Interpreting the yield of transit surveys: are there groups in the known transiting planets population?^*

¹ Observatoire de la Côte d'Azur, Laboratoire Cassiopée, CNRS UMR 6202, BP 4229, 06304 Nice cedex 4, France e-mail: ffressin@cfa.harvard.edu
² Observatoire Français des Conjonctures Économiques (OFCE), 250 rue Albert Einstein, 06560 Valbonne, France

Received: 30 April 2008
Accepted: 24 December 2008

Abstract

Context. Each transiting planet discovered is characterized by 7 measurable quantities, that may or may not be linked. This includes those relative to the planet (mass, radius, orbital period, and equilibrium temperature) and those relative to the star (mass, radius, effective temperature, and metallicity). Correlations between planet mass and period, surface gravity and period, planet radius and star temperature have been previously observed among the 31 known transiting giant planets. Two classes of planets have been previously identified based on their Safronov number.

Aims. We use the CoRoTlux transit surveys to compare simulated events to the sample of discovered planets and test the statistical significance of these correlations. Using a model proved to be able to match the yield of OGLE transit survey, we generate a large sample of simulated detections, in which we can statistically test the different trends observed in the small sample of known transiting planets.

Methods. We first generate a stellar field with planetary companions based on radial velocity discoveries, use a planetary evolution model assuming a variable fraction of heavy elements to compute the characteristics of transit events, then apply a detection criterion that includes both statistical and red noise sources. We compare the yield of our simulated survey with the ensemble of 31 well-characterized giant transiting planets, using different statistical tools, including a multivariate logistic analysis to assess whether the simulated distribution matches the known transiting planets.

Results. Our results satisfactorily match the distribution of known transiting planet characteristics. Our multivariate analysis shows that our simulated sample and observations are consistent to 76%. The mass vs. period correlation for giant planets first observed with radial velocity holds with transiting planets. The correlation between surface gravity and period can be explained as the combined effect of the mass vs. period lower limit and by the decreasing transit probability and detection efficiency for longer periods and higher surface gravity. Our model also naturally explains other trends, like the correlation between planetary radius and stellar effective temperature. Finally, we are also able to reproduce the previously observed apparent bimodal distribution of planetary Safronov numbers in 10% of our simulated cases, although our model predicts a continuous distribution. This shows that the evidence for the existence of two groups of planets with different intrinsic properties is not statistically significant.