The HU Aqr planetary system hypothesis revisited

TL;DR

This study revisits the planetary hypothesis for HU Aqr, using new eclipse timing data to challenge previous models and suggest more complex, possibly non-coplanar planetary configurations or stellar activity effects.

Contribution

It provides an updated analysis of HU Aqr's eclipse timings, ruling out simple coplanar 2-planet models and proposing the possibility of complex, non-coplanar planetary systems or stellar activity influences.

Findings

Previous planetary models are inconsistent with new data.

Coplanar 2-planet configurations are dynamically unstable.

Stable, non-coplanar 3-planet configurations are possible.

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 1Gyr, 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.