Observational Constraints on the Origin of the Elements. X. Combining Non–Local Thermodynamic Equilibrium and Machine Learning for Chemical Diagnostics of 4 Million Stars in the 4MIDABLE-HR Survey

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Abstract We present the 4MOST-HR resolution non–local thermal equilibrium (NLTE) Payne artificial neural network (ANN), trained on 404,793 new FGK spectra with 16 elements computed in NLTE. This network will be part of the Stellar Abundances and atmospheric Parameters Pipeline (SAPP), which will analyze 4 million stars during the 5 yr long 4MOST consortium 4: 4MOST MIlky way Disc And BuLgE High-Resolution (4MIDABLE-HR) survey. A fitting algorithm using this ANN is also presented that is able to fully automatically and self-consistently derive both stellar parameters and elemental abundances. The ANN is validated by fitting 121 observed spectra of low-mass FGKM-type stars, including main-sequence dwarf, subgiant, and giant stars down to [Fe/H] ≈ −3.3 degraded to a 4MOST-HR resolution of R ≈ 20,000 and comparing the derived abundances with the output of the classical radiative transfer code TSFitPy. We are able to recover all 18 elemental abundances with a bias of <0.13 and spread of <0.16 dex, although the typical values are <0.09 dex for most elements. These abundances are compared to the OMEGA+ Galactic chemical evolution model, showcasing for the first time the expected performance and results obtained from high-resolution spectra of the quality expected to be obtained with 4MOST. The expected Galactic trends are recovered, and we highlight the potential of using many chemical elements to constrain the formation history of the Galaxy.

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