2014A&A...569L...4C

The hydrogen-deficient supergiants known as R Coronae Borealis (RCB) stars might be the result of a double-degenerate merger of two white dwarfs (WDs), or a final helium shell flash in a planetary nebula central star. In this context, any information on the geometry of their circumstellar environment and, in particular, the potential detection of elongated structures, is of great importance. We obtained near-IR observations of V854 Cen with the AMBER recombiner located at the Very Large Telescope Interferometer (VLTI) array with the compact array (B≤35m) in 2013 and the long array (B≤140m) in 2014. At each time, V854 Cen was at maximum light. The H- and K-band continua were investigated by means of spectrally dependant geometric models. These data were supplemented with mid-IR VISIR/VLT images. A dusty slightly elongated overdensity is discovered both in the H- and K-band images. With the compact array, the central star is unresolved (Θ≤2.5mas), but a flattened dusty environment of 8x11 mas is discovered whose flux increases from about ∼20% in the H band to reach about ∼50% at 2.3µm, which indicates hot (T∼1500K) dust in the close vicinity of the star. The major axis is oriented at a position angle (PA) of 126±29°. Adding the long-array configuration dataset provides tighter constraints on the star diameter (Θ≤1.0mas), a slight increase of the overdensity to 12x15mas and a consistent PA of 133±49°. The closure phases, sensitive to asymmetries, are null and compatible with a centro-symmetric, unperturbed environment excluding point sources at the level of 3% of the total flux in 2013 and 2014. The VISIR images exhibit a flattened aspect ratio at the 15-20% level at larger distances (∼1") with a position angle of 92±19°, marginally consistent with the interferometric observations. This is the first time that a moderately elongated structure has been observed around an RCB star. These observations confirm the numerous suggestions for a bipolar structure proposed for this star in the literature, which were mainly based on polarimetric and spectroscopic observations.