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The chemodynamics of prograde and retrograde Milky Way stars

Abstract : Context. The accretion history of the Milky Way is still unknown, despite the recent discovery of stellar systems that stand out in terms of their energy-angular momentum space, such as Gaia-Enceladus-Sausage. In particular, it is still unclear how these groups are linked and to what extent they are well-mixed. Aims. We investigate the similarities and differences in the properties between the prograde and retrograde (counter-rotating) stars and set those results in context by using the properties of Gaia-Enceladus-Sausage, Thamnos/Sequoia, and other suggested accreted populations. Methods. We used the stellar metallicities of the major large spectroscopic surveys (APOGEE, Gaia-ESO, GALAH, LAMOST, RAVE, SEGUE) in combination with astrometric and photometric data from Gaia's second data-release. We investigated the presence of radial and vertical metallicity gradients as well as the possible correlations between the azimuthal velocity, v φ , and metallicity, [M/H], as qualitative indicators of the presence of mixed populations. Results. We find that a handful of super metal-rich stars exist on retrograde orbits at various distances from the Galactic center and the Galactic plane. We also find that the counter-rotating stars appear to be a well-mixed population, exhibiting radial and vertical metallicity gradients on the order of ∼ − 0.04 dex kpc −1 and −0.06 dex kpc −1 , respectively, with little (if any) variation when different regions of the Galaxy are probed. The prograde stars show a v φ − [M/H] relation that flattens-and, perhaps, even reverses as a function of distance from the plane. Retrograde samples selected to roughly probe Thamnos and Gaia-Enceladus-Sausage appear to be different populations yet they also appear to be quite linked, as they follow the same trend in terms of the eccentricity versus metallicity space.
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Georges Kordopatis, Alejandra Recio-Blanco, Mathias Schultheis, Vanessa Hill. The chemodynamics of prograde and retrograde Milky Way stars. Astronomy and Astrophysics - A&A, EDP Sciences, 2020, 643, pp.A69. ⟨10.1051/0004-6361/202038686⟩. ⟨insu-03001942⟩

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