Deep infrared imaging of close companions to austral A- and F-type stars^⋆,⋆⋆

¹ Laboratoire d’astrophysique de Grenoble, Université Joseph Fourier, CNRS (UMR 5571), BP 53, 38041 Grenoble Cedex 9, France
e-mail: david.ehrenreich@obs.ujf-grenoble.fr
² European Southern Observatory, Alonso de Cordova 3107, Vitacura Casilla 19001, Santiago 19, Chile

Received: 9 April 2010
Accepted: 24 June 2010

Abstract

The search for substellar companions around stars with different masses along the main sequence is critical to understanding the different processes leading to the formation of low-mass stars, brown dwarfs, and planets. In particular, the existence of a large population of low-mass stars and brown dwarfs physically bound to early-type main-sequence stars could imply that the massive planets recently imaged at wide separations (10–100 AU) around A-type stars are disc-born objects in the low-mass tail of the binary distribution and are thus formed via gravitational instability rather than by core accretion. Our aim is to characterize the environment of early-type main-sequence stars by detecting substellar companions between 10 and 500 AU. The sample stars are also surveyed with radial velocimetry, providing a way to determine the impact of the imaged companions on planets at  ≲10 AU. High-contrast and high angular resolution near-infrared images of a sample of 38 southern A- and F-type stars were obtained between 2005 and 2009 with the instruments NaCo on the Very Large Telescope and PUEO on the Canada-France-Hawaii Telescope. Direct and saturated imaging were used in the J to K_s bands to probe the faint circumstellar environments with contrasts of  ~5 × 10^-2 to 10^-4 at separations of and 1″, respectively. Using coronagraphic imaging, we achieved contrasts between 10^-5 and 10^-6 at separations  >5″̣ Multi-epoch observations were performed to distinguish comoving companions from background contaminants. This survey is sensitive to companions of A and F stars in the brown dwarf to low-mass star mass regime. About 41 companion candidates were imaged around 23 stars. Follow-up observations for 83% of these stars allowed us to identify a large number of background contaminants. We report detection of 7 low-mass stars with masses between 0.1 and 0.8 M_⊙ in 6 multiple systems: the discovery of an M2 companion around the A5V star HD 14943 and detection of HD 41742B around the F4V star HD 41742 in a quadruple system. We resolve the known companion of the F6.5V star HD 49095 as a short-period binary system composed of 2 M/L dwarfs. We also resolve the companions to the astrometric binaries ι Crt (F6.5V) and 26 Oph (F3V), and identify an M3/M4 companion to the F4V star o Gru, associated with an X-ray source. The global multiplicity fraction measured in our sample of A and F stars is  ≥ 16%. This has a probable impact on the radial velocity measurements performed on the sample stars.