CNO ABUNDANCES AND THE EVOLUTIONARY STATUS OF 3 A-TYPE SUPERGIANTS

Abundance results are presented for three massive (approximately 10 M.), Population I A-type supergiants; HD 13476 (A3 Iab), HD 46300 (A0 Ib), and HD 87737 (A0 Ib). Equivalent widths of weak CNO and metal lines have been collected from high signal-to-noise CCD spectra. Atmospheric parameters (T(eff) and gravity) are determined from spectroscopic indicators since Stromgren colors are not well calibrated for these stars. Observed Hy profiles are compared to those generated from Kurucz's line-blanketed model atmospheres, and ionization equilibrium of Mg I/Mg II is used to choose the final model parameters. Ionization equilibrium of Fe I/Fe II is also calculated but not used explicitly since large non-LTE effects are predicted for Fe I lines. We find these A supergiants have roughly solar metal abundances (i.e., [M/H] = +0.1 to -0.3 for Fe, Mg, Ti, and Cr). There are large enrichments of nitrogen ([N/H]approximately + 0.8), accompanied by slight carbon and possibly oxygen depletions. These are consistent with the predictions for CNO-processed envelopes of post-red supergiant stars that have undergone the first dredge-up mixing event. In support of this, the sum [(C + N +, O)/Fe] = 0.0 to 0.1 for all three stars and the nitrogen enhancement appears correlated with the carbon and oxygen deficits as expected for CNO-cycled gas. However, the ratios [N/C] and [N/O] are larger than most first dredge-up predictions by 0.3-0.6 dex. This may indicate additional mixing of CNO-cycled gas, significant mixing of ON-cycled gas to drive up the nitrogen abundances, or that CNO-cycling alone is not responsible for the abundance pattern. Thus, the nitrogen enhancements indicate a complex chemical evolutionary history for these stars, or a systematic error in the classical analysis of N I lines in A supergiant atmospheres, for example, large non-LTE corrections.