Period and light curve fluctuations of the Kepler Cepheid V1154 Cyg

TL;DR

This study analyzes Kepler data of the Cepheid V1154 Cyg, revealing significant cycle-to-cycle period fluctuations and light curve instabilities, which challenge the assumption of Cepheids as precise astrophysical clocks and impact binary detection.

Contribution

It provides the first detailed analysis of period fluctuations in V1154 Cyg using Kepler data, highlighting the intrinsic variability and its implications for binary searches.

Findings

Cycle-to-cycle period scatter of 0.015-0.02 days.

Light curve shape instability correlates with period variations.

Period jitter limits binary companion detection.

Abstract

We present a detailed period analysis of the bright Cepheid-type variable star V1154 Cygni (V =9.1 mag, P~4.9 d) based on almost 600 days 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 days over the 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.