The HU Aqr planetary system hypothesis revisited

Abstract

We study the mid-egress eclipse timing data gathered for the cataclysmic binary HU Aquarii during the years 1993-2014. The (O-C) residuals were previously attributed to a single ∼ 7 Jupiter mass companion in ∼ 5 au orbit or to a stable 2-planet system with an unconstrained outermost orbit. We present 22 new observations gathered between June, 2011 and July, 2014 with four instruments around the world. They reveal a systematic deviation of ∼ 60-120 seconds from the older ephemeris. We re-analyse the whole set of the timing data available. Our results provide an erratum to the previous HU Aqr planetary models, indicating that the hypothesis for a third and fourth body in this system is uncertain. The dynamical stability criterion and a particular geometry of orbits rule out coplanar 2-planet configurations. A putative HU Aqr planetary system may be more complex, e.g., highly non-coplanar. Indeed, we found examples of 3-planet configurations with the middle planet in a retrograde orbit, which are stable for at least 1 Gyr, and consistent with the observations. The (O-C) may be also driven by oscillations of the gravitational quadrupole moment of the secondary, as predicted by the Lanza et al. modification of the Applegate mechanism. Further systematic, long-term monitoring of HU Aqr is required to interpret the (O-C) residuals.

The conjectured S-type retrograde planet in ν Octantis: more evidence including four years of iodine-cell radial velocities

“…Several trial runs of the observations revealed many local minima which could easily trap our Markov chains depending on our set of initial conditions. It is possible, therefore, that our results are based on samples from a local rather than the true global minimum, which, based on the results of Goździewski et al (2013), would not be too surprising.…”

“…A Bulirsch-Stoer integrator, similarly used by Goździewski et al (2013) in their detailed analysis of the ν Oct system, is appropriate given the assumed geometry includes a nearby massive secondary-star perturber, and which makes for instance the 'hybrid' integrator unsuitable. Nothing remained dynamically stable past our integration baseline.…”

“…Gayon-Markt & Bois 2009;Goździewski et al 2015) whilst no choice exists for ν Oct. Nu Octantis (HD 205478 HIP 107089 HR 8254) has a slightly evolved early K-type primary with a late-dwarf or white-dwarf secondary (Table 1). The possibility of a planet was first mentioned in Ramm (2004) with 156 RVs and barely one binary orbit observed.…”

“…The retrograde-orbit scenario for ν Oct has been explored in increasing complexity by Quarles, Cuntz & Musielak (2012) and Goździewski et al (2013). Both studies conclude that stable ret- Table 2.…”

“…As well as being the first to re-analyse the RVs with a selfconsistent Newtonian model, Goździewski et al (2013) also emphasised the complex chaotic space of narrow intersecting stability zones and relatively small islands of long-term stability. Furthermore, their stable models had to be significantly shifted from the N -body and Keplerian best-fit models, implying the conjectured planet was inconsistent with the then available data.…”