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The abundances of carbon, nitrogen, and oxygen in the atmospheres of Cepheid variables - Evidence for helium enrichment
Luck, R. E.; Lambert, D. L.
AA(Louisiana State University, Baton Rouge, La.)
Astrophysical Journal, Part 1, vol. 245, May 1, 1981, p. 1018-1034. Research supported by the Robert A. Welch Foundation (ApJ Homepage)
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NASA/STI Keywords:
Abundance, Cepheid Variables, Stellar Atmospheres, Stellar Evolution, Stellar Spectra, Supergiant Stars, Atmospheric Models, Carbon, Enrichment, Heavy Elements, Helium, Metallic Stars, Nitrogen, Oxygen, Red Giant Stars, Spectrum Analysis, Stellar Mass Ejection, Stellar Rotation, Stellar Spectrophotometry
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An analysis is presented of the abundances of carbon, nitrogen, oxygen and 14 heavier elements in the atmospheres of 14 Cepheid variables and five nonvariable supergiants. Model atmospheres and a spectrum synthesis code were used to derive CNO and metal abundances as well as effective temperatures, surface gravities, microturbulences and macroturbulences for the program stars on the basis of Reticon spectra obtained in 100-A spectral regions centered at 5380, 6300, 6385, 6560 and 8710 A. The CNO abundances obtained indicate a level of atmospheric mixing with CN-cycle and ON-cycle processed material beyond that predicted by standard calculations for both the Cepheids and nonvariable supergiants, particularly evident in a reduction of O abundances. A possible explanation for the mixing level is suggested in which core convection or meridional mixing induced by rapid rotation caused extensive mixing into the H-burning shell and ON processing zone when the Cepheids were main-sequence OB stars. The O depletion is noted to imply a significant He enrichment, corresponding to an envelope He mass fraction of up to 0.8 in the double-mode Cepheid TU Cas.

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