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Fig. 8

image

Same as Fig.6. The 1300 K thermodynamic equilibrium distribution corresponding to the lower energy levels is shown by a straight dashed line. The disagreement between this line and the observations related to the higher energy levels shows that indeed LTE is not reached for all levels. Table 7 explains at least qualitatively why such a situation is encountered, the two higher groups of energy levels are increasingly sensitive to radiation from the stellar photospheric flux, as shown by the two vertical jumps underlined by the two vertical arrows. Conversely, the lowest levels have five well-aligned levels and must be less sensitive to radiation and thus are mostly populated by another mechanism, likely collisions (see text).

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