Tracking down R Coronae Borealis stars from their mid-infrared WISE colours

Abstract

R Coronae Borealis stars (RCBs) are hydrogen-deficient and carbon-rich supergiant stars. They are very rare, as only $\sim 50$ are actually known in our Galaxy. Interestingly, RCBs are strongly suspected to be the evolved merger product of two white dwarfs and could therefore be an important tool to understand Supernovae type Ia in the double degenerate scenario. Constraints on the spatial distribution and the formation rate of such stars are needed to picture their origin and test it in the context of actual population synthesis results. To do so, it is crucial to increase significantly the number of known RCBs. With an absolute magnitude ${\mathrm{M}}_{\mathrm{V}}\sim -5$ and a bright/hot circumstellar shell made of amorphous carbon grains, RCBs are really distinctive stars. Mono-epoch mid-infrared data can help us to discriminate RCBs among other dust-producing stars. The aim is to produce from the WISE and 2MASS infrared catalogues a new catalogue of reasonable size, enriched with RCB stars. Colour-Colour cuts used on all stars detected are the main selection criteria. The selection efficiency was monitored using the 52 known RCBs. It has been found that selection cuts in mid-infrared colour-colour diagrams are a very efficient method of discriminating RCBs from other stars. An RCB enriched catalogue made of only 1602 stars, with a high detection efficiency of about 77%, was produced. Spectral energy distributions of 49 known RCBs and 5 known HdCs are also presented with estimates of their photosphere and circumstellar shell temperatures. The newly released WISE all sky catalogue has proven to be a valuable resource in finding RCB stars. Actual scenarios predict that between 100 and 500 RCBs exist in our Galaxy. The newly created RCB enriched catalogue is an important step forward to significantly increase the number of known RCB stars and therefore better understand their origin.

16 pages, 12 figures, Accepted in A&A

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