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We present $\gtrsim 15,000$ metal-rich (${\rm [Fe/H]}>-0.2$dex) A and F starswhose surface abundances deviate strongly from Solar abundance ratios andcannot plausibly reflect their birth material composition. These stars areidentified by their high [Ba/Fe] abundance ratios (${\rm [Ba/Fe]}>1.0$dex) inthe LAMOST DR5 spectra analyzed by Xiang et al. (2019). They are almostexclusively main sequence and subgiant stars with $T_{\rm eff}\gtrsim6300$K.Their distribution in the Kiel diagram ($T_{\rm eff}$--$\log g$) traces a sharpborder at low temperatures along a roughly fixed-mass trajectory (around$1.4M_\odot)$ that corresponds to an upper limit in convective envelope massfraction of around $10^{-4}$. Most of these stars exhibit distinctly enhancedabundances of iron-peak elements (Cr, Mn, Fe, Ni) but depleted abundances of Mgand Ca. Rotational velocity measurements from GALAH DR2 show that the majorityof these stars rotate slower than typical stars in an equivalent temperaturerange. These characteristics suggest that they are related to the so-calledAm/Fm stars. Their abundance patterns are qualitatively consistent with thepredictions of stellar evolution models that incorporate radiativeacceleration, suggesting they are a consequence of stellar internal evolutionparticularly involving the competition between gravitational settling andradiative acceleration. These peculiar stars constitute 40% of the wholepopulation of stars with mass above 1.5$M_\odot$, affirming that "peculiar"photospheric abundances due to stellar evolution effects are a ubiquitousphenomenon for these intermediate-mass stars. This large sample of Ba-enhancedchemically peculiar A/F stars with individual element abundances provides thestatistics to test more stringently the mechanisms that alter the surfaceabundances in stars with radiative envelopes.

Chemically peculiar A and F stars with enhanced s-process and iron-peak elements: stellar radiative acceleration at work