VLTI observations of the dust geometry around R Coronae Borealis stars

TL;DR

This study uses high-resolution interferometry to investigate dust structures around R Coronae Borealis stars, revealing asymmetric dust distributions likely formed by random clump ejections, with implications for understanding their dust formation processes.

Contribution

First high-angular resolution interferometric observations of RCB stars' dust environments, providing insights into dust geometry and formation mechanisms.

Findings

Asymmetric dust structures observed in RY Sgr and possibly in V CrA and V854 Cen.

Dust likely forms in randomly ejected clumps creating a spherical distribution over time.

Current data insufficient to determine preferred dust ejection planes.

Abstract

We are investigating the formation and evolution of dust around the hydrogen-deficient supergiants known as R Coronae Borealis (RCB) stars. We aim to determine the connection between the probable merger past of these stars and their current dust-production activities. We carried out high-angular resolution interferometric observations of three RCB stars, namely RY Sgr, V CrA, and V854 Cen with the mid-IR interferometer, MIDI on the VLTI, using two telescope pairs. The baselines ranged from 30 to 60 m, allowing us to probe the dusty environment at very small spatial scales (~ 50 mas or 400 stellar radii). The observations of the RCB star dust environments were interpreted using both geometrical models and one-dimensional radiative transfer codes. From our analysis we find that asymmetric circumstellar material is apparent in RY Sgr, may also exist in V CrA, and is possible for V854 Cen. Overall, we find that our observations are consistent with dust forming in clumps ejected randomly around the RCB star so that over time they create a spherically symmetric distribution of dust. However, we conclude that the determination of whether there is a preferred plane of dust ejection must wait until a time series of observations are obtained.