A SEARCH FOR EVIDENCE OF THE NEON-SODIUM CYCLE IN THE CEPHEID X CYGNI

We have determined abundances of sodium, aluminum, silicon, calcium, and iron from 16 red spectra of 10 Å mm⁻¹ of X Cyg to search for a sodium enhancement that might be expected from partial operation of the neon-sodium hydrogen burning cycle. Such burning is possible if the low oxygen and very high nitrogen abundances in X Cyg and other supergiants reported by Luck and Lambert (1981) are correct. We find no significant enhancement of sodium indicating that insufficient material that may have burned hydrogen at temperatures in the 30-50 × ⁶6 K range has reached the surface. Hence the composition appears to have been altered by material that has been subject to at least 20 × 10² Ê but not 30 + 10⁶ K. The requirement that composition be independent of phase assists in establishing the range of temperature variation in the 16-day cepheid X Cyg.