Playing CHESS with stars. I. Search for similar stars in large spectroscopic data sets

Abstract

Context: Massive amounts of spectroscopic data obtained by stellar surveys are feeding an ongoing revolution in our knowledge of stellar and Galactic astrophysics. Analysing these data sets to extract the best possible astrophysical parameters on short time scales represents a considerable challenge. Aims: The differential analysis method is known to return the most precise results in the spectroscopic analyses of F-, G-, and K-type stars. However, it can only be applied to stars with similar parameters. Our goal is to present a procedure that significantly simplifies the identification of spectra from stars with similar atmospheric parameters within extensive spectral datasets. This approach allows for the quick application of differential analyses in these samples, thus enhancing the precision of the results. Methods: We used projection maps created by the t-SNE dimensionality reduction algorithm applied directly to the spectra using pixels as dimensions. For testing the method, we used more than 7300 high-resolution UVES spectra of about 3000 stars in the field-of-view towards open and globular clusters. As reference, we used 1244 spectra of 274 stars with well-determined and high-quality atmospheric parameters. Results: We calibrated a spectral similarity metric that can identify stars in a t-SNE projection map with parameters that differ by $\pm$ 200 K, $\pm$ 0.3 dex, and $\pm$ 0.2 dex in effective temperatures, surface gravities, and metallicities, respectively. We achieved completeness between 74-98 % and typical purity between 39-54 % in this selection. With this, we will drastically facilitate the detection of stars with similar spectra for a successful differential analysis. We used this method to evaluate the accuracy and precision of four atmospheric parameter catalogues, identifying the regions of the parameter space where spectral analysis methods needs improvement.