Validity of the composite atom model. We show the population level ratio of the lower energy state of the Ca IIλ854.2 nm transition between ⁴⁰Ca and ⁴⁴Ca normalized by the ratio at z = 0 (see text). ⁴⁰Ca and ⁴⁴Ca are the two most abundant Ca isotopes. The orange curve indicates the formation height of the intensity minimum of the Ca IIλ854.2 nm line. The shown data are from the IM computation of snapshotmuram_en_499000_379s. The light yellow background indicates where no data are plotted.