Stellar Population Astrophysics (SPA) with the TNG. CNO abundances in 28 open clusters

Our main aim with this work was to enlarge the pool of open clusters with determined carbon, nitrogen, and oxygen abundances in evolved giants to further advance chemical clocks in stellar age determinations. High-resolution spectra were analysed using a differential model atmosphere method. Carbon abundances were derived using spectral synthesis of the $ ̊m C _2$ band heads at 5135 and 5635.5 Å. The CN features at 6470--6490 Å were analysed to determine the abundances of nitrogen. The oxygen abundances were determined from the i line at 6300 Å. We provide abundances of C, N, and O for 88 giants in 28 open clusters and in two stars of the association Theia,1214. The results were compared with theoretical predictions and used for the analysis of the C/N relations with age, and we investigated the origin of the two Theia,1214 stars. Precise age dating requires separate age calibrations of C/N ratios for the first-ascent giants of the lower part of the red giant branch and for the red clump stars to account for the additional abundance alterations during the post-red giant branch luminosity bump evolution. In the first-ascent giant stars with larger turn-off masses, the observed C/N ratios are slightly higher than those predicted by standard models, and the obtained C/N versus age relation is flatter in the young age regime than in previous studies. We doubt that the two stars we investigated in the Theia,1214 association belong to the tail of NGC,752. Most probably, they are field stars.