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Recent observational results have demonstrated an increase in the surface Naabundance that correlates with stellar mass for red giants between 2 and 3Msun. This trend supports evolutionary mixing processes as the explanation forNa overabundances seen in some red giants. In this same mass range, the surfaceAl abundance was shown to be constant. Our main aim was to extend theinvestigation of the Na and Al surface abundances to giants more massive than 3Msun. We sought to establish accurately whether the Na abundances keepincreasing with stellar mass or a plateau is reached. In addition, weinvestigated whether mixing can affect the surface abundance of Al in giantsmore massive than 3 Msun. We obtained new high-resolution spectra of 20 giantsin the field of 10 open clusters; 17 of these stars were found to be members of9 clusters. The giants have masses between 2.5 Msun and 5.6 Msun. Abundances ofC, N, and O were determined using spectrum synthesis. The abundances of Na andAl were corrected for non-local thermodynamic equilibrium effects (non-LTE).Moreover, to extend the mass range of our sample, we collected from theliterature high-quality C, N, O, and Na abundances of 32 Galactic Cepheids withaccurate masses in the range between 3 Msun and 14 Msun. The surface abundancesof C, N, O, Na, and Al were compared to predictions of stellar evolution modelswith and without the inclusion of rotation-induced mixing. The surfaceabundances of most giants and Cepheids of the sample can be explained by modelswithout rotation. For giants above ~ 2.5 Msun, the Na abundances reach aplateau level of about [Na/Fe] ~ 0.20-0.25 dex (in non-LTE). Our resultssupport previous works that found models with rotation to overestimate themixing effects in intermediate-mass stars. [abridged]

astronomy and astrophysics

Deep secrets of intermediate-mass giants and supergiants: Models with  rotation seem to overestimate mixing effects on the surface abundances of C,  N, and Na