An Analysis of Metallic-line Stars with Emission Lines Based on LAMOST

Am stars are chemically peculiar A-type stars with weak Ca II K and enhanced metal lines. Emission lines in Am stars are extremely rare and their origin poorly understood. Using LAMOST DR12, we identify emission-line components (H α , [S II ], [N II ]) in a subset of Am stars. To investigate this phenomenon, we first apply five classification methods to obtain 52,939 high-confidence Am candidates from A- and early F-type (F0–F2) stars (S/N g ≥ 50). After cross-matching with published catalogs and emission-line detection, we further isolate 340 emission-line Am stars. To uncover their origin, we cross-match with existing catalogs and use Gaia DR3 astrometric indicators (ruwe, astrometric excess noise, ipd_frac_multi_peak). Binary interaction is the dominant mechanism: 119 sources are identified as binaries and 60 as variable stars. We systematically analyze the color–magnitude, spatial, and atmospheric parameter ( T eff , log g , [Fe/H]) distributions of these emission-line Am stars. They overlap well with normal Am stars on the main sequence, concentrate toward the Galactic plane (∣ b ∣ ≤ 30 ∘ ), and show no significant offsets in T eff , log g , or [Fe/H]. These results provide a first observational characterization of this rare subpopulation and establish binary interaction as the primary driver of emission lines in Am stars.