Tracing the formation of the Milky Way through ultra metal-poor stars

Abstract

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 ([Fe/H] < −4.0 dex) available in the literature. Assuming that these stars are old (> 11 Gyr) 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 ∼ 20 kpc 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 (> 30 kpc), hinting at later accretion from dwarf galaxies. Of particular interest, we find evidence that a significant fraction of all known UMP stars (∼ 26%) are on prograde orbits confined within 3 kpc of the Milky Way plane (J z < 100 km s −1 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 Pristine survey – X. A large population of low-metallicity stars permeates the Galactic disc

“…More importantly, the sample exhibits a strong asymmetry between prograde (J φ > 0) and retrograde (J φ < 0) stars, where prograde stars dominate with an angular momentum up to the Sun's value. The bottom panels of Figure 1 show the same action plot divided into 4 metallicity bins, respectively the UMP stars populated only by the 42 stars from Sestito et al (2019), the −4.0 < [Fe/H] ≤ −3.0 bin, the −3.0 < [Fe/H] ≤ −2.5 regime, and, to be complete, the bin with −2.5 < [Fe/H] ≤ −2.0, where the signature of a disk population was already discovered (Beers et al 2002;Reddy & Lambert 2008;Ruchti et al 2011;Li & Zhao 2017). Separating the sample in these metallicity bins makes it evident that the prograde stars that remain close to the MW plane inhabit all [Fe/H] ranges.…”

“…This Bayesian method to infer distance does not require a reliable parallax measurement, but does take into account all parallax information available (even negative values). As discussed in Sestito et al (2019), the choice of the MW density prior affects the results only when the distance PDF has two solutions ( i.e., both a dwarf and a giant solution) changing the probabilities associated to the two solutions, but not the values of the distances. After finding that a significant fraction of UMP stars reside close to the MW plane (Sestito et al 2019), we therefore chose a MW density prior composed by the sum of a halo component described by a power law, and a disk component described by an exponential distribution law.…”

“…As discussed in Sestito et al (2019), the choice of the MW density prior affects the results only when the distance PDF has two solutions ( i.e., both a dwarf and a giant solution) changing the probabilities associated to the two solutions, but not the values of the distances. After finding that a significant fraction of UMP stars reside close to the MW plane (Sestito et al 2019), we therefore chose a MW density prior composed by the sum of a halo component described by a power law, and a disk component described by an exponential distribution law. Subsequently, we derive the orbit using the Galpy code (Bovy 2015) providing it with the inferred distances, the radial velocities and the exquisite Gaia DR2 proper motion.…”

“…For the gravitational potential, we use a more massive halo (1.2 · 10 12 M ⊙ ) compared to MWPo-tential14 from Galpy (0.8 · 10 12 M ⊙ ) in agreement with the value from Bland-Hawthorn & Gerhard (2016), an exponentially cut-off bulge, a Miyamoto Nagai Potential disc, and a Navarro et al (1997) dark matter halo. The Local Standard of Rest circular velocity, Sun peculiar motion, and distance from the Galactic centre are the same assumed by Sestito et al (2019) (see also references therein). The table with the inferred orbital parameters is provided as Online material.…”

“…Recent work by Sestito et al (2019) has shown the orbital properties of the 42 most pristine stars known in the ultra metal-poor regime (UMP, [Fe/H] < −4.0) using the photometric and kinematic data of the Data Release 2 (DR2) of the Gaia satellite. Surprisingly, roughly a quarter of those stars orbit close or within the plane of the MW disk.…”