Origin of apparent period variations in eclipsing post-common-envelope binaries

Abstract

Context. Apparent period variations detected in several eclipsing, close-compact binaries are frequently interpreted as being caused by circumbinary giant planets. This interpretation raises the question of the origin of the potential planets that must have either formed in the primordial circumbinary disk, together with the host binary star, and survived its evolution into a close-compact binary or formed in a post-common-envelope circumbinary disk that remained bound to the post-common-envelope binary (PCEB). Aims. Here we combine current knowledge of planet formation and the statistics of giant planets around primordial and evolved binary stars with the theory of close-compact binary star evolution aiming to derive new constraints on possible formation scenarios. Methods. We compiled a comprehensive list of observed eclipsing PCEBs, estimated the fraction of systems showing apparent period variations, reconstructed the evolutionary history of the PCEBs, and performed binary population models of PCEBs to characterize their main sequence binary progenitors. We reviewed the currently available constraints on the fraction of PCEB progenitors that host circumbinary giant planets. Results. We find that the progenitors of PCEBs are very unlikely to be frequent hosts of giant planets ( < ∼ 10 per cent), while the frequency of PCEBs with observed apparent period variations is very high (∼90 per cent). Conclusions. The variations in eclipse timings measured in eclipsing PCEBs are probably not caused by first-generation planets that survived common-envelope evolution. The remaining options for explaining the observed period variations are second-generation planet formation or perhaps variations in the shape of a magnetically active secondary star. We suggest observational tests for both options.

Planet formation from the ejecta of common envelopes

“…For the CVs, we find very good agreement in all three cases, with mass differences of less than a factor of 2. For the system DP Leo, we note that Schwope et al (2002) provide different stellar masses than Pandel et al (2002) and Beuermann et al (2011), but the resulting planetary masses are both comparable to the value derived by Zorotovic & Schreiber (2013).…”

“…In the systems including a WD, a particularly good agreement with the masses given by Zorotovic & Schreiber (2013) is achieved for the systems NN Ser and RR Cae, where the difference in mass is less than a factor of 2 and the orbital radius in between the cases considered here. For V 471 Tau and QS Vir, our model underpredicts the masses by a factor of 3−5.…”

“…To explore the relevance of this scenario for other systems, we applied the model proposed here to the PCEB-systems marked by Zorotovic & Schreiber (2013) as candidates for planets, assuming that the interpretation of the LTT signals as caused by planetary companions is indeed correct. We note that, given the long orbital periods in combination with the rather short observational coverage, parameters of several of the planetary candidates are preliminary and may change.…”

“…The two-planet solution was, however, shown to be dynamically unstable (Wittenmyer et al 2013), while the one-planet solution was shown to be stable (Beuermann et al 2012a). For this system, we thus deviate from Zorotovic & Schreiber (2013) by adopting the one-planet solution in the following.…”

“…A final conclusion about the stability of planetary orbits in Hu Aqr is currently pending Hinse et al 2012), and more data are required to understand the eclipse-time variations in QS Vir (Parsons et al 2010b). Overall, Zorotovic & Schreiber (2013) list 12 planet candidates in PCEB systems.…”