AR Aur A+B is a close binary of astrophysical interest because dissimilar surface compositions are reported between similar late B-type dwarfs. A new spectroscopic study on this system was carried out based on the disentangled spectra, in order to determine their atmospheric parameters and elemental abundances. The effective temperature and microturbulence (determined from the equivalent widths of Fe ii lines) turned out to be (11,150 K, 0.9 km s−1) and (10,650 K, 0.1 km s−1) for A and B, respectively. The chemical abundances of 28 elements were then derived while taking into account the non-LTE effect for Z ≤ 15 elements (Z: atomic number). The following trends were elucidated for [X/H] (abundance of X relative to the Sun): (1) Qualitatively, [X/H] shows a rough global tendency to increase with Z, with the gradient steeper for A than for B. (2) However, considerable dispersion is involved for A, since prominently large peculiarities are seen in specific elements reflecting the characteristics of HgMn stars (e.g., very deficient N, Al, Sc, Ni; markedly overabundant P, Mn). (3) In contrast, the Z-dependence of [X/H] for B tends to be nearly linear with only a small dispersion. These observational facts may serve as a key to understanding the critical condition for the emergence of the chemical anomaly.

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