2013MNRAS.432..769T

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 ( ∼ 2-16 d) 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 ( ∼ 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 Fei and Feii lines, we derived C, N, O and Na abundances from Ci 7111/7113/7115/7116/7119, Oi 6155-6158, Ni 8680/8683/8686 and Nai 6154/6161 lines by using the spectrum-synthesis fitting technique, while taking into account the non-local thermodynamic equilibrium effect. The resulting abundances of these elements for 12 programme stars turned out to show remarkably small star-to-star dispersions ( ≲ 0.1-0.2dex) without any significant dependence upon the pulsation period: near-solar Fe ([Fe/H] ∼ 0.0), moderately underabundant C ([C/H] ∼ -0.3), appreciably overabundant N ([N/H] ∼ +0.4-0.5) and mildly supersolar Na ([Na/H] ∼ +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 of the absolute magnitude, i.e. also of the stellar mass.