N III lambdalambda4640, 4097 lines in Of stars

Detailed calculations based on non-LTE plane-parallel model atmospheres show that the N iii emission lines at XX4634, 4640, 4641 observed in Of stars are produced primarliy by dielectronic recombination to 3d 3D followed by the 3d-3p transition in a compact atmosphere. The 3P state in turn is drained by two-electron transitions to the 2p2 levels. We find that X4640 is in emission for Teff < 37,0000 K for main-sequence objects (55,0000 K for giants), while X4097 remains strongly in absorption, in accordance with observations. The calculated equivalent widths are in substantial agreement with the observed values for those Of stars with He II X4686 in absorption, i.e., for those objects designated by Walborn as O((f)).