THE INTERESTING PROBLEM OF THE NLTE NITROGEN ABUNDANCE IN VEGA

We investigate the NLTE formation of nitrogen lines in Vega and presen t a new estimate for the nitrogen abundance based on detailed spectrum synthesis. The NLTE problem is very complex such that our final estim ate of log epsilon(N) = 7.6 is very uncertain. The statistical equilib rium of nitrogen is almost completely controlled by the UV flux around 1200 Angstrom, which is sensitive to both the carbon abundance and th e shape of Lyman alpha. We found that Ly alpha must be included when c omputing bound-bound rates, a fact not realized before. The sensitivit y 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 alpha reduces this sensitivity considerably. Most accessible N I lines are in the wings of broad Paschen lines which cau se the NLTE abundance corrections to be different for lines of the sam e multiplet that occur at different distances from the Paschen line ce nter. The nitrogen statistical equilibrium is also slightly sensitive to the collisional excitation cross-sections. For our best estimate, t he NLTE corrections are considerable, typically Delta[N/H] = -0.4. An LTE sulphur abundance of log epsilon(S) = 6.93 (or [S/H] = -0.3) has a lso been determined. Considering all of the available abundances for V ega, and their likely uncertainties, we find that Vega continues to lo ok like a mild Lambda Bootis star.