| Description |
The unexpected discovery that the R Coronae Borealis (RCB) stars have isotopic abundances of 18O which are up to 500 times greater than those seen in other stars has increased the likelihood that these rare hydrogen-deficient carbon stars are the result of a double degenerate white dwarf (WD) merger (DD) rather than a final helium shell flash (FF). But other pieces of the puzzle have yet to be explained. For instance, a few RCB stars, including R CrB itself, show enhanced Li abundances, which favors the FF scenario. Also, IRAS images show that several RCB stars possess large dust shells which are consistent with fossil Planetary Nebula (PN) shells where the gas is no longer ionized. If these are PN shells, then that would point strongly toward the FF scenario. But other explanations for the dust shells include the FF outburst itself and new dust from the recent RCB phase. In addition, newly acquired Herschel/Spire images have called into question the reality of the shells themselves. The much improved spatial resolution over previous IRAS and MIPS images shows that some or all of the R CrB 100-micron shell is due to a cluster of galaxies lying directly behind the star. It is imperative that new high spatial resolution PACS and Spire images be obtained to verify the existence of these RCB star dust shells, as well as to analyze the morphology of the shells to deduce their nature and help decide between the DD and the FF scenarios. Furthermore, the new Herschel images, covering the wavelength range 70-500 microns, will be used together with archival IRAS, Spitzer and other datasets to produce SEDs for each star which will be modeled using Monte Carlo radiative transfer codes to determine the total dust mass, and total mass loss overthe lifetime of the stars. It is a very exciting prospect that the RCB stars could be shown to be the product of a WD merger. This small group of stars would become even more interesting as they would be low-mass analogies to Type Ia supernovae. |
