CNO Abundances in A-supergiants:\\A Test of their Evolutionary Status

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Session 36 -- Giant and Supergiant Stars
Oral presentation, Monday, 9, 1995, 2:00pm - 3:30pm

[36.02D] CNO Abundances in A-supergiants:\\A Test of their Evolutionary Status

Kim A. Venn (U Sternwarte-Muenchen, MPIA, U Texas-Austin)

Various evolution scenarios predict vastly different histories for the 10-20 M$_\odot$ stars. To provide a new observational constraint of these theories, we have calculated the elemental abundances in 22 intermediate-mass, Galactic, A-supergiants, with special attention to the abundances of carbon and nitrogen which can be effected by evolutionary changes.

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The {\sl metal} abundances, calculated assuming LTE, are solar to within $\pm$0.2~dex. Detailed NLTE line formation calculations have been carried out for nitrogen and carbon. For carbon, NLTE corrections (= log~$\epsilon$(X)$_{NLTE}$$-$log~$\epsilon$(X)$_{LTE}$) are significant, ranging from $-$0.15 in the F0 supergiants to $-$0.5 in the A3 supergiants; with log~$\epsilon$(C/H)$_{NLTE}$ = 8.21~$\pm$0.11 for 14 A3-F0 supergiants. For nitrogen, we have constructed a new, detailed model atom (Lemke \& Venn, 1994). NLTE corrections are quite large even for {\sl weak} spectral lines, ranging from $-$1.0 in the A0 supergiants to $-$0.3 in the F0 supergiants; with log~$\epsilon$(N/H)$_{NLTE}$ = 8.06~$\pm$0.18 for 22 A0-F0 supergiants. The NLTE nitrogen abundances eliminate a {\sl very} strong dependence of the LTE abundances on the effective temperature.

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When the NLTE nitrogen and carbon abundances of the A3-F0 supergiants are compared to those of the main-sequence B-stars, we find (log~$\epsilon$(N/C)$_{A-supergiants}-$ log~$\epsilon$(N/C)$_{B-dwarfs}$)= +0.33 $\pm$0.24. This value is significantly less than the first dredge-up abundances predicted by several evolution scenarios. However, the non-zero N/C ratio suggests that the A-supergiants have undergone some {\sl partial} mixing of CN-cycled gas. This is similar to recent abundance results for some B-type supergiants (Gies \& Lambert 1992, Lennon 1994), suggesting that partial mixing may occur near the main-sequence (possibly by turbulent diffusive mixing as discussed by Maeder, 1987, and Denissenkov, 1993). We conclude that the intermediate-mass, A-supergiants in the Galaxy have evolved {\sl directly} from the main-sequence.

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