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The RCB star V854 Cen is surrounded by a hot dusty shell

Aims. The hydrogen-deficient supergiants known as R Coronae Borealis Stars might be the result of a double degenerate merger of two white dwarfs (WDs), or a final helium shell flash in a PN 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 \object{V854\,Cen} with the {{\sc AMBER}} recombiner located at the Very Large Telescope Interferometer ({{\sc VLTI}}) array with the compact array (B≤35m) in 2013 and the long array (B≤140m) in 2014. At each time, \object{V854\,Cen} was at maximum light. The H- and K-band continua were investigated by means of spectrally-dependant geometric models. These data are supplemented with mid-IR {{\sc VISIR}}/VLT images. Results. A dusty slightly elongated over-density is discovered both in the H and K-band images. With the compact array, the central star is unresolved (Θ≤2.5\,mas), but a flattened dusty environment of 8×11 mas is discovered whose flux raises from about ∼20\% in the H band to reach about ∼50\% at 2.3\micron, indicative of the presence of hot (T∼1500\,K) dust in the close vicinity of the star. The major axis is oriented at a position angle (P.A.) of 126±29\deg. Adding the long array configuration dataset provides tighter constraints on the star diameter (Θ≤1.0\,mas), a slight increase of the over-density representing 12×15 mas and a consistent P.A. of 133±49\deg. 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 at larger distances (∼1\arcsec) a flattened aspect ratio at the 15-20\% level with a position angle of 92±19\deg, 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 this star for a bipolar structure proposed in the literature, mainly based on polarimetric and spectroscopic observations.

Authors: Olivier Chesneau (LAGRANGE), Florentin Millour (LAGRANGE), Orsola De Marco, S.N. Bright (LAGRANGE), Alain Spang (LAGRANGE), D. P. K. Banerjee (PRL), N. M. Ashok (PRL), T. Kaminski (MPIFR), John P. Wisniewski, Anthony Meilland (LAGRANGE), Eric Lagadec (LAGRANGE)

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