The VLT/NaCo large program to probe the occurrence of exoplanets and brown dwarfs at wide orbits^⋆,⋆⋆

II. Survey description, results, and performances

¹ UJF-Grenoble1/CNRS-INSU, Institut de Planétologie et d’Astrophysique de Grenoble UMR 5274, 38041, Grenoble, France
e-mail: Gael.Chauvin@obs.ujf-grenoble.fr
² Aix-Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France
³ Max-Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁴ INAF – Osservatorio Astronomico di Padova, Vicolo dell Osservatorio 5, 35122 Padova, Italy
⁵ LESIA, Observatoire de Paris Meudon, 5 Pl. J. Janssen, 92195 Meudon, France
⁶ Department of Physics & Astronomy, College of Charleston, 58 Coming Street, Charleston, SC 29424, USA
⁷ Geneva Observatory, University of Geneva, Chemin des Mailettes 51, 1290 Versoix, Switzerland
⁸ Institute for Astronomy, ETH Zurich, Wolfgang-Pauli-Strasse 27, 8093 Zurich, Switzerland
⁹ INAF Osservatorio Astronomico di Capodimonte via Moiarello 16, 80131 Napoli, Italy
¹⁰ European Southern Observatory, Casilla 19001, Santiago 19, Chile
¹¹ European Southern Observatory, Karl Schwarzschild St, 2, 85748 Garching, Germany
¹² INAF – Catania Astrophysical Observatory, via S. So a 78, 95123 Catania, Italy

Received: 3 February 2014
Accepted: 9 April 2014

Abstract

Context. Young, nearby stars are ideal targets for direct imaging searches for giant planets and brown dwarf companions. After the first-imaged planet discoveries, vast efforts have been devoted to the statistical analysis of the occurence and orbital distributions of giant planets and brown dwarf companions at wide (≥5–6 AU) orbits.

Aims. In anticipation of the VLT/SPHERE planet-imager, guaranteed-time programs, we have conducted a preparatory survey of 86 stars between 2009 and 2013 to identify new faint comoving companions to ultimately analyze the occurence of giant planets and brown dwarf companions at wide (10–2000 AU) orbits around young, solar-type stars.

Methods. We used NaCo at VLT to explore the occurrence rate of giant planets and brown dwarfs between typically 0.1 and 8′′. Diffraction-limited observations in H-band combined with angular differential imaging enabled us to reach primary star-companion brightness ratios as small as 10^-6 at 1.5′′. Repeated observations at several epochs enabled us to discriminate comoving companions from background objects.

Results. During our survey, twelve systems were resolved as new binaries, including the discovery of a new white dwarf companion to the star HD 8049. Around 34 stars, at least one companion candidate was detected in the observed field of view. More than 400 faint sources were detected; 90% of them were in four crowded fields. With the exception of HD 8049 B, we did not identify any new comoving companions. The survey also led to spatially resolved images of the thin debris disk around HD 61005 that have been published earlier. Finally, considering the survey detection limits, we derive a preliminary upper limit on the frequency of giant planets for the semi-major axes of [10, 2000] AU: typically less than 15% between 100 and 500 AU and less than 10% between 50 and 500 AU for exoplanets that are more massive than 5 M_Jup and 10 M_Jup respectively, if we consider a uniform input distribution and a confidence level of 95%.

Conclusions. The results from this survey agree with earlier programs emphasizing that massive, gas giant companions on wide orbits around solar-type stars are rare. These results will be part of a broader analysis of a total of ~210 young, solar-type stars to bring further statistical constraints for theoretical models of planetary formation and evolution.