Fig. 10. Variation of Na I level departure coefficients for stars with different parameters. (Top row ): typical subdwarf with = 5200 K, = 4.50, [Fe/H] = 0.0 (left ); subgiant with = 5500 K, = 3.50, [Fe/H] = 0.0 (middle ); hot turnoff star with = 6500 K, = 4.00, [Fe/H] = 0.0 (right ). (Bottom row ): as above, but [Fe/H] = -2.