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Abstract

An extensive non–LTE abundance analysis based on Na i 5890/5896 doublet lines was carried out for a large unbiased sample of |$\sim $| 120 A-type main-sequence stars (including 23 Hyades stars) covering a wide |$v_{\rm e}$|sin |$i$| range of |$\sim $| 10–300 km s|$^{-1}$|⁠, with an aim to examine whether the Na abundances in such A dwarfs can be reliably established from these strong Na|$\;$|i D lines. The resulting abundances ([Na|$/$|H]|$_{58}$|⁠), which were obtained by applying the |$T_{\rm eff}$|-dependent microturbulent velocities of |$\xi \sim$| 2–4 km s|$^{-1}$| with a peak at |$T_{\rm eff}$||$\sim $| 8000 K (typical for A stars), turned out to be generally negative with a large diversity (from |$\sim-$|1 to |$\sim $|0), while showing a sign of |$v_{\rm e}$|sin |$i$|-dependence (decreasing toward higher rotation). However, the reality of this apparently subsolar trend is very questionable, since these [Na|$/$|H]|$_{58}$| are systematically lower by |$\sim $|0.3–0.6 dex than more reliable [Na|$/$|H]|$_{61}$| (derived from weak Na|$\;$|I 6154/6161 lines for sharp-line stars). Considering the large |$\xi $|-sensitivity of the abundances derived from these saturated Na|$\;$|I D lines, we regard that [Na|$/$|H]|$_{58}$| must have been erroneously underestimated, suspecting that the conventional |$\xi $| values are improperly too large, at least for such strong high-forming Na|$\;$|I 5890/5896 lines, presumably due to the depth-dependence of |$\xi $| decreasing with height. The nature of the atmospheric turbulent velocity field in mid-to-late A stars needs to be further investigated before we can determine reliable sodium abundances from these strong resonance D lines.

© 2009 Astronomical Society of Japan
Subject
stars: abundances stars: atmospheres stars: chemically peculiar stars: early-type stars: rotation
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