Testing the planetary models of HU Aquarii

Abstract

We present new eclipse observations of the polar (i.e. semi-detached magnetic white dwarf + M-dwarf binary) HU Aqr, and mid-egress times for each eclipse, which continue to be observed increasingly early. Recent eclipses occurred more than 70 seconds earlier than the prediction from the latest model that invoked a single circumbinary planet to explain the observed orbital period variations, thereby conclusively proving this model to be incorrect. Using ULTRACAM data, we show that mid-egress times determined for simultaneous data taken at different wavelengths agree with each other. The large variations in the observed eclipse times cannot be explained by planetary models containing up to three planets, because of poor fits to the data as well as orbital instability on short time scales. The peak-to-peak amplitude of the O-C diagram of almost 140 seconds is also too great to be caused by Applegate's mechanism, movement of the accretion spot on the surface of the white dwarf, or by asynchronous rotation of the white dwarf. What does cause the observed eclipse time variations remains a mystery.

The Occurrence and Architecture of Exoplanetary Systems

“…One serious problem is that in at least four cases, the proposed planetary orbits are dynamically unstable, casting doubt on the planetary hypothesis for those systems and leaving one wondering whether all the signals are the result of some other physical phenomenon. In one of the best studied systems, HU Aquarii, additional timing data refuted the planetary hypothesis (Bours et al 2014), although the true origin of the timing variations is unknown.…”

Long-term eclipse timing of white dwarf binaries: an observational hint of a magnetic mechanism at work

“…On top of this, there are additional features near the white dwarf eclipse caused by the eclipse of the accretion disc and bright spot in non-magnetic systems and intermediate polars, and of the hot spot on the white dwarf's surface in polars. Therefore we chose to approach the fitting as done in Bours et al (2014b), where the ingress and egress of the white dwarf eclipse are fit by a least-squares approach using a function that is composed of a sigmoid and a straight line,…”

“…Clearly, the eclipse time variations in this binary are complex, and what causes them remains to be discovered. There are a few other binaries in the timing programme that show similarly large O-C variations, such as V471 Tau (Hardy et al 2015) and HU Aqr (Bours et al 2014b).…”

“…The luminosities of these stars can be considered as their maximum energy budget available to drive the magnetic cycles, their physical distortions, and therefore the binary's orbital period variations. For certain white dwarf + M-dwarf binaries in which large variations of this kind have been observed, the energy budget is seemingly insufficient (Brinkworth et al 2006;Bours et al 2014b). However, note that a modified version of Applegate's mechanism predicts that orbital period variations can be induced while only re-quiring a fraction of the energy of the original mechanism of Applegate (1992), see Lanza et al (1998) and Lanza (2006).…”

Multi‐epoch time‐resolved photometry of the eclipsing polar CSS081231:071126+440405

“…In addition, magnetic braking of the binary and gravitational radiation will occur but their strength is not enough to make an observable impact on the time-scale of a scientific career. Some objects show strong, quasi-regular variability in their eclipse arrival times that cannot be explained by current models (see the claimed discovery of a planetary system around HU Aqr; Bours et al 2014;Goździewski et al 2012;Horner et al 2011;Qian et al 2011;Schwarz et al 2009;Schwope & Thinius 2014;Wittenmyer et al 2012). …”