Density sensitive C II lines in cool stars of low gravity

It is shown that the relative intensities of emission lines within the multiplet uv 0.01 of C II, around 2325 Å, are sensitive to electron density in the range $$10^9 \gt N_\text e \gt 10^7 \text{cm}^{-3}$$. The lines therefore offer a valuable method for measuring N_e in the chromospheres of late-type giants and supergiants. Calculated line ratios are compared with those observed in a range of objects.