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

Abstract

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 (Θ ≤ 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 (Θ ≤ 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.

Mid-infrared variations of R Coronae Borealis stars

“…In this simple picture, puffs are formed and expand away from the star, i.e., there is no mechanism for storing dust (and gas) near the star as in a circumstellar disk. Chesneau et al (2014) from interferometric observations of V854 Cen show that this star's dust is concentrated in an elongated structure which, as the authors note, may reflect a bipolar wind off the star. Quasi-stable dusty disks have been found as circumbinary features for some binary stars.…”

Unidentified emission features in the R Coronae Borealis star V854 Centauri

“…A bipolar geometry has been suggested in literature (e.g. Rao & Lambert 1993a;Chesneau et al 2014). If the other UFs are part of the same outflow, we would expect them to show a shift of ∼0.3 Å between their closest and furthest detection.…”

Olivier Chesneau's Work on Massive Stars

“…This kind of object can evolve rapidly up until a merger event, like e.g. the V838 Mon event (Chesneau et al 2014b). This is it on HR5171, because as O. Chesneau wrote in an email to all the co-authors in one of his typical enthusiastic messages, "Explaining all the developments of the HR 5171 A project would be a (too) long (and wonderful!)…”