The AMBRE Project: Spectrum normalisation influence on Mg abundances in the metal-rich Galactic disc

Context. The abundance of α-elements provides an important fossil signature in Galactic archaeology to trace the chemical evolution of the different disc populations. High-precision chemical abundances are crucial to improving our understanding of the chemodynamical properties present in the Galaxy. However, deriving precise abundance estimations in the metal-rich disc ([M/H] > 0 dex) is still challenging. Aims. The aim of this paper is to analyse different error sources affecting magnesium abundance estimations from optical spectra of metal-rich stars. Methods. We derived Mg abundances for 87522 high-resolution spectra of 2210 solar neighbourhood stars from the AMBRE Project, and selected the 1172 best parametrised stars with more than four repeated spectra. For this purpose, the GAUGUIN automated abundance estimation procedure was employed. Results. The normalisation procedure has a strong impact on the derived abundances, with a clear dependence on the stellar type and the line intensity. For non-saturated lines, the optimal wavelength domain for the local continuum placement should be evaluated using a goodness-of-fit criterion, allowing mask-size dependence with the spectral type. Moreover, for strong saturated lines, applying a narrow normalisation window reduces the parameter-dependent biases of the abundance estimate, increasing the line-to-line abundance precision. In addition, working at large spectral resolutions always leads to better results than at lower ones. The resulting improvement in the abundance precision makes it possible to observe both a clear thin-thick disc chemical distinction and a decreasing trend in the magnesium abundance even at supersolar metallicities. Conclusions. In the era of precise kinematical and dynamical data, optimising the normalisation procedures implemented for large spectroscopic stellar surveys would provide a significant improvement to our understanding of the chemodynamical patterns of Galactic populations