLIMPSE data to overcome
these problems and target candidate inner bulge metal-poor giants for
moderate-resolution spectroscopy with AAT/AAOmega. We used those data to select
three confirmed metal-poor giants ($[mathrm{Fe/H}]=-3.15,-2.56,-2.03$) for
follow-up high-resolution Magellan/MIKE spectroscopy. A comprehensive orbit
analysis using Gaia DR2 astrometry and our measured radial velocities confirms
that these stars are tightly bound inner bulge stars. We determine the
elemental abundances of each star and find high titanium and iron-peak
abundances relative to iron in our most metal-poor star. We propose that the
distinct abundance signature we detect is a product of nucleosynthesis in the
Chandrasekhar-mass thermonuclear supernova of a CO white dwarf accreting from a
helium star with a delay time of about 10 Myr. Even though chemical evolution
is expected to occur quickly in the bulge, the intense star formation in the
core of the nascent Milky Way was apparently able to produce at least one
Chandrasekhar-mass thermonuclear supernova progenitor before chemical evolution
advanced beyond $[mathrm{Fe/H}]sim-3$.
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