Ariane Schad, Wiebke Herzberg, and Markus Roth

Solar-like oscillations exhibit a regular pattern of frequencies. This pattern is dominated by the small and large frequency separations between modes. The accurate determination of these parameters is of great interest, because they give information about e.g. the evolutionary state and the mass of a star. Here, we present a robust method to determine the large and small frequency separations for time series with low signal-to-noise ratio. For this purpose, we analyse a time series of the star KIC 5184732 from the NASA Kepler satellite by employing a combination of Fourier and Hilbert transform. We use the analytic signal of the filtered time series to compute the signal envelope. Spectral analysis of the signal envelope then reveals frequency differences of dominant modes in the periodogram of the stellar time series. With this method, the large frequency separation $\Delta\nu$ can be extracted from the envelope spectrum even for data of poor signal-to-noise ratio. A modification of the method allows for an overview of regularities in a periodogram.

Rudi Komm, Frank Hill

The properties of solar acoustic oscillations are known to vary with the solar cycle. With 20 years of continuous observations by GONG, we are now in the position to carry out a comparison of the variation of the p mode parameters between the last two solar cycles. We present results of our analyses of the widths, amplitudes, areas (width $\times$ amplitude), and background amplitudes of solar p modes, as well as the corresponding physical quantities for modes of harmonic degree $10\leq l \leq 150$ of the global solar oscillations. We investigate the variation of these parameters for different ranges of mode frequency and harmonic degree. The fractional change in mode widths is largest for modes in the frequency range 2400-3300 $\mu$Hz, regardless of harmonic degree, with an amplitude of $\sim 10\%$ between solar minimum and maximum. The fractional change in mode amplitude exhibits a peak-to-peak amplitude of $\sim 30\%$ over solar cycle 23. Also, we find hysteresis in mode amplitudes over cycle 23 for some combinations of mode frequencies and harmonic degree. The physical quantities which correspond to the mode parameters (mean square velocity power, total mode energy, mode energy supply rate) are investigated as a function of time for different ranges of harmonic degree. We find that the mode energy supply rate decreased by $\sim 20\%$ over the last 20 years.

Ariane Schad, and Markus Roth

Several hundred stars were observed in the short cadence mode of the Kepler satellite during the nominal mission phase. This generated a large pool of data which can provide insight into the characteristics of stellar activity cycles of solar-type stars through the methods of asteroseismology. From helioseismology it is known that the frequencies of solar acoustic oscillations (p modes) are positively correlated with the solar magnetic activity cycle. Evidence for a similar behaviour in the p modes of a star, which was observed by the CoRoT satellite, was provided by [1]. This showed that it is feasible to trace activity cycles of stars in their p mode frequencies. We analyse the Kepler time series of a set of solar-type stars with the aim to find signatures of stellar magnetic activity. We divide the time series of each star into shorter sub-series in order to analyse the temporal evolution of the p mode frequencies. The sub-series’ periodograms are cross-correlated to retrieve the shift of p mode frequencies over time. The errors on the shifts are computed by a resampling approach of the periodogram. We find significant frequency shifts, indicating stellar magnetic activity, for all stars we investigated. For the most prominent example, KIC 8006161, we find that, not unlike in the solar case, frequency shifts are smallest for the lowest and largest for the highest p mode frequencies.