Abstract:
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 AlI
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
AlI/AlII 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 AlI 3944/3961 lines
for 63 A-type dwarfs (7000 ≲ Teff ≲ 10000 K) of
comparatively lower rotational velocities (vesin i ≲
100 km s-1). The following results were obtained.
(1) The non-LTE corrections (Δ) are positive (reflecting the
importance of overionization) and significantly large (0.3 ≲ Δ ≲
1.0 dex depending on Teff; generally
Δ3944 < Δ3961). (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).
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