High-contrast optical imaging of companions: the case of the brown dwarf binary HD 130948 BC

¹ Instituto de Astrofisica de Canarias, C/ Via Lactea s/n, La Laguna, 38200 Tenerife , Spain
e-mail: labadie@iac.es
² Departamento de Astrofisica, Universidad de La Laguna, 38205 La Laguna, Tenerife, Islas Canarias, Spain
³ Universidad Politecnica de Cartagena, Campus Muralla del Mar, Cartagena, 30202 Murcia, Spain
⁴ Laboratoire de Physique Subatomique et de Cosmologie, 53 avenue des Martyrs, 38026 Grenoble, France
⁵ Max-Planck-Institut für Sonnensystemforschung, Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany
⁶ Consejo Superior de Investigaciones Cientificas, Spain

Received: 5 March 2010
Accepted: 27 September 2010

Abstract

Context. High-contrast imaging at optical wavelengths is limited by the modest correction of conventional near-IR optimized AO systems. We take advantage of new fast and low-readout-noise detectors to explore the potential of fast imaging coupled to post-processing techniques to detect faint companions of stars at small angular separations. Aims. We have focused on I-band direct imaging of the previously detected brown dwarf binary HD 130948 BC, attempting to spatially resolve the L2+L2 system considered as a benchmark for the determination of substellar objects dynamical masses. Methods. We used the lucky-imaging instrument FastCam at the 2.5-m Nordic Telescope to obtain quasi diffraction-limited images of HD 130948 with  ~0.1′′ resolution. In order to improve the detectability of the faint binary in the vicinity of a bright (I = 5.19 ± 0.03) solar-type star, we implemented a post-processing technique based on wavelet transform filtering of the image, which allows us to strongly enhance the presence of point-like sources in regions where the primary halo generally dominates. Results. We detect for the first time the binary brown dwarf HD 130948 BC in the optical band I with a SNR ~9 at 2.561′′ ± 0.007′′ (46.5 AU) from HD 130948 A and confirm in two independent datasets (2008 May 29 and July 25) that the object is real, as opposed to time-varying residual speckles. We do not resolve the binary, which can be explained by astrometric results posterior to our observations, which predict a separation below the telescope resolution. We reach a contrast of ΔI = 11.30 ± 0.11 at this distance, and estimate a combined magnitude for this binary I = 16.49 ± 0.11 and a I − J color of 3.29 ± 0.13. At 1′′, we reach a detectability 10.5 mag fainter than the primary after image post-processing. Conclusions. We obtain on-sky validation of a technique based on speckle imaging and wavelet-transform post-processing, which improves the high-contrast capabilities of speckle imaging. The I − J color measured for the BD companion is slightly bluer, but still consistent with what is typically found for L2 dwarfs (~3.4–3.6).