The inner debris disc of Vega as seen by CHARA/FLUOR

Using the FLUOR near-infrared beam combiner installed at the CHARA Array (Mt Wilson, CA), we have obtained high-precision interferometric measurements of Vega, a prototypical debris-disk star known to be surrounded by large amounts of cold dust at 80-100 AU in a ring-like structure. The combination of short and long baselines has allowed us to separately resolve the stellar photosphere and its immediate neighbourhood inside by the FLUOR field-of-view of 1’’ (~8 AU at the distance of Vega). Our observations show a significant deficit in visibility at short baselines with respect to the expected visibility of a simple uniform disk stellar model, suggesting the presence of an extended source of emission around Vega. We propose that the visibility deficit is most likely due to the presence of hot circumstellar dust in the inner part of the debris disk, with a flux ratio of 1.29+/-0.19% between the integrated dust emission and the stellar photosphere in K band (2.15 microns). Using this piece of information together with archival photometric and interferometric data in the near- and mid-infrared, we derive the expected physical properties of the dust grains by modelling their infrared Spectral Energy Distribution. Small, highly refractive grains are suspected to be located at only a few 0.1 AU from Vega, with a total mass of about 1e-7 Earth mass. Their presence is thought to be an indicator of major dynamical perturbations currently ongoing in the Vega system.