We present \gtrsim 15 , 000 metal-rich ( { [ Fe / H ] } > -0.2 dex ) A and F stars whose surface abundances deviate strongly from Solar abundance ratios and can not 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 \citet Xiang2019 .
They are almost exclusively main sequence and subgiant stars with T _ { eff } \gtrsim 6300 K. Their distribution in the Kiel diagram ( T _ { eff } – \log g ) traces a sharp border at low temperatures along a roughly fixed-mass trajectory ( around 1.4 M _ { \odot } ) 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 M _ { \odot } , 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 envelopes .