Enhanced models for stellar Doppler noise reveal hints of a 13-year activity cycle of 55 Cancri

Abstract

We consider the impact of Doppler noise models on the statistical robustness of the exoplanetary radial-velocity fits. We show that the traditional model of the Doppler noise with an additive jitter can generate large non-linearity effects, decreasing the reliability of the fit, especially in the cases when a correleated Doppler noise is involved. We introduce a regularization of the additive noise model that can gracefully eliminate its singularities together with the associated non-linearity effects. We apply this approach to Doppler time-series data of several exoplanetary systems. It demonstrates that our new regularized noise model yields orbital fits that have either increased or at least the same statistical robustness, in comparison with the simple additive jitter. Various statistical uncertainties in the parametric estimations are often reduced, while planet detection significance is often increased. Concerning the 55 Cnc five-planet system, we show that its Doppler data contain significant correlated ("red") noise. Its correlation timescale is in the range from days to months, and its magnitude is much larger than the effect of the planetary N-body perturbations in the radial velocity (these perturbations thus appear undetectable). Characteristics of the red noise depend on the spectrograph/observatory, and also show a cyclic time variation in phase with the public Ca II H & K and photometry measurements. We interpret this modulation as a hint of the long-term activity cycle of 55 Cnc, similar to the Solar 11-year cycle. We estimate the 55 Cnc activity period by $12.6{\pm }_{1.0}^{2.5}$ yrs, with the nearest minimum presumably expected in 2014 or 2015.

21 pages, 12 figures, 3 tables; Accepted for publication in MNRAS; the PlanetPack2 code implementing these new Doppler noise models is available at http://sourceforge.net/projects/planetpack/

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