A near-infrared interferometric survey of debris disk stars. I. Probing the hot dust content around ɛ Eridani and τ Ceti with CHARA/FLUOR

Context: The quest for hot dust in the central region of debris disks requires high resolution and high dynamic range imaging. Near-infrared interferometry is a powerful means to directly detect faint emission from hot grains.
Aims: We probed the first 3 AU around τ Ceti and ɛ Eridani with the CHARA array (Mt Wilson, USA) in order to gauge the 2 μm excess flux emanating from possible hot dust grains in the debris disks and to also resolve the stellar photospheres.
Methods: High precision visibility amplitude measurements were performed with the FLUOR single mode fiber instrument and telescope pairs on baselines ranging from 22 to 241 m of projected length. The short baseline observations allow us to disentangle the contribution of an extended structure from the photospheric emission, while the long baselines constrain the stellar diameter.
Results: We have detected a resolved emission around τ Cet, corresponding to a spatially integrated, fractional excess flux of 0.98±0.21 × 10^-2 with respect to the photospheric flux in the K'-band. Around ɛ Eri, our measurements can exclude a fractional excess of greater than 0.6× 10^-2 (3σ). We interpret the photometric excess around τ Cet as a possible signature of hot grains in the inner debris disk and demonstrate that a faint, physical or background, companion can be safely excluded. In addition, we measured both stellar angular diameters with an unprecedented accuracy: Θ_LD(τ Cet)= 2.015 ± 0.011 mas and Θ_LD(ɛ Eri)=2.126 ± 0.014 mas.