Table 3: Fe I and Fe II abundances for the reference stars and the Sun for different assumptions concerning the efficiency of hydrogen collisions. Differential abundances [Fe/H] $_{\rm diff}$ are given, except in the case of the Sun where log  $\varepsilon $(Fe) is specified. $\sigma $ refers to the line-to-line scatter which is a factor $\sqrt n$ larger than the error of the mean. Note that the scatter in [Fe II/H] $_{\rm diff}$ is significantly reduced. Like in the case of HD 140283 discussed above, a strong imbalance between Fe I and Fe II results for all metal-poor stars if $S_{\rm H}$ = 0 is assumed. Instead, the ionization equilibrium points towards $S_{\rm H}$ = 3. Procyon cannot be fitted by any model.
Object $S_{\rm H}$ = 0 $S_{\rm H}$ = 3 LTE # of LTE # of
  [Fe I/H] $\pm $ $\sigma $ [Fe I/H] $\pm $ $\sigma $ [Fe I/H] $\pm $ $\sigma $ lines [Fe II/H] $\pm $ $\sigma $ lines
HD 103095 -1.21 $\pm $ 0.03 -1.35 $\pm $ 0.04 -1.33 $\pm $ 0.04 41 -1.36 $\pm $ 0.03 15
HD19445 -1.67 $\pm $ 0.07 -2.07 $\pm $ 0.08 -2.07 $\pm $ 0.08 58 -2.08 $\pm $ 0.05 16
HD 140283 -2.00 $\pm $ 0.07 -2.43 $\pm $ 0.09 -2.46 $\pm $ 0.09 62 -2.43 $\pm $ 0.05 13
HD 84937 -1.78 $\pm $ 0.07 -2.15 $\pm $ 0.09 -2.18 $\pm $ 0.09 61 -2.16 $\pm $ 0.05 16
Procyon -0.08 $\pm $ 0.06 -0.10 $\pm $ 0.08 -0.11 $\pm $ 0.07 52 -0.03 $\pm $ 0.04 23
Sun 7.54 $\pm $ 0.08 7.49 $\pm $ 0.08 7.46 $\pm $ 0.07 96 7.53 $\pm $ 0.10 37


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