3D non-local thermodynamic equilibrium magnesium abundances reveal a distinct halo population

Vol. 688
8. Stellar atmospheres

3D non-local thermodynamic equilibrium magnesium abundances reveal a distinct halo population

Chemical abundances of stars are the fossil record of their formation environment and can be used to disentangle stellar populations with different origins, especially when they are measured with high precision. One example is the discovery of two major populations in the Milky Way halo, high-Mg stars and low-Mg stars, which are respectively considered to have formed in the Milky Way and in dwarf galaxies, disrupted by and accreted onto our Galaxy.

The authors have employed state-of-the-art modeling of stellar spectra in 3D stellar atmospheres under nonlocal thermodynamic equilibrium (non-LTE) for precise and accurate Mg abundance measurements. They find that the 3D non-LTE analysis significantly improves the consistency of the Mg abundances derived from Mg I and Mg II lines. When the 3D non-LTE analysis is applied to the Mg abundance of Milky Way halo stars in the solar neighborhood, the authors find further subpopulations among the low-Mg stars that also show different kinematics and hence have different origins. This work demonstrates the importance of 3D non-LTE analysis for studying stellar populations in the Milky Way using the chemical abundances of stars.