Spitzer Space Telescope - General Observer Proposal #30380


Looking for Dual Dust Chemistry in the Shells of Hot R Coronae
Borealis Stars

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                                                    
Don Pollacco, Queen's University Belfast                              
Michael Barlow, University College London                             

Science Category: evolved stars/pn/sne 
 Observing Modes: IrsPeakupImage IrsStare 
  Hours Approved: 1.0 

Abstract:
Detecting the presence of dual dust chemistry will establish a
definitive link between RCB stars and the [WC] central stars of PNe.
More importantly, given the established relationship between dual dust
chemistry and binarity, this program could be the first to firmly
determine a link between the RCB phenomenon and binarity, constituting
a large step towards resolving the origin of RCB stars and the
phenomenon of H-deficiency on the AGB.  The RCB stars are rare
hydrogen-deficient carbon-rich supergiants, consistent with being
post-AGB stars.  Evolutionary scenarios have been suggested including,
a merger of two stars, or a final helium shell flash.  While most RCB
stars are relatively cool (<7000  K), a few are significantly hotter
(~20,000  K).  Two of these stars, V348 Sgr and HV 2671 show
similarities to the [WC] central stars of PNe such as CPD -56 8032,
which shows RCB-like dust formation.  CPD -56 8032 shows emission
features of PAHs as well as crystalline silicates, indicating a dual
dust chemistry, i.e., the simultaneous presence of both C-rich and
O-rich dust.  These systems may all be binaries in which the O-rich
silicates are trapped in a disk as a result of a past mass transfer
event, with the C-rich particles being more widely distributed in the
nebula as a result of recent ejections of C-rich material.  We will
use the IRS to obtain spectra in the Short-Low and Long-Low modes to
look for evidence of a similar dual dust chemistry in the shells of
the two hot RCB stars, V348 Sgr and the LMC star, HV 2671.  In the
large nebula around V348 Sgr, we may be able to see a spatial
separation between the O-rich dust in a disk close to the star and the
C-rich dust spread through the nebula, which would strongly support
the binary/disk model for dual dust chemistry.  For a very small
expenditure of Spitzer Space Telescope time, we may be able to provide
for the first time a real connection between the RCB and [WC] central
stars.