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

Research School of Astronomy and Astrophysics, Australian National University, Cotter Rd, Weston Creek, ACT 2611, Australia
e-mail: tisserand@mso.anu.edu.au

Received: 11 August 2011
Accepted: 24 November 2011

Abstract

Context. R Coronae Borealis stars (RCBs) are hydrogen-deficient and carbon-rich supergiant stars. They are very rare, with only  ~50 actually known in our Galaxy. Interestingly, RCBs are strongly suspected of being the evolved merger product of two white dwarfs and could therefore be an important tool for understanding 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.

Aims. It is crucial to increase the number of known RCBs significantly. With an absolute magnitude M_V ~  −5 and a bright/hot circumstellar shell made of amorphous carbon grains, RCBs are so distinctive that we should nowadays be able to find them everywhere in our Galaxy using publicly available catalogues. In the optical, the search is difficult because RCBs are known to undergo unpredictable photometric declines; however, mono-epoch mid-infrared data can help us to discriminate RCBs among other dust-producing stars. The aim is to produce from the mid-infrared WISE and near-infrared 2MASS catalogues a new catalogue of reasonable size, enriched with RCB stars.

Methods. Colour–colour cuts used on all stars detected are the main selection criteria. The selection efficiency was monitored using the 52 known RCBs located in the sky area covered by the WISE first preliminary data release.

Results. It has been found that selection cuts in mid-infrared colour–colour diagrams are a very efficient method of distinguishing RCBs from other stars. An RCB enriched catalogue made of only 1602 stars was produced, with a high detection efficiency of about 77%. Spectral energy distributions of 49 known RCBs and 5 known HdCs are also presented with estimates of their photosphere and circumstellar shell temperatures.

Conclusions. 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 towards significantly increasing the number of known RCB stars and therefore better understanding their origin.