  Although anomalous surface abundances are often observed in A-type
main-sequence stars (known as chemically peculiar stars; e.g., metallic line
stars or Am stars), our understanding about the behavior of aluminium is still
insufficient. Actually, even whether Al is overabundant or underabundant in Am
stars is not clarified. This is presumably because most of the previous studies
employed the Al I 3944/3961 lines with the assumption of LTE, despite that a
considerable non-LTE effect is expected in this resonance doublet. With an aim
to shed light on this issue, extensive statistical-equilibrium calculations on
Al I/Al II were carried out for a wide range of atmospheric parameters, based
on which the non-LTE Al abundances were determined by applying the
spectrum-fitting technique to the Al I 3944/3961 lines for 63 A-type dwarfs
(7000 < Teff < 10000 K) of comparatively lower rotational velocities (vsini <
100 km/s). The following results were obtained. (1) The non-LTE corrections
(Delta) are positive (reflecting the importance of overionization) and
significantly large (0.3 < Delta < 1.0 dex depending on Teff; generally
Delta_3944 < Delta_3961). (2) By applying these corrections (and indispensable
inclusion of Balmer line wings as background opacity), consistent non-LTE
abundances for both lines could be obtained, and the serious zero-point
discrepancy (considerably negative [Al/H] for normal metallicity stars of
[Fe/H]~ 0) found in old studies has been settled. (3) Al abundances of A-type
stars are almost in proportion to [Fe/H] (tending to be overabundant in Am
stars) with an approximate relation of [Al/H]~1.2[Fe/H]. which is qualitatively
consistent with the prediction of the diffusion theory (suggesting an Al excess
in the photosphere of Am stars).
