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Fig. 14

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Underpopulation of the hydrogen n = 4 level (compared to n = 5) in the outer photospheres of late-type O-dwarfs with very thin winds (pure H/He model atmosphere). Left: departure coefficients for n = 1,2,4,5 (dashed-dotted, solid, dotted, dashed), accounting for all processes (radiative and collisional). Overplotted are the corresponding departure coefficients (red, blue, green, magenta) as resulting from a NLTE solution discarding the collisional processes. Note the underpopulation of n = 4 compared to n = 5 in the outer photosphere (responsible for the line core emission in Brα), which is no longer present in the wind. Right: departure coefficients of n = 2,4,5 (solid, dotted, dashed) in the outermost photosphere (corresponding to the region embraced by dotted lines on the left), as resulting from the complete NLTE solution and various approximations, the latter all without collisions: 2: nebula approximation (Case A); 3: as 2, but including excitation/induced deexcitation from resonance lines (roughly Case B); 4: as 3, but including ionization from excited states; 5: as 4, but including excitation/deexcitation from all lines with lower level n ≤ 3; 6: as 4, but including excitation/deexcitation from all lines with lower level n ≤ 5 (see text).

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