Automatic line selection for abundance determination in large stellar spectroscopic surveys

Abstract

Context. Over the last years, new multiplex spectrographs having observed or planning to observe several millions of stars have emerged. The optimisation of these instruments (regarding resolution or wavelength range), their associated surveys (choice of instrumental setup), or their parameterisation pipelines require methods that estimate which wavelengths, or pixels, contain useful information.Aims. We propose a method that establishes the usefulness of an atomic spectral line, where usefulness is defined by the purity of the line and its detectability. We show two applications of our code: a) optimising an instrument, by comparing the number of detected useful lines at a given wavelength range and resolution, and b) optimising the line-list for a given setup, in the sense of creating a golden subsample, choosing the least blended lines detectable at different signal-to-noise ratios. Methods. The method compares pre-computed normalised synthetic stellar spectra containing all of the elements and molecules with spectra containing the lines of specific elements alone. Then, the flux ratios between the full spectrum and the element spectrum are computed to estimate the line purities. The method identifies automatically (i) the line's central wavelength, (ii) its detectability based on its depth and a given signal-to-noise threshold and (iii) its usefulness based on the purity ratio defined above. Results. We apply this method to compare the three WEAVE high-resolution setups (Blue: 404−465 nm, Green: 473−545 nm, Red: 595−685 nm), and find that the Green+Red setup both allows one to measure more elements and contains more numerous useful lines. However, there is a disparity in terms of which elements are detected over each of the setups, which we characterise. We also study the performances of high-resolution (R ∼ 20 000) and low-resolution (R ∼ 6 000) spectra covering the entire optical wavelength range. Assuming a purity threshold of 60 per cent, we find that the high-resolution setup contains a much wealthier selection of lines, for any of the considered elements, whereas the low-resolution has a "loss" of 50 to 90 per cent of the lines (depending on the nucleosynthetic channel considered) even when the signal-to-noise is increased. Conclusions. The method presented provides a vital diagnostic of where to focus to get the most out of a spectrograph, and is easy to implement for future instruments that have not decided yet their final configuration, or for pipelines that require line masks.