With an aim of investigating the nature of evolution-induced mixing in the envelope of evolved intermediate-mass stars , we carried out an extensive spectroscopic study for 12 Cepheid variables of various pulsation periods ( \sim 2–16 days ) to determine the photospheric abundances of C , N , O , and Na , which are the key elements for investigating how the H-burning products are salvaged from the interior , based on 122 high-dispersion echelle spectra ( \sim 10 per target ) of wide wavelength coverage collected at Bohyunsan Astronomical Observatory .
Having established the relevant atmospheric parameters corresponding to each phase spectroscopically from the equivalent widths of Fe i and Fe ii lines , we derived C , N , O , and Na abundances from C i 7111/7113/7115/7116/7119 , O i 6155–8 , N i 8680/8683/8686 , and Na i 6154/6161 lines by using the spectrum-synthesis fitting technique , while taking into account the non-LTE effect .
The resulting abundances of these elements for 12 program stars turned out to show remarkably small star-to-star dispersions ( \la 0.1–0.2dex ) without any significant dependence upon the pulsation period : near-solar Fe ( [ Fe/H ] \sim 0.0 ) , moderately underabundant C ( [ C/H ] \sim - 0.3 ) , appreciably overabundant N ( [ N/H ] \sim +0.4–0.5 ) , and mildly supersolar Na ( [ Na/H ] \sim + 0.2 ) .
We conclude the following implications from these observational facts : ( 1 ) These CNO abundance trends can be interpreted mainly as due to the canonical dredge-up of CN-cycled material , while any significant non-canonical deep mixing of ON-cycled gas is ruled out ( though only a slight mixing may still be possible ) .
( 2 ) The mild but definite overabundance of Na suggests that the NeNa-cycle product is also dredged up .
( 3 ) The extent of mixing-induced peculiarities in the envelope of Cepheid variables is essentially independent on the absolute magnitude ; i.e. , also on the stellar mass .