Searching for faint companions with VLTI/PIONIER
I. Method and first results⋆
O. Absil1⋆⋆, J.-B. Le Bouquin2, J.-P. Berger3, A.-M. Lagrange2, G. Chauvin2,4, B. Lazareff2, G. Zins2, P. Haguenauer3, L. Jocou2, P. Kern2, R. Millan-Gabet5, S. Rochat2 and W. Traub6
Département d’Astrophysique, Géophysique et Océanographie, Université de Liège, 17 allée du Six Août, 4000 Liège, Belgium
2 Institut de Planétologie et d’Astrophysique de Grenoble (IPAG) UMR 5274, UJF-Grenoble 1/CNRS-INSU, Grenoble, France
3 European Southern Observatory, Casilla 19001, Santiago 19, Chile
4 Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
5 NASA Exoplanet Science Institute (NExScI), California Institute of Technology, Pasadena, CA 91125, USA
6 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
Received: 16 July 2011
Accepted: 21 September 2011
Context. A new four-telescope interferometric instrument called PIONIER has recently been installed at VLTI. It provides improved imaging capabilities together with high precision.
Aims. We search for low-mass companions around a few bright stars using different strategies, and determine the dynamic range currently reachable with PIONIER.
Methods. Our method is based on the closure phase, which is the most robust interferometric quantity when searching for faint companions. We computed the χ2 goodness of fit for a series of binary star models at different positions and with various flux ratios. The resulting χ2 cube was used to identify the best-fit binary model and evaluate its significance, or to determine upper limits on the companion flux in case of non-detections.
Results. No companion is found around Fomalhaut, tau Cet and Regulus. The median upper limits at 3σ on the companion flux ratio are respectively of 2.3 × 10-3 (in 4 h), 3.5 × 10-3 (in 3 h) and 5.4 × 10-3 (in 1.5 h) on the search region extending from 5 to 100 mas. Our observations confirm that the previously detected near-infrared excess emissions around Fomalhaut and tau Cet are not related to a low-mass companion, and instead come from an extended source such as an exozodiacal disk. In the case of del Aqr, in 30 min of observation, we obtain the first direct detection of a previously known companion, at an angular distance of about 40 mas and with a flux ratio of 2.05 × 10-2 ± 0.16 × 10-2. Due to the limited u,v plane coverage, its position can, however, not be unambiguously determined.
Conclusions. After only a few months of operation, PIONIER has already achieved one of the best dynamic ranges world-wide for multi-aperture interferometers. A dynamic range up to about 1:500 is demonstrated on unresolved targets, but significant improvements are still required to reach the ultimate goal of directly detecting hot giant extrasolar planets.
Key words: techniques: interferometric / binaries: close / stars: low mass / brown dwarfs / planetary systems
© ESO, 2011