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

TL;DR

This study investigates whether Na I 5890/5896 lines can reliably determine sodium abundances in A-type stars, revealing significant underestimation issues due to microturbulence effects and emphasizing the need for improved atmospheric models.

Contribution

It provides a detailed non-LTE analysis of Na I lines in A-type stars, highlighting the limitations of current microturbulence assumptions for strong resonance lines.

Findings

Na abundances from Na I D lines are systematically underestimated.

Abundance estimates are sensitive to microturbulence, especially for saturated lines.

Reliable sodium abundances require better understanding of atmospheric turbulence.

Abstract

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 v_e 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 T_eff-dependent microturbulent velocities of \xi ~2--4 km/s with a peak at T_eff ~ 8000 K (typical for A stars), turned out generally negative with a large diversity (from ~-1 to ~0), while showing a sign of v_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 ~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 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.