Period and light-curve fluctuations of the Kepler Cepheid V1154 Cygni

Derekas, A. and Szabó, Gy. M. and Berdnikov, L. and Szabó, Róbert and Smolec, R and Kiss, L. L. and Szabados, László and Chadid, M. and Evans, Nancy Remage and Kinemuchi, K. and Nemec, J.M. and Seader, S.E. and Smith, J.C. and Tenenbaum, P. (2012) Period and light-curve fluctuations of the Kepler Cepheid V1154 Cygni. Monthly Notices of the Royal Astronomical Society. ISSN 0035-8711

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Abstract

We present a detailed period analysis of the bright Cepheid-type variable star V1154 Cygni (V1154 Cyg; V = 9.1 mag, P ≈ 4.9 d) based on almost 600 d of continuous observations by the Kepler space telescope. The data reveal significant cycle-to-cycle fluctuations in the pulsation period, indicating that classical Cepheids may not be as accurate astrophysical clocks as commonly believed: regardless of the specific points used to determine the O − C values, the cycle lengths show a scatter of 0.015–0.02 d over 120 cycles covered by the observations. A very slight correlation between the individual Fourier parameters and the O − C values was found, suggesting that the O − C variations might be due to the instability of the light-curve shape. Random-fluctuation tests revealed a linear trend up to a cycle difference 15, but for long term, the period remains around the mean value. We compare the measurements with simulated light curves that were constructed to mimic V1154 Cyg as a perfect pulsator modulated only by the light travel time effect caused by low-mass companions. We show that the observed period jitter in V1154 Cyg represents a serious limitation in the search for binary companions. While the Kepler data are accurate enough to allow the detection of planetary bodies in close orbits around a Cepheid, the astrophysical noise can easily hide the signal of the light-time effect.

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