1996A&A...309..558L

We investigate the NLTE formation of nitrogen lines in Vega and present a new estimate for the nitrogen abundance based on detailed spectrum synthesis. The NLTE problem is very complex such that our final estimate of logε(N)=7.6 is very uncertain. The statistical equilibrium of nitrogen is almost completely controlled by the UV flux around 1200A, which is sensitive to both the carbon abundance and the shape of Lyman α. We found that Ly α must be included when computing bound-bound rates, a fact not realized before. The sensitivity of the NLTE effect to the global metallicity found by Takeda (1992a) is entirely due to the corresponding change of the carbon abundance. The inclusion of Ly α reduces this sensitivity considerably. Most accessible Ni lines are in the wings of broad Paschen lines which cause the NLTE abundance corrections to be different for lines of the same multiplet that occur at different distances from the Paschen line center. The nitrogen statistical equilibrium is also slightly sensitive to the collisional excitation cross-sections. For our best estimate, the NLTE corrections are considerable, typically {DELTA}[N/H]=-0.4. An LTE sulphur abundance of logε(S)=6.93 (or [S/H]=-0.3) has also been determined. Considering all of the available abundances for Vega, and their likely uncertainties, we find that Vega continues to look like a mild Lambda Bootis star.