Transit timing analysis of the exoplanets TrES-1 and TrES-2^*

¹ Instituto de Astrofísica de Canarias, vía Lactea s/n, 38205 La Laguna, Spain e-mail: mrabus@iac.es
² Departamento de Astrofísica, Universidad de La Laguna, 38205 La Laguna, Tenerife, Spain
³ Laboratoire d'Astrophysique de Marseille (UMR 6110), Technopôle de Marseille-Étoile, 13388 Marseille, France
⁴ Observatoire de Genève, Université de Genève, 1290 Sauverny, Switzerland

Received: 1 April 2009
Accepted: 28 August 2009

Abstract

Aims. The aim of this work is a detailed analysis of transit light curves from TrES-1 and TrES-2, obtained over a period of three to four years, in order to search for variabilities in observed mid-transit times and to set constraints on the presence of additional third bodies.

Methods. Using the IAC 80 cm telescope, we observed transits of TrES-1 and TrES-2 over several years. Based on these new data and previously published work, we studied the observed light curves and searched for variations in the difference between observed and calculated (based on a fixed ephemeris) transit times. To model possible transit timing variations, we used polynomials of different orders, simulated O–C diagrams corresponding to a perturbing third mass, and we used sinusoidal fits. For each model we calculated the  residuals and the false alarm probability (FAP).

Results. For TrES-1, we can exclude planetary companions (>1 ) in the 3:2 and 2:1 MMRs having high FAPs based on our transit observations from the ground. Likewise, a light time effect caused, e.g., by a 0.09  mass star at a distance of 7.8 AU is possible. As for TrES-2, we find a better ephemeris of T_c = 2 453 957.63512(28) + 2.4706101(18)  Epoch and a good fit for a sine function with a period of 0.2 days, compatible with a moon around TrES-2 and an amplitude of 57 s, but it is not a uniquely low  value that would indicate a clear signal. In both cases, TrES-1 and TrES-2, we are able to put upper constraints on the presence of additional perturbers masses. We also conclude that any sinusoidal variations that might be indicative of exomoons need to be confirmed with higher statistical significance by further observations, noting that TrES-2 is in the field-of-view of the Kepler Space Telescope.