TheGaia-ESO Survey: characterisation of the [α/Fe] sequences in the Milky Way discs

Abstract

Context. High-resolution spectroscopic surveys of stars indicate that the Milky Way thin and thick discs follow different paths in the chemical space defined by [α/Fe] vs. [Fe/H], possibly suggesting different formation mechanisms for each of these structures. Aims. We investigate, using the Gaia-ESO Survey internal Data-Release 2, the properties of the double sequence of the Milky Way discs, which are defined chemically as the high-α and low-α populations. We discuss their compatibility with discs defined by other means, such as metallicity, kinematics, or positions. Methods. This investigation uses two different approaches: in velocity space, for stars located in the extended solar neighbourhood; and, in chemical space, for stars at different ranges of Galactocentric radii and heights from the Galactic mid-plane. The separation we find in velocity space allows us to investigate, using a novel approach, the extent of metallicity of each of the two chemical sequences, without making any assumption about the shape of their metallicity distribution functions. Then, using the separation in chemical space, adopting the magnesium abundance as a tracer of the α-elements, we characterise the spatial variation of the slopes of the [α/Fe]−[Fe/H] sequences for the thick and thin discs and the way in which the relative proportions of the two discs change across the Galaxy. Results. We find that the thick disc, defined as the stars tracing the high-α sequence, extends up to super-solar metallicities ([Fe/H] ≈ +0.2 dex), and the thin disc, defined as the stars tracing the low-α sequence, extends at least down to [Fe/H] ≈ −0.8 dex, with hints pointing towards even lower values. Radial and vertical gradients in α-abundances are found for the thin disc, with mild spatial variations in its [α/Fe]−[Fe/H] paths, whereas for the thick disc we do not detect any spatial variations of this kind. This is in agreement with results obtained recently from other high-resolution spectroscopic surveys. Conclusions. The small variations in the spatial [α/Fe]−[Fe/H] paths of the thin disc do not allow us to distinguish between formation models of this structure. On the other hand, the lack of radial gradients and [α/Fe]−[Fe/H] variations for the thick disc indicate that the mechanism responsible for the mixing of metals in the young Galaxy (e.g. radial stellar migration or turbulent gaseous disc) was more efficient before the (present) thin disc started forming.

Compact Multi-planet Systems are more Common around Metal-poor Hosts

“…The effect is more pronounced for more massive stars, but affects all elements heavier than helium roughly equally. Stars with lower initial metallicity should also have a higher ratio of α-elements to iron (Kordopatis et al 2015), so we can use the Si/Fe ratio as a function of [Fe/H] to see if the increase in frequency of compact multis is due to age or inherently low metallicity (Figure 2). At both low metallicity and high Si/Fe, none of the hosts are hot-Jupiters and an increasing fraction are compact multi-planet systems.…”

“…Stars of lower metallicity and higher Si/Fe ratios are generally older or members of the galactic thick-disk population (Kordopatis et al 2015). This could point to a changing mix of planet architectures based on formation time and location.…”

The temporal evolution of neutron-capture elements in the Galactic discs

“…The chemical evolution of the Galactic components is often investigated through [X/Y]-[Fe/H] plots that give a picture of the progressive and differential enrichment of the X and Y elements and that can be used to calibrate the rate of star formation rates in the Galaxy and the yields from different progenitors (e.g., Gilmore et al 1989;Reddy et al 2006;Shen et al 2015 , it is also true that the [X/Y]-[Fe/H] distributions contain a direct information on how the differential production of the X and Y elements varied with the metallicity of the progenitors or among populations with different chemical histories (Kordopatis et al 2015;Rojas-Arriagada et al 2016;Bekki & Tsujimoto 2017). However, when studying the chemical evolution of the Galaxy, it is fundamental to bear in mind that [Fe/H] in a given population does not always increases with time (Bensby et al 2007(Bensby et al , 2014Haywood et al 2013).…”

Cosmic phylogeny: reconstructing the chemical history of the solar neighbourhood with an evolutionary tree

“…One reason is that in the [α/Fe] -[Fe/H] diagram, there is an 'intermediate-α' population lying between the thin and thick-disk populations (Edvardsson et al 1993), probably related to our orange population. This population is either split between the thin and thick disks (Kordopatis et al 2015) or simply rejected from the analyses (Hawkins et al 2015;. This becomes especially uncertain at solar metallicities, where the [α/Fe] of the two populations overlap (see discussions in e.g.…”