Transiting exoplanets from the CoRoT space mission^⋆

XXIV. CoRoT-25b and CoRoT-26b: two low-density giant planets

¹ Aix Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France
e-mail: josemanuel.almenara@oamp.fr
² Institut d’Astrophysique de Paris, 98bis boulevard Arago, 75014 Paris, France
³ Department of Astronomy, New Mexico State University, PO Box 30001, MSC 4500, Las Cruces, NM 88003-8001, USA
⁴ NASA Postdoctoral Program Fellow, Ames Research Center, PO Box 1, Moffett Field, CA 94035, USA
⁵ Research and Scientific Support Department, ESTEC/ESA, PO Box 299, 2200 AG Noordwijk, The Netherlands
⁶ Instituto de Astrofisica de Canarias, 38205 La Laguna, Tenerife, Spain
⁷ Universidad de La Laguna, Dept. de Astrofísica, 38200 La Laguna, Tenerife, Spain
⁸ Thüringer Landessternwarte, Sternwarte 5, Tautenburg 5, 07778 Tautenburg, Germany
⁹ Observatoire de la Côte d’Azur, Laboratoire Cassiopée, BP 4229, 06304 Nice Cedex 4, France
¹⁰ Institute of Planetary Research, German Aerospace Center, Rutherfordstrasse 2, 12489 Berlin, Germany
¹¹ Department of Physics, Denys Wilkinson Building Keble Road, Oxford, OX1 3RH, UK
¹² Observatoire de l’Université de Genève, 51 chemin des Maillettes, 1290 Sauverny, Switzerland
¹³ LESIA, Obs de Paris, Place J. Janssen, 92195 Meudon Cedex, France
¹⁴ INAF − Osservatorio Astronomico di Torino, via Osservatorio 20, 10025 Pino Torinese, Italy
¹⁵ Institut d’astrophysique spatiale, Université Paris-Sud 11 & CNRS, UMR 8617, Bât. 121, 91405 Orsay, France
¹⁶ McDonald Observatory, The University of Texas at Austin, Austin, TX 78712, USA
¹⁷ University of Vienna, Institute of Astronomy, Türkenschanzstr. 17, 1180 Vienna, Austria
¹⁸ IAG, Universidade de Sao Paulo, Brazil
¹⁹ Observatoire de Haute Provence, 04670 Saint Michel l’ Observatoire, France
²⁰ University of Liège, Allée du 6 août 17, Sart Tilman, Liège 1, Belgium
²¹ Space Research Institute, Austrian Academy of Science, Schmiedlstr. 6, 8042 Graz, Austria
²² Wise Observatory, Tel Aviv University, 69978 Tel Aviv, Israel
²³ Institut für Astrophysik, Georg-August-Universität, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
²⁴ Rheinisches Institut für Umweltforschung an der Universität zu Köln, Aachener Strasse 209, 50931 Köln, Germany
²⁵ Center for Astronomy and Astrophysics, TU Berlin, Hardenbergstr. 36, 10623 Berlin, Germany
²⁶ LUTH, Observatoire de Paris, CNRS, Université Paris Diderot, 5 place Jules Janssen, 92195 Meudon, France
²⁷ Department of Physics and Astronomy, Aarhus University, 8000 Aarhus C, Denmark

Received: 13 March 2013
Accepted: 24 May 2013

Abstract

We report the discovery of two transiting exoplanets, CoRoT-25b and CoRoT-26b, both of low density, one of which is in the Saturn mass-regime. For each star, ground-based complementary observations through optical photometry and radial velocity measurements secured the planetary nature of the transiting body and allowed us to fully characterize them. For CoRoT-25b we found a planetary mass of 0.27 ± 0.04 M_Jup, a radius of 1.08_-0.10^+0.3 R_Jup and hence a mean density of 0.15_-0.06^+0.15 g cm^-3. The planet orbits an F9 main-sequence star in a 4.86-day period, that has a V magnitude of 15.0, solar metallicity, and an age of 4.5_-2.0^+1.8-Gyr. CoRoT-26b orbits a slightly evolved G5 star of 9.06 ± 1.5-Gyr age in a 4.20-day period that hassolar metallicity and a V magnitude of 15.8. With a mass of 0.52 ± 0.05 M_Jup, a radius of 1.26_-0.07^+0.13 R_Jup, and a mean density of 0.28_-0.07^+0.09 g cm^-3, it belongs to the low-mass hot-Jupiter population. Planetary evolution models allowed us to estimate a core mass of a few tens of Earth mass for the two planets with heavy-element mass fractions of 0.52_-0.15^+0.08 and 0.26_-0.08^+0.05, respectively, assuming that a small fraction of the incoming flux is dissipated at the center of the planet. In addition, these models indicate that CoRoT-26b is anomalously large compared with what standard models could account for, indicating that dissipation from stellar heating could cause this size.