Coronagraphic search for wide substellar companions among members of the Ursa Major moving group^⋆,⋆⋆

¹ Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
e-mail: ammler@mps.mpg.de
² Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany
³ Astrophysikalisches Institut und Universitäts-Sternwarte Jena, Schillergäßchen 2-3, 07745 Jena, Germany
⁴ Centro de Astronomia e Astrofísica da Universidade de Lisboa, Observatório Astronómico de Lisboa, Tapada da Ajuda, 1349-018 Lisboa, Portugal
⁵ Centro de Astrofísica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
⁶ Georg-August-Universität, Institut für Astrophysik. Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
⁷ Faculty of Natural Sciences, University of Split, Teslina 12. 21000 Split, Croatia
⁸ Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
⁹ Abbe Center of Photonics, Friedrich-Schiller-Universität Jena, Max-Wien-Platz 1, 07743 Jena, Germany

Received: 28 May 2015
Accepted: 9 March 2016

Abstract

Context. We present the results of a survey to detect low-mass companions of Ursa Major (UMa) group members, carried out in 2003–2006 with NACO at the ESO VLT. While many extra-solar planets and planetary candidates have been found in close orbits around stars by the radial velocity and the transit methods, direct detections at wider orbits are rare. The UMa group, a young nearby stellar association at an age of about 200–600 Myr, has not yet been addressed as a whole although its members represent a very interesting sample to search for and characterize substellar companions by direct imaging.

Aims. Our goal was to find or to provide detection limits on wide substellar companions around nearby UMa group members using high-resolution imaging.

Methods. We searched for faint companions around 20 UMa group members within 30 pc. The primaries were placed below a semi-transparent coronagraph, a rarely used mode of NACO, to increase the dynamic range of the images. In most cases, second epoch images of companion candidates were taken to check whether they share common proper motion with the primary.

Results. Our coronagraphic images rule out substellar companions around the stars of the sample. A typical dynamical range of 13–15 mag in the K_s band was achieved at separations beyond 3′′ from the star. Candidates as faint as K_s ≈ 20 were securely identified and measured. The survey is most sensitive between separations of 100 and 200 au but only on average because of the very different target distance. Field coverage reaches about 650 au for the most distant targets. Most of the 200 candidates detected in the covered fields are visible in two epochs and were rejected because they are distant background objects.