Disentangling 2:1 resonant radial velocity orbits from eccentric ones and a case study for HD 27894
Abstract
Context. In radial velocity (RV) observations, a pair of extrasolar planets near a 2:1 orbital resonance can be misinterpreted as a single eccentric planet, if data are sparse and measurement precision insufficient to distinguish between these models.
Aims: Using the Exoplanet Orbit Database (EOD), we determine the fraction of alleged singleplanet RV detected systems for which a 2:1 resonant pair of planets is also a viable model and address the question of how the models can be disentangled.
Methods: By simulation we quantified the mismatch arising from applying the wrong model. Model alternatives are illustrated using the supposed singleplanet system HD 27894 for which we also study the dynamical stability of near2:1 resonant solutions.
Results: Using EOD values of the data scatter around the fitted singleplanet Keplerians, we find that for 74% of the 254 putative singleplanet systems, a 2:1 resonant pair cannot be excluded as a viable model, since the error due to the wrong model is smaller than the scatter. For 187 EOD stars χ^{2}probabilities can be used to reject the Keplerian models with a confidence of 95% for 54% of the stars and with 99.9% for 39% of the stars. For HD 27894 a considerable fit improvement is obtained when adding a lowmass planet near half the orbital period of the known Jovian planet. Dynamical analysis demonstrates that this system is stable when both planets are initially placed on circular orbits. For fully Keplerian orbits a stable system is only obtained if the eccentricity of the inner planet is constrained to < 0.3.
Conclusions: A large part of the allegedly RV detected singleplanet systems should be scrutinized in order to determine the fraction of systems containing near2:1 resonant pairs of planets. Knowing the abundance of such systems will allow us to revise the eccentricity distribution for extrasolar planets and provide direct constraints for planetary system formation.
This research has made use of the Exoplanet Orbit Database and the Exoplanet Data Explorer at http://www.exoplanets.org
 Publication:

Astronomy and Astrophysics
 Pub Date:
 May 2015
 DOI:
 10.1051/00046361/201525872
 arXiv:
 arXiv:1503.07769
 Bibcode:
 2015A&A...577A.103K
 Keywords:

 celestial mechanics;
 planetary systems;
 techniques: radial velocities;
 stars: individual: HD 27894;
 Astrophysics  Solar and Stellar Astrophysics;
 Astrophysics  Earth and Planetary Astrophysics
 EPrint:
 12 pages, 8 figures, one of them composed by two files, accepted by A&