Introduction
The merging process of two white dwarfs (WDs) is a rare astrophysical phenomenon that concludes the life of a close stellar binary system. This process is the source of a very interesting new class of stars or phenomena that are much studied nowadays. For example, the fusion of two helium white dwarfs, i.e. low mass WDs (purple area, total mass lower than 0.5 solar mass), gives birth to a new type of hot sub-dwarf star (called SdB and SdO), which allows us to explain the excess UV emission observed in elliptical galaxies (Podsiadlowski et al. 2008). At the other side of the mass scale, the fusion of two massive CO white dwarfs (orange and green area, total mass higher than 1.44 solar masses) can generate a thermonuclear explosion like the most famous Supernovae type Ia (SnIa) (Sim et al. 2010; Pakmor et al. 2012) or create neutron stars after a gravitational collapse (AIC) (Nomoto & Kondo 1991). In the intermediary mass range, when the total mass of the system is lower than the Chandrasekhar mass (i.e. corresponding to the fusion of a helium and a CO white dwarf), the final product is still not known despite the fact that synthetic stellar populations models predict a fusion rate similar to the low and the high mass range: about one fusion every 200 years per galaxy.
The best candidates are a class of supergiant stars called R Coronae borealis (RCB) (Webbink 1984; Jeffery et al. 2011; Shen et al. 2012; Staff et al. 2012). The duration of the lifetime of these newly created objects is estimated to be in the order of 100 thousand years (Saio & Jeffery, 2002). They are known to possess a peculiar atmosphere made of Helium at 99%, poor in Hydrogen but rich in Carbon. RCB stars are rare. Only 76 are actually known in our galaxy and 22 in the Magellanic Clouds (Tisserand et al., 2013). This rarity, their clear peculiar atmospheric chemical abundances and the presence in high quantity of unexpected isotopic elements, such as oxygen 18, strengthen RCBs’ status as strong candidates for being the product of white dwarf mergers in the intermediary mass range.
RCB stars are then thought to be born from the ashes (i.e. WDs) of classical stars. Their large distribution in absolute luminosity (-5 < Mv < -3.5 mag.) and temperatures (from 4000 to 10000 K) (see Tisserand, P. et al. 2009) also suits well the merger scenario. Indeed, the input WDs would have had a range in mass, which would explains the range in luminosity, and the RCBs would be observed at different ages after the merger phase, which would explains the range in temperature. During the lifetime of an RCB star, it is expected that the atmosphere gets smaller and by consequence hotter.
The goal of our research is understand the origin and the evolution path of RCB stars, to use them to probe the mass regime near the Chadrasekhar limit and therefore to test the hypothesis of the existence of sub-Chandrasekhar Supernovae SNIa via the double-degenerate scenario. Actually, the models show that supernovae type Ia explosion could occur when the mass of the system is a low as 1.1 solar mass. These explosions are by nature transitory phenomena and difficult to model due to the difficulty to obtain informations before the explosion: “double-degenerate”, “single-degenerate” scenario or a mixed of both ? In inverse, RCB stars are stable and bright objects. They can be studied more easily to in fine bring constraint to white dwarf merger models. A more precise spectroscopic study of their atmosphere show that there exists numerous anomalies concerning the abundance of elements such as Nitrogen, Oxygen, Fluor, Silicium, Sulfur, but also Lithium (Asplund, 2000 ; Pandey et al., 2006). These abundances are very different to classical stars crossing the AGB phase. All these particularities indicate that a cataclysmic event, that has either ejected or consume the original envelope rich in Hydrogen, has occurred.
Actually, our research is concentrating on finding new RCB stars in our Galaxy and the Magellanic Clouds to estimate their formation rate. Among more than 500 millions objects catalogued in the all sky infra-red surveys 2MASS and WISE, we have selected 2356 stars candidates. There stars need now to be follow-up spectroscopically to determine their nature. The list and their updated status is accessible on the main page. A description of the selection process of these candidates is given in an article: Tisserand et al. (2016).