Aims: 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.
Methods: 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 = 35 m) in 2013 and the long array (B = 140 m) 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.
Results: A dusty slightly elongated overdensity is discovered both in the H- and K-band images. With the compact array, the central star is unresolved (T = 2.5 mas), but a flattened dusty environment of 8 × 11 mas is discovered whose flux increases from about 20% in the H band to reach about ∼50% at 2.3μm, which indicates hot (T ∼ 1500 K) 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 (T = 1.0 mas), a slight increase of the overdensity to 12 × 15 mas 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.
Conclusions: 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.