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We use Gaia DR2 astrometric and photometric data, published radial velocities and MESA models to infer distances, orbits, surface gravities, and effective temperatures for all ultra metal-poor stars ($FeH<-4.0$ 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 42 stars (18 dwarf stars and 24 giants or sub-giants) are currently within $sim20kpc$ of 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 ($sim26$%) 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 might have formed in the early disc and have been dynamically heated, or 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.
The first stars are predicted to have formed within 200 million years after the Big Bang, initiating the cosmic dawn. A true first star has not yet been discovered, although stars with tiny amounts of elements heavier than helium (metals) have been f
We present a chemo-dynamical analysis of low-resolution ($R sim 1300$) spectroscopy of stars from the AAOmega Evolution of Galactic Structure (AEGIS) survey, focusing on two key populations of carbon-enhanced metal-poor (CEMP) stars within the disk s
We explore the origin of a population of stars recently detected in the inner parsec of the Milky Way Nuclear Cluster (NC), which exhibit sub-solar metallicity and a higher rotation compared to the dominant population. Using state-of-the-art $N$-body
We present a low metallicity map of the Milky Way consisting of $sim$111,000 giants with $-3.5 lesssim$ [Fe/H] $lesssim -$0.75, based on public photometry from the second data release of the SkyMapper survey. These stars extend out to $sim$7kpc from
RR Lyrae stars being distance indicators and tracers of old population serve as excellent probes of the structure, formation, and evolution of our Galaxy. Thousands of them are being discovered in ongoing wide-field surveys. The OGLE project conducts