HELIUM CHEMISTRY OF ONC SUPERGIANTS

We present high-resolution, high signal-to-noise spectroscopic observa tions of the Galactic late-O supergiants HD 154811 (OC9.7 Iab), HD 154 368 (O9.5 Iab), and HD 123008 (ON9.5 Iab). The spectra of these stars exemplify the carbon and nitrogen line-strength anomalies characterist ic of the ONC supergiants. A hydrogen/helium non-LTE model-atmosphere analysis is carried out using extensive model atoms. Helium abundances (by number) of 0.09, 0.13, and 0.17 are obtained for the OC, normal O , and ON stars respectively. These results are consistent with Walborn 's hypothesis that OC supergiants have a normal (i.e., cosmic) helium abundance whereas the morphologically-normal O supergiants and, to a g reater extent, the ON supergiants exhibit the products of nucleosynthe tic processing at their surfaces. We note that the diagnostic line He II lambda 4200 is probably contaminated by a N III line in late-O supe rgiants.