Direct imaging of a massive dust cloud around R Coronae Borealis

Abstract

We present recent polarimetric images of the highly variable star R CrB using ExPo and archival WFPC2 images from the HST. We observed R CrB during its current dramatic minimum where it decreased more than 9 mag due to the formation of an obscuring dust cloud. Since the dust cloud is only in the line-of-sight, it mimics a coronograph allowing the imaging of the star's circumstellar environment. Our polarimetric observations surprisingly show another scattering dust cloud at approximately 1.3 or 2000 AU from the star. We find that to obtain a decrease in the stellar light of 9 mag and with 30% of the light being reemitted at infrared wavelengths (from R CrB's SED) the grains in R CrB's circumstellar environment must have a very low albedo of approximately 0.07%. We show that the properties of the dust clouds formed around R CrB are best fitted using a combination of two distinct populations of grains size. The first are the extremely small 5 nm grains, formed in the low density continuous wind, and the second population of large grains (∼0.14 μm) which are found in the ejected dust clouds. The observed scattering cloud, not only contains such large grains, but is exceptionally massive compared to the average cloud.

Detection of an asymmetry in the envelope of the carbon Mira R Fornacis using VLTI/MIDI

“…Using the determined separation of 42 mas, and assuming that the blob moved with a constant velocity of 5 km s −1 over the last 30 years (a reasonable value for the inner envelope, predicted by dynamic model computations; Nowotny et al 2010), it is worthwhile to mention that the event of 1983 would correspond to the moment at which such a structure was formed near the surface of the star. Recent results from investigations of R CrB stars (Jeffers et al 2012) indicate the presence of dust clouds with different grain-sizes. Nevertheless, Bright et al (2011) did not detect any differential phase signature for these objects, arguing that the signatures of dust clumps are not strong enough to induce significant spectral signatures like the one we observed.…”

“…On the other hand, these obscuration events have similarities with those observed for R Coronae Borealis stars (R CrB; Clayton et al 2012). In the latter case, the third and the fourth scenario would be more likely explanations (Bright et al 2011;Jeffers et al 2012 This Letter presents high angular resolution observations in the mid-infrared that allow us to probe the dust-forming region of the nearby AGB star R For. Observations close in time, with different position angles and baseline lengths, the differential phase information, and the comparison with geometric models can help to distinguish possible deviations from a spherical structure.…”

Imaging diagnostics for transitional discs

“…It produces common observables for the study of the disc, such as SEDs or emission and scattering images, at the desired wavelengths and inclination angles. The code is widely used to compare theoretical models of discs with observations (see Mulders et al 2010Mulders et al , 2011Mulders & Dominik 2012;de Vries et al 2012;Canovas et al 2012;Jeffers et al 2012;Lombaert et al 2012;Honda et al 2012;Min et al 2012;Mulders et al 2013, for some examples).…”

Imaging the circumstellar environment of the young T Tauri star SU Aurigae

“…Both steps rely on the assumption that the central star is unpolarised, an assumption that has also been extensively used (e.g. Perrin et al 2008;Quanz et al 2011;Canovas et al 2012;Jeffers et al 2012). The resulting values of polarisation should be treated as lower limits of the true polarisation signal of the disk and circumstellar environment of SU Aur, as discussed by Min et al (2012).…”