Can Sodium Abundances of A-Type Stars Be Reliably Determined from Na I 5890/5896 Lines?

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An extensive non-LTE abundance analysis based on Na I 5890/5896 doublet lines was carried out for a large unbiased sample of 120 A-type main-sequence stars (including 23 Hyades stars) covering a wide vₑ sin i range of 10--300 km/s, 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 Tₑff-dependent microturbulent velocities of ξ 2--4 km/s with a peak at Tₑff 8000 K (typical for A stars), turned out generally negative with a large diversity (from -1 to 0), while showing a sign of vₑ 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 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 ξ-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 ξvalues are improperly too large at least for such strong high-forming Na I 5890/5896 lines, presumably due to the depth-dependence of ξdecreasing with height. The nature of atmospheric turbulent velocity field in mid-to-late A stars would have to be more investigated before we can determine reliable sodium abundances from these strong resonance D lines.