CHEMICAL ABUNDANCES AND THE EVOLUTIONARY STATUS OF 22 GALACTIC A-TYPE SUPERGIANTS

Abstract

ABSTRACT The A-type supergiants have an interesting location on the HR-diagram for testing stellar evolution theories since various evolution scenarios describe vastly different histories for these stars. In particular, the predicted abundances of carbon and nitrogen in the stellar atmospheres differ significantly since these stars may have evolved directly from the main-sequence or may be returning from the red giant branch where they would undergo the first dredge-up of CN-cycled H-burned gas from the stellar interior. This thesis provides new elemental abundances for a large group of Galactic A-type supergiants in the 5 to 20 solar mass range, while also addressing the difficulties in determining reliable abundances in these stars. The atmospheric analysis of each star was performed using the most recent Kurucz LTE model atmospheres. Atmospheric parameters (Teff and log g) have been determined from ionization equilibrium of weak Mg I and Mg II lines and fitting the wings of the H-gamma line profiles; the final parameters were chosen at the intersection of the loci of possible Teff-gravity values from each indicator. Calculations show that NLTE effects on the weak Mg I and Mg II spectral lines used in this analysis are small, therefore they are ideal atmospheric parameter indicators. The metal abundances, log epsilon (O, Mg, Si, Ca, Sc, Ti, Cr, Fe, Ni) are solar to within about ±0.2~dex, calculated assuming LTE. Overabundances of Na are found, which are discussed as a combination of possible NLTE effects and/or pollution of newly synthesized Na from a NeNa proton capture reaction that could occur in the stellar core. Otherwise, we see no evidence of slight overall metal enrichments in these young stars that might be expected due to Galactic chemical evolution. More details of the atmospheric and LTE metal abundance analyses can be found in Venn 1994 (ApJS, 99, 659). The carbon and nitrogen abundances are examined in order to study the evolutionary status of these stars. LTE abundances show that nitrogen is enriched and carbon depleted, as expected after the first dredge-up, however the quantities are much greater than predicted and there is a very strong trend in the nitrogen abundances with Teff. New NLTE line formation calculations have been carried out for nitrogen and carbon to investigate these results (details in Venn 1994 ApJ, 449, 839). For nitrogen, it was necessary to construct a new and detailed N I/II model atom (which was calibrated with respect to Vega, see Lemke & Venn 1995, A&A, in press). Application to the A-supergiants shows that departures from LTE strongly affect the nitrogen abundances, even for weak lines. NLTE corrections (=log (X)NLTE - log epsilonXLTE) for the weak lines used in this analysis range from -1.0 dex in the A0 supergiants to -0.3 in the F0 supergiants. The NLTE corrections remove the strong trend with effective temperature seen in the LTE nitrogen abundances. The average NLTE abundance is log epsilon (N)NLTE = 8.05 ± 0.19 for the 22 A0-F0 supergiants analyzed. For carbon, the Sturenburg & Holweger (1990, A&A, 237, 125) model atom was adopted and tested extensively. Again, the NLTE corrections are significant, ranging from -0.1 in the F0 stars to -0.5 in the A3 stars. For the hotter stars, we show that the only observable C I lines, near 9100A, do not yield reliable elemental abundances. The mean NLTE carbon abundance is log epsilon (C)NLTE =8.14 ± 0.13 for the 14 A3-F0 supergiants. The A-supergiant NLTE abundances are compared to the mean carbon and nitrogen abundances in unevolved B-stars in the same mass range, which are not the same as solar ([C/H]B*= -0.35 and [N/H]B*= -0.21). We find log epsilon (N/C)A I - log epsilon (N/C)B* =+0.38 ± 0.26. This value is significantly less than the first dredge-up ratio (~0.65 for 10 solar mass stars and ~0.98 for 20 solar mass stars) predicted by several evolution scenarios, including the popular Geneva tracks. However, the non-zero [N/C] ratio suggests that the A-type supergiants have undergone some partial mixing of CN-cycled gas, which is not currently predicted by Galactic metallicity evolution tracks. Therefore, Galactic A-type supergiants are not evolving from the red giant branch - they have evolved directly from the main-sequence, but have undergone some mixing of CN-cycled gas not currently predicted by stellar evolution theory.