A frozen super-Earth orbiting a star at the bottom of the main sequence ^⋆

¹ Institut d’Astrophysique de Paris, UMR 7095 CNRS – Université Pierre & Marie Curie, 98 bis blv Arago, 75014 Paris, France
e-mail: dkubas@gmail.com
² European Southern Observatory, Casilla 19001, Vitacura 19, Santiago, Chile
³ University College of London, Deparment of Physics and Astronomy, Gower Street, London, WC1E 6BT, UK
⁴ University of Notre Dame, Department of Physics, 225 Nieuwland Science Hall Notre Dame, USA
⁵ University of Tasmania, School of Mathematics and Physics, Private Bag 37, GPO Hobart, Tas 7001, Australia
⁶ Dipartimento di Fisica, Università degli Studi di Roma “Tor Vergata”, Rome, Italy
⁷ Department of Astronomy, 610 Space Sciences Building, Cornell University, Ithaca, NY 14853, USA
⁸ Sagan Fellow  
⁹ Institute for Advanced Study, Einstein Drive, Princeton, NJ 08540, USA
¹⁰ Department of Astronomy, Ohio State University, 140 W. 18th Ave., Columbus, OH 43210, USA
¹¹ Department of Earth and Space Science, Osaka University, Osaka 560-0043, Japan
¹² Observatoire Midi-Pyrénées, UMR 5572, 14 avenue Edouard Belin, 31400 Toulouse, France
¹³ Institute of Information and Mathematical Sciences, Massey University, Private Bag 102-904, North Shore Mail Centre, Auckland, New Zealand
¹⁴ Department of Physics and Astrophysics, Faculty of Science, Nagoya University, Nagoya 464-8602, Japan
¹⁵ Warsaw University Observatory, Al. Ujazdowskie 4, 00-478 Warszawa, Poland
¹⁶ Laboratoire d’Astrophysique de Grenoble, UMR 5571 Université Joseph Fourier/CNRS, BP 53, 38051 Grenoble Cedex 9, France

Received: 28 September 2010
Accepted: 7 February 2012

Abstract

Context. Microlensing is a unique method to probe low mass exoplanets beyond the snow line. However, the scientific potential of the new microlensing planet discovery is often unfulfilled due to lack of knowledge of the properties of the lens and source stars. The discovery light curve of the super Earth MOA-2007-BLG-192Lb suffers from significant degeneracies that limit what can be inferred about its physical properties.

Aims. High resolution adaptive optics images allow us to solve this problem by resolving the microlensing target from all unrelated background stars, yielding the unique determination of magnified source and lens fluxes. This estimation permits the solution of our microlens model for the mass of the planet and its host and their physical projected separation.

Methods. We observed the microlensing event MOA-2007-BLG-192  at high angular resolution in JHKs with the NACO adaptive optics system on the VLT while the object was still amplified by a factor 1.23 and then at baseline 18 months later. We analyzed and calibrated the NACO photometry in the standard 2MASS system in order to accurately constrain the source and the lens star fluxes.

Results. We detect light from the host star of MOA-2007-BLG-192Lb, which significantly reduces the uncertainties in its characteristics as compared to earlier analyses. We find that MOA-2007-BLG-192L is most likely a very low mass late type M-dwarf (0.084_-0.012^+0.015 M_⊙) at a distance of 660_-70⁺¹⁰⁰ pc orbited by a 3.2_-1.8^+5.2 M_⊕ super-Earth at 0.66_-0.22^+0.51 AU. We then discuss the properties of this cold planetary system.