Volume 555, July 2013
|Number of page(s)||8|
|Section||Planets and planetary systems|
|Published online||12 July 2013|
The quest for companions to post-common envelope binaries
IV. The 2:1 mean-motion resonance of the planets orbiting NN Serpentis⋆
Institut für Astrophysik, Georg-August-Universität, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
Received: 6 October 2012
Accepted: 9 May 2013
We present 69 new mid-eclipse times of the young post-common envelope binary (PCEB) NN Ser, which was previously suggested to possess two circumbinary planets. We have interpreted the observed eclipse-time variations in terms of the light-travel time effect caused by two planets, exhaustively covering the multi-dimensional parameter space by fits in the two binary and ten orbital parameters. We supplemented the fits by stability calculations for all models with an acceptable χ2. An island of secularly stable 2:1 resonant solutions exists, which coincides with the global χ2 minimum. Our best-fit stable solution yields current orbital periods Po = 15.47 yr and Pi = 7.65 yr and eccentricities eo = 0.14 and ei = 0.22 for the outer and inner planets, respectively. The companions qualify as giant planets, with masses of 7.0 MJup and 1.7 MJup for the case of orbits coplanar with that of the binary. The two-planet model that starts from the present system parameters has a lifetime greater than 108 yr, which significantly exceeds the age of NN Ser of 106 yr as a PCEB. The resonance is characterized by libration of the resonant variable Θ1 and circulation of , the difference between the arguments of periapse of the two planets. No stable nonresonant solutions were found, and the possibility of a 5:2 resonance suggested previously by us is now excluded at the 99.3% confidence level.
Key words: binaries: close / binaries: eclipsing / white dwarfs / stars: individual: NN Serpentis / planets and satellites: detection / planets and satellites: dynamical evolution and stability
Table 1 is available in electronic form at http://www.aanda.org
© ESO, 2013
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