Formation of Al II lines and photospheric aluminium abundances in B-type stars

The aluminium abundances of early-to-late B-type main-sequence stars in the effective temperature range of 10000 K < T eff < 22000 K (comprising normal stars as well as chemically peculiar HgMn stars) were spectroscopically determined, with an aim of getting information about the galactic gas composition at the time of their formation from their photospheric abundances.For this purpose, two Al ii lines at 6243 and 4663 were employed, for which the non-LTE effect was taken into account based on detailed statisticalequilibrium calculations.The non-LTE effect of these Al ii lines generally acts in the direction of weakening (i.e., profile becomes shallower) caused by a decrease of line opacity (due to overionization) along with an enhanced line source function (overexcitation), and this effect tends to become progressively larger with an increase in T eff as well as with a decrease in log g (surface gravity).Regarding the Al ii 6243 line, while the non-LTE calculation qualitatively reproduces its overall behavior (e.g., transition from absorption to emission at early B-type), some T eff -dependent systematic trend remains unremoved in the non-LTE abundances of normal stars, which means that non-LTE corrections evaluated for this line are quantitatively insufficient.Meanwhile, for the case of the Al ii 4663 line, which is more advantageous than the 6243 line in the sense that it is stronger without showing any emission, the resulting non-LTE abundances of ordinary B stars are almost constant at the solar abundance (A 6.5) over the wide T eff range ( 10000-20000 K), suggesting that the abundances derived from this line are successfully non-LTE-corrected and trustable.Therefore, according to the results from the Al ii 4663 line, we may conclude that the Al abundance of the galactic gas in the recent past (several times 10 7 -10 8 yr ago) is almost consistent with the solar composition.As to the Al abundances of HgMn stars (T eff < 15000 K), our analysis confirmed that this element is conspicuously deficient (by 0.5-2 dex in comparison with the Sun) in the photosphere of these chemically peculiar stars, as already reported in previous studies.