2012A&A...541A..51K

It was recently discovered that the period spacings of mixed pressure/gravity dipole modes in red giants permit a distinction between the otherwise unknown evolutionary stage of these stars. The Kepler space mission is reaching continuous observing times long enough to also start studying the fine structure of the observed pressure-mode spectra. In this paper, we aim to study the signature of stellar evolution on the radial and pressure-dominated l=2 modes in an ensemble of red giants that show solar-type oscillations. We use established methods to automatically identify the mode degree of l=0 and 2 modes and measure the large (Δν_c) and small (δν₀₂) frequency separation around the central radial mode. We then determine the phase shift ε_c of the central radial mode, i.e. the linear offset in the asymptotic fit to the acoustic modes. Furthermore we measure the individual frequencies of radial modes and investigate their average curvature. We find that ε_c is significantly different for red giants at a given Δν_c but which burn only H in a shell (RGB) than those that have already ignited core He burning. Even though not directly probing the stellar core the pair of local seismic observables (Δν_c, ε_c) can be used as an evolutionary stage discriminator that turned out to be as reliable as the period spacing of the mixed dipole modes. We find a tight correlation between ε_c and Δν_c for RGB stars and unlike less evolved stars we find no indication that ε_c depends on other properties of the star. It appears that the difference in ε_c between the two populations becomes smaller and eventually indistinguishable if we use an average of several radial orders, instead of a local, i.e. only around the central radial mode, large separation to determine the phase shift. This indicates that the information on the evolutionary stage is encoded locally, more precisely in the shape of the radial mode sequence. This shape turns out to be approximately symmetric around the central radial mode for RGB stars but asymmetric for core He burning stars. We computed radial mode frequencies for a sequence of red-giant models and find them to qualitatively confirm our findings. We also find that, at least in our models, the local Δν is an at least as good and mostly better proxy for both the asymptotic spacing and the large separation scaled from the model density than the average Δν. Finally, we investigate the signature of the evolutionary stage on δν₀₂ and quantify the mass dependency of this seismic parameter.