Other
Scientific paper
Jan 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994phdt........10v&link_type=abstract
PhD Dissertation, Texas Univ. Austin, TX United States
Other
1
A Stars, Supergiant Stars, Chemical Evolution, Galactic Evolution, Atmospheric Models, Metallicity, Stellar Mass, Local Thermodynamic Equilibrium, H Gamma Line, Turbulent Mixing
Scientific paper
The chemical composition of 22 massive, Galactic, A-type supergiants is presented in order to study the evolutionary status of these stars. Various evolution scenarios describe vastly different histories for these stars, which can be distinguished by the atmospheric abundances that they predict. Chemical abundances are determined from observations of weak optical spectral lines gathered at the McDonald Observatory An atmospheric analysis is performed for each star adopting the most recent Kurucz LTE model atmospheres. Atmospheric parameters (Teff, log g, xi) are determined from Mg I/II equilibrium and fitting theoretical H-gamma line profiles; we chose the parameters where the loci of possible Teff-gravity values from each indicator intersect. We show that NLTE effects on the Mg I and Mg II abundances are small, but NLTE can strongly affect Fe I abundances such that Fe I/II equilibrium is a poor atmospheric indicator. The metal abundances are calculated assuming LTE. Most metal abundances are solar to within +/-0.2 dex. Overabundances of Na are found in the A-type supergiant atmospheres; we discuss this as combination of possible NLTE effects and/or pollution of newly synthesized Na from a NeNa proton capture reaction that could occur in the stellar cores. Otherwise, we see no evidence of slight overall abundance enrichments relative to solar in these young stars that might be expected due to Galactic chemical evolution. NLTE line formation calculations have been carried out for the nitrogen and carbon abundances. For carbon, we adopt the Sturenburg & Holweger (1992) model atom and extensively test the effects of the atomic data on the resultant NLTE corrections. We find significant NLTE corrections (=log epsilon(XNLTE - log epsilon(X)LTE) for the cooler supergiants that range from -0.15 in the F0 stars to -0.5 in the A3 stars. The mean NLTE carbon abundance is log epsilon(C/H)NLTE = 8.21 +/- 0.11 for the 14 A3-F0 supergiants. For the hotter stars, we show that the only C I lines that we have observed near 9100 A do not yield reliable elemental abundances. For nitrogen, we have constructed a new, detailed model atom. Application of this model to the A-type supergiants shows that departures from LTE strongly affect the nitrogen abundances. NLTE corrections for weak lines are quite large, ranging from -1.0 in the A0 supergiants to -0.3 in the F0 supergiants. The average NLTE nitrogen abundances is log epsilon(N/H)NLTE = 8.06 +/- 0.18 for the 22 A0-F0 supergiants. The NLTE nitrogen abundances do not show the strong dependence we observed in 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 I - log epsilon(N/C)B*) = +0.33 +/- 0.24. This value is significantly less than the first dredge-up abundances (approximately +0.65 for 10 solar mass stars predicted by several evolution scenarios. However, the N/C ratio suggests that the A-type supergiants have undergone some partial mixing of CN-cycled gas. This is similar to recent abundance results for some B-type supergiants, suggesting that partial mixing may occur near the main-sequence (possibly by turbulent diffusive mixing). We conclude that the 5 to 20 solar mass A-type supergiants in the Galaxy probably have evolved directly from the main-sequence.
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