Same as Fig. 10, but showing the sensitivity of the results to a change in the star formation efficiency f_star. If the carbon output from the stars is kept fixed, then a change in f_star translates to a proportional change in the metallicity, as shown in the upper right panel. However, the carbon yield from stars is uncertain, and thus for the other panels an increase of f_star by some factor may also be viewed as a decrease in the carbon yield by the same factor, while keeping the IGM metallicity fixed.