More rare supergiant stars on our Galaxy’s map
Hydrogen-deficient and carbon-rich, with an atmosphere of 99% helium, it is assumed that the so-called R Coronae Borealis stars (RCBs) are the result of CO and He white dwarves merging. Only 55 of these stars used to be known in our Galaxy but a new publication by CAASTRO researcher Patrick Tisserand at the ANU in Canberra, along with his overseas colleagues, has now boosted this number to 76 in the Milky Way, plus 22 in the Magellanic Clouds.
The team used the “ASAS-3 south” (All Sky Automated Survey, located at Las Campanas Observatory) optical dataset – resulting from observations made between 2000 and 2010 – to identify candidate stars. In their analysis, the team applied four independent techniques that exploited different properties, such as light curve variability, or drew on different candidate catalogues. As the final step of their follow-up, they obtained spectroscopic measurements of 104 RCB candidates with the Wide Field Spectrograph at Siding Spring Observatory in New South Wales. The spectroscopic success rate was ~90% for bright RCBs (magnitude ~13, corresponding to distances of ~20 kiloparsec), and the team also confirmed two previously known RCB candidates.
It appears as if RCBs are mainly located in the Galactic bulge. A few of the new discoveries reveal high infra-red excesses which means that they have a dusty shell surrounding them. In addition to their analysis of RCBs, the researchers also looked for so-called DY Per stars, carbon-rich supergiants which might be the cooler counterpart of RCBs; they found two new candidate DY Per stars in the ASAS-3 data.