Testing the Breakdown of the Asteroseismic Scaling Relations in Luminous Red Giants

Nearly all cool, evolved stars are solar-like oscillators, and fundamental stellar properties can be inferred from these oscillations with asteroseismology. Scaling relations are commonly used to relate global asteroseismic properties-the frequency of maximum power nu max and the large-frequency separation Delta nu-to stellar properties. Mass, radius, and age can then be inferred with the addition of stellar spectroscopy. There is excellent agreement between seismic radii and fundamental data on the lower red giant branch and red clump. However, the scaling relations appear to breakdown in luminous red-giant stars. We attempt to constrain the contributions of the asteroseismic parameters to the observed breakdown. We test the nu max and Delta nu scaling relations separately, by using stars of known mass and radius in star clusters and the Milky Way's high-alpha sequence. We find evidence that the Delta nu scaling relation contributes to the observed breakdown in luminous giants more than the nu max relation. We test different methods of mapping the observed Delta nu to the mean density via a correction factor, F Delta nu, and find a approximate to 1%-3% difference in the radii in the luminous giant regime, depending on the technique used to measure F Delta nu. The differences between the radii inferred by these two techniques are too small on the luminous giant branch to account for the inflated seismic radii observed in evolved giant stars. Finally, we find that the F Delta nu correction is insensitive to the adopted mixing length, chosen by calibrating the models to observations of Teff .