New R Coronae Borealis stars discovered in OGLE-III Galactic bulge fields from their mid- and near-infrared properties

Abstract

Context. An R Coronae Borealis (RCB) star is a rare type of supergiant star that is increasingly suspected to be the evolved merger product of two white dwarfs. Lately, many RCBs have been found distributed in a thin disk structure embedded inside the Galactic bulge. This unexpectedly high density may provide additional insight into the nature and age of RCB stars. Aims. We apply and test a new technique to find RCB stars based on their particular infrared emission due to circumstellar shell. We attempt to demonstrate that RCB stars can be identified without performing a light curve analysis, which would simplified the search outside optically monitored fields. Methods. We selected RCB candidates based on their near-infrared excess and on their particular mid-infrared shells emission, using photometric data from the 2MASS and Spitzer/GLIMPSE surveys. Each candidates OGLE light curves were then visually inspected and we selected for spectroscopy follow-up those that underwent large and rapid declines. Results. We discover two new R Coronae Borealis stars, but also indicate four new possible candidates. We emphasize that all of the 7 known RCB stars located in both the Spitzer/GLIMPSE and OGLE-III fields were also re-discovered, which illustrates the high efficiency of our analysis. Conclusions. The proposed new technique to find RCB stars has been successful. It can now be extended to larger areas, in particular where the interstellar extinction is too high to be monitored by optical microlensing surveys, such as the inner part of the Galactic bulge.