Spitzer Space Telescope - General Observer Proposal #30029


The Shells of R Coronae Borealis Stars:  Fossil Planetary Nebulae?

Principal Investigator: Geoffrey Clayton
           Institution: Lousiana State University

Co-Investigators: 
Karl Gordon, Steward Observatory                                      
Orsola De Marco, American Museum of Natural History                   
Warrick Lawson, Australian Defence Force Academy                      
Howard Bond, STScI                                                    
Ben Sugerman, STScI                                                   

Science Category: evolved stars/pn/sne 
 Observing Modes: MipsScan 
  Hours Approved: 7.2 

Abstract:
IRAS observations detected extended arcminute-size shells around five
R Coronae Borealis (RCB) stars but due to its poor spatial resolution
no detailed maps of these shells exist.  R CrB, itself, has an 18'
shell at 100  microns.  Twenty years later, this shell is still
unexplained.  But the shell around R CrB and other RCB stars hold
clues to the evolutionary history of these stars.  The RCB stars are
rare hydrogen-deficient and carbon-rich supergiants.  Their rarity may
stem either from them being in an extremely rapid phase of evolution,
or from them being in an evolutionary phase that most stars are able
to avoid.  Two contending evolutionary scenarios have been suggested
to account for RCB stars:  they arise from a merger of two white
dwarfs, or from a single star which has evolved to become a
planetary-nebula central star and only then undergoes a final helium
shell flash which returns it to a cool supergiant configuration.  The
proposed MIPS maps will provide images of the RCB IRAS shells, which
are fossil records of previous evolutionary stages of these stars.
For instance, an old Planetary Nebula shell would no longer be ionized
and is now seen in IR emission from dust.  Understanding the RCB stars
is a key test of for any theory which aims to explain the evolution of
post AGB stars and hydrogen deficiency.