Xiang, Mao-ShengRix, Hans-WalterTing, Yuan-SenLudwig, H.-G.Coronado, JohannaZhang, MengZhang, Hua-WeiBuder, SvenDal Tio, Piero2022-07-272022-07-270004-637Xhttp://hdl.handle.net/1885/269965We present 15,000 metal-rich ([Fe/H] > −0.2 dex) A and F stars whose surface abundances deviate strongly from solar abundance ratios and cannot plausibly reflect their birth material composition. These stars are identified by their high [Ba/Fe] abundance ratios ([Ba/Fe] > 1.0 dex) in the LAMOST DR5 spectra analyzed by Xiang et al. They are almost exclusively main-sequence and subgiant stars with Teff  6300 K. Their distribution in the Kiel diagram (Teff–log g) traces a sharp border at low temperatures along a roughly fixed-mass trajectory (around 1.4 Me) that corresponds to an upper limit in convective envelope mass fraction of around 10−4 . Most of these stars exhibit distinctly enhanced abundances of iron-peak elements (Cr, Mn, Fe, Ni) but depleted abundances of Mg and Ca. Rotational velocity measurements from GALAH DR2 show that the majority of these stars rotate slower than typical stars in an equivalent temperature range. These characteristics suggest that they are related to the so-called Am/Fm stars. Their abundance patterns are qualitatively consistent with the predictions of stellar evolution models that incorporate radiative acceleration, suggesting they are a consequence of stellar internal evolution, particularly involving the competition between gravitational settling and radiative acceleration. These peculiar stars constitute 40% of the whole population of stars with mass above 1.5 Me, affirming that “peculiar” photospheric abundances due to stellar evolution effects are a ubiquitous phenomenon for these intermediate-mass stars. This large sample of Ba-enhanced, chemically peculiar A/F stars with individual element abundances provides the statistics to test more stringently the mechanisms that alter the surface abundances in stars with radiative envelopesH.W.R. and H.G.L. acknowledge funding by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation)—Project-ID 138713538 —SFB 881 (“The Milky Way System,” subproject A03 & A04). Y.S.T. is grateful to be supported by the NASA Hubble Fellowship grant HST-HF2-51425.001 awarded by the Space Telescope Science Institute. H.W.Z. and M.Z. acknowledge the National Natural Science Foundation of China 11973001 and National Key R&D Program of China No. 2019YFA0405504. S.B. acknowledges funds from the Alexander von Humboldt Foundation in the framework of the Sofja Kovalevskaja Award endowed by the Federal Ministry of Education and Research as well as support by the Australian Research Council (grants DP150100250 and DP160103747). Parts of this research were supported by the Australian Research Council (ARC) Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), through project number CE170100013.application/pdfen-AU© 2020. The American Astronomical SocietyAm starsBarium starsChemically peculiar starsStellar typesHeavy metal starsChemical abundancesStellar abundancesStellar diffusionStellar processesStellar evolutionRadiative processesS-process202010.3847/1538-4357/ab99a52021-08-01