Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994aas...185.3602v&link_type=abstract
American Astronomical Society, 185th AAS Meeting, #35.06; Bulletin of the American Astronomical Society, Vol. 26, p.1366
Astronomy and Astrophysics
Astronomy
Scientific paper
Various evolution scenarios predict vastly different histories for the 10-20 M_sun 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. The metal abundances, calculated assuming LTE, are solar to within +/-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 +/-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 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 +/-0.18 for 22 A0-F0 supergiants. The NLTE nitrogen abundances eliminate a very strong dependence of the LTE abundances on the effective temperature. 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 +/-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 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 directly from the main-sequence.
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