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

O Chesneau,A Spang,S N Bright,F Millour,O De Marco,E Lagadec,W J De Wit,D Mékarnia

doi:10.1051/0004-6361/201424173

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$\leq$35m) in 2013 and the long array (B$\leq$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. 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 ($\Theta\leq2.5$\,mas), but a flattened dusty environment of $8 \times 11$ mas is discovered whose flux increases from about $\sim$20% in the $H$ band to reach about $\sim$50% at 2.3$\micron$, which indicates hot (T$\sim$1500\,K) dust in the close vicinity of the star. The major axis is oriented at a position angle (P.A.) of 126$\pm$29$\deg$. Adding the long-array configuration dataset provides tighter constraints on the star diameter ($\Theta\leq1.0$ mas), a slight increase of the overdensity to $12 \times 15$ mas and a consistent P.A. of 133$\pm$49$\deg$. The closure phases, sensitive to asymmetries, are null and compatible with a centro-symmetric, unperturbed environment excluding point sources at the level of 3% of the total flux in 2013 and 2014. The VISIR images exhibit a flattened aspect ratio at the 15-20% level at larger distances ($\sim$1$\arcsec$) with a position angle of 92$\pm$19$\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 a bipolar structure proposed for this star in the literature, which were mainly based on polarimetric and spectroscopic observations.

Highlights

The R Coronae Borealis (RCB) stars are rare hydrogendeficient carbon-rich supergiants, best known for their spectacular declines in brightness at irregular intervals (Clayton 2012; De Marco et al 2002)
Two evolutionary scenarios have been suggested for producing an RCB star, a double-degenerate merger of two white dwarfs (WD), or a final helium-shell flash in a planetary nebula central star
A moderately flattened dusty environment was discovered around V854 Cen

Summary

Introduction

The R Coronae Borealis (RCB) stars are rare hydrogendeficient carbon-rich supergiants, best known for their spectacular declines in brightness at irregular intervals (Clayton 2012; De Marco et al 2002). The discovery that RCB stars have large amounts of 18O is interpreted as a serious argument in favor of the merger scenario (Clayton et al 2011, 2007). Extensive polarimetric observations have been reported (Kameswara Rao & Raveendran 1993; Whitney et al 1992). Another striking aspect of this star is the fast wind that was spectroscopically detected, which reaches several hundreds of km s−1 (Clayton et al 2013, 2003, 1993; Lawson et al 1999; Kameswara Rao & Lambert 1993; Lawson 1992). The bright point-source is clearly surrounded by a fainter structure with a largest extent of ∼3

Analysis

Observations

Findings

Discussion