Sparse aperture masking at the VLT^⋆

I. Faint companion detection limits for the two debris disk stars HD 92945 and HD 141569

¹ LESIA, CNRS/UMR-8109, Observatoire de Paris, UPMC, Université Paris Diderot, 5 place Jules Janssen, 92195 Meudon, France
e-mail: sylvestre.lacour@obspm.fr
² Sydney Institute for Astronomy, School of Physics, The University of Sydney, NSW 2006, Australia
³ ESO, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
⁴ UJF-Grenoble 1/CNRS-INSU, Institut de planétologie et d’Astrophysique de Grenoble (IPAG) UMR 5274, 38041 Grenoble, France
⁵ Centro de Astrobiología (INTA-CSIC); LAEFF, PO Box 78, 28691 Villanueva de la Cañada, Spain

Received: 14 February 2011
Accepted: 20 June 2011

Abstract

Aims. Observational data on companion statistics around young stellar systems is needed to flesh out the formation pathways for extrasolar planets and brown dwarfs. Aperture masking is a new technique that is able to address an important part of this discovery space.

Methods. We observed the two debris disk systems HD 92945 and HD 141569 with sparse aperture masking (SAM), a new mode offered on the NaCo instrument at the VLT. A search for faint companions was performed using a detection strategy based on the analysis of closure phases recovered from interferograms recorded on the Conica camera.

Results. Our results demonstrate that SAM is a very competitive mode in the field of companion detection. We obtained 5σ high-contrast detection limits at λ/D of 2.5 × 10^-3 (ΔL′ = 6.5) for HD 92945 and 4.6 × 10^-3 (ΔL′ = 5.8) for HD 141569. According to brown dwarf evolutionary models, our data impose an upper mass boundary for any companion for the two stars to, respectively, 18 and 22 Jupiter masses at minimum separations of 1.5 and 7 AU. The detection limits is mostly independent of angular separation, until reaching the diffraction limit of the telescope.

Conclusions. We have placed upper limits on the existence of companions to our target systems that fall close to the planetary mass regime. This demonstrates the potential for SAM mode to contribute to studies of faint companions. We furthermore show that the final dynamic range obtained is directly proportional to the error on the closure phase measurement. At the present performance levels of 0.28 degree closure phase error, SAM is among the most competitive techniques for recovering companions at scales of one to several times the diffraction limit of the telescope. Further improvements to the detection threshold can be expected with more accurate phase calibration.