AN INTERFEROMETRIC STUDY OF THE FOMALHAUT INNER DEBRIS DISK. I. NEAR-INFRARED DETECTION OF HOT DUST WITH VLTI/VINCI

The innermost parts of dusty debris disks around main sequence stars are currently poorly known due to the high contrast and small angular separation with their parent stars. Using near-infrared interferometry, we aim to detect the signature of hot dust around the nearby A4V star Fomalhaut, which has already been suggested to harbor a warm dust population in addition to a cold dust ring located at about 140AU. Archival data obtained with the VINCI instrument at the VLTI are used to study the fringe visibility of the Fomalhaut system at projected baseline lengths ranging from 4m to 140m in the K band. A signiflcant visibility deflcit is observed at short baselines with respect to the expected visibility of the sole stellar photosphere. This is interpreted as the signature of resolved circumstellar emission, producing a relative ∞ux of 0:88%§0:12% with respect to the stellar photosphere. While our interferometric data cannot directly constrain the morphology of the excess emission source, complementary data from the literature allow us to discard an ofi-axis point-like object as the source of circumstellar emission. We argue that the thermal emission from hot dusty grains located within 6AU from Fomalhaut is the most plausible explanation for the detected excess. Our study also provides a revised limb-darkened diameter for Fomalhaut (µLD = 2:223 § 0:022mas), taking into account the efiect of the resolved circumstellar emission. Subject headings: Circumstellar matter | techniques: interferometric | stars: individual (Fomalhaut)

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