Oscillations in G-Type Giants

Abstract

Precise radial-velocity measurements of 4 G-type giants, 11 Com, $\zeta$ Hya, $\epsilon$ Tau, and $\eta$ Her, were carried out. Short-term variations with amplitudes of 1–7 m s$^{-1}$ and periods of 3–10 hr were detected. A period analysis shows that the individual power distribution has a Gaussian shape, and that their peak frequencies ($\nu_{\rm max}$) are in a good agreement with a prediction by the scaling law. With using a pre-whitening procedure, significant frequency peaks of more than 3$\ \sigma$ were extracted for these giants. From these peaks, we determined the large frequency separation by constructing the highest peak distribution of the collapsed power spectrum, which is also in good agreement with what the scaling law for the large separation predicts. Echelle diagrams of the oscillation frequency were created based on the extracted large separations, which is very useful to clarify the properties of the oscillation modes. In these echelle diagrams, odd-even mode sequences are clearly seen. Therefore, it is certain that in these G-type giants, non-radial modes are detected in addition to the radial mode. As a consequence, these properties of oscillation modes are shown to follow what Dziembowski et al. (2001, MNRAS, 328, 601) and Dupret et al. (2009, A&A, 506, 57) theoretically predicted. The damping times for these giants were estimated with the same method as that developed by Stello et al. (2004, Sol. Phys., 220, 207). The relation of the $Q$-value (ratio of damping time to period) to the period was discussed by adding data of other stars, ranging from dwarfs to giants.