Tracing the formation of the Milky Way through ultra metal-poor stars. (arXiv:1811.03099v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Sestito_F/0/1/0/all/0/1">Federico Sestito</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Longeard_N/0/1/0/all/0/1">Nicolas Longeard</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Martin_N/0/1/0/all/0/1">Nicolas F. Martin</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Starkenburg_E/0/1/0/all/0/1">Else Starkenburg</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Fouesneau_M/0/1/0/all/0/1">Morgan Fouesneau</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hernandez_J/0/1/0/all/0/1">Jonay I. Gonzalez Hernandez</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Arentsen_A/0/1/0/all/0/1">Anke Arentsen</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ibata_R/0/1/0/all/0/1">Rodrigo Ibata</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Aguado_D/0/1/0/all/0/1">David S. Aguado</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Carlberg_R/0/1/0/all/0/1">Raymond G. Carlberg</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Jablonka_P/0/1/0/all/0/1">Pascale Jablonka</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Navarro_J/0/1/0/all/0/1">Julio F. Navarro</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Tolstoy_E/0/1/0/all/0/1">Eline Tolstoy</a>
We use Gaia DR2 astrometric and photometric data, combined with MESA
isochrone and luminosity function models and radial velocities from literature
to infer distances, orbits, surface gravities, and effective temperatures for
all ultra metal-poor stars ($FeH< -4$ dex) available in the literature.
Assuming that these stars are old ($>11Gyr$) and that they are expected to
belong to the Milky Way halo, we find that these 41 stars (18 dwarf stars and
23 giants or sub-giants) are currently within $sim20$ kpc from the Sun and
that they map a wide variety of orbits. A large fraction of those stars remains
confined to the inner parts of the halo and was likely formed or accreted
early-on in the history of the Milky Way, while others have larger apocentres
($>30kpc$), hinting at later accretion from dwarf galaxies. Of particular
interest, we find evidence that a significant fraction of all known UMP stars
($sim29$%) are on prograde orbits confined within $3kpc$ of the Milky Way
plane ($J_z < 100 kms kpc$). One intriguing interpretation is that these
stars belonged to the massive building block(s) of the proto-Milky Way that
formed the backbone of the Milky Way disc. Alternatively, they may have been
brought into the Milky Way by one or more accretion events whose orbit was
dragged into the plane by dynamical friction before disruption. The combination
of the exquisite Gaia DR2 data and surveys of the very metal-poor sky opens an
exciting era in which we can trace the very early formation of the Milky Way.
We use Gaia DR2 astrometric and photometric data, combined with MESA
isochrone and luminosity function models and radial velocities from literature
to infer distances, orbits, surface gravities, and effective temperatures for
all ultra metal-poor stars ($FeH< -4$ dex) available in the literature.
Assuming that these stars are old ($>11Gyr$) and that they are expected to
belong to the Milky Way halo, we find that these 41 stars (18 dwarf stars and
23 giants or sub-giants) are currently within $sim20$ kpc from the Sun and
that they map a wide variety of orbits. A large fraction of those stars remains
confined to the inner parts of the halo and was likely formed or accreted
early-on in the history of the Milky Way, while others have larger apocentres
($>30kpc$), hinting at later accretion from dwarf galaxies. Of particular
interest, we find evidence that a significant fraction of all known UMP stars
($sim29$%) are on prograde orbits confined within $3kpc$ of the Milky Way
plane ($J_z < 100 kms kpc$). One intriguing interpretation is that these
stars belonged to the massive building block(s) of the proto-Milky Way that
formed the backbone of the Milky Way disc. Alternatively, they may have been
brought into the Milky Way by one or more accretion events whose orbit was
dragged into the plane by dynamical friction before disruption. The combination
of the exquisite Gaia DR2 data and surveys of the very metal-poor sky opens an
exciting era in which we can trace the very early formation of the Milky Way.
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