Using chemical tagging to redefine the interface of the Galactic disc and halo
Abstract
We present a chemical abundance distribution study in 14 α, odd-Z, even-Z, light, and Fe-peak elements of approximately 3200 intermediate-metallicity giant stars from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey. The main aim of our analysis is to explore the Galactic disc-halo transition region within -1.20 < [Fe/H] < -0.55 as a means to study chemical difference (and similarities) between these components. In this paper, we show that there is an α-poor and α-rich sequence within both the metal-poor and intermediate-metallicity regions. Using the Galactic rest-frame radial velocity and spatial positions, we further separate our sample into the canonical Galactic components. We then studied the abundances ratios of Mg, Ti, Si, Ca, O, S, Al, C+N, Na, Ni, Mn, V, and K for each of the components and found the following: (1) the α-poor halo subgroup is chemically distinct in the α-elements, particularly O, Mg, S, Al, C+N, and Ni, from the α-rich halo, consistent with the literature confirming the existence of an α-poor accreted halo population; (2) the canonical thick disc and halo are not chemically distinct in all elements indicating a smooth transition between the thick disc and halo; (3) a subsample of the α-poor stars at metallicities as low as [Fe/H] ∼ -0.85 dex are chemically and dynamically consistent with the thin disc indicating that the thin disc may extend to lower metallicities than previously thought; and (4) the locations of the most metal-poor thin disc stars are consistent with a negative radial metallicity gradient. Finally, we used our analysis to suggest a new set of chemical abundance planes ([α/Fe], [C+N/Fe], [Al/Fe], and [Mg/Mn]) that may be able to chemically label the Galactic components in a clean and efficient way independent of kinematics.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- October 2015
- DOI:
- 10.1093/mnras/stv1586
- arXiv:
- arXiv:1507.03604
- Bibcode:
- 2015MNRAS.453..758H
- Keywords:
-
- Galaxy: abundances;
- Galaxy: halo;
- Galaxy: structure;
- Astrophysics - Astrophysics of Galaxies;
- Astrophysics - Solar and Stellar Astrophysics
- E-Print:
- 18 Pages, 10 Figures, Accepted for Publication in MNRAS