Several additional paragraphs have been added near the figures to help explain the plots appearing in these figures. For the stationary atom case, the limits on y_{m0} are determined from inequality (106) and it is indeed possible for y_{m0} to be less than y_{min}. The manner in which we arrived at the regions of validity of the approximations is now described in more detail.
Authors’ response We would like to thank the referee for his/her careful reading of the manuscript and the helpful comments.