Dust around R Coronae Borealis stars: I. Spitzer/IRS observations
  Spitzer/IRS spectra from 5 to 37 um for a complete sample of 31 R Coronae
Borealis stars (RCBs) are presented. These spectra are combined with optical
and near-infrared photometry of each RCB at maximum light to compile a spectral
energy distribution (SED). The SEDs are fitted with blackbody flux
distributions and estimates made of the ratio of the infrared flux from
circumstellar dust to the flux emitted by the star. Comparisons for 29 of the
31 stars are made with the IRAS fluxes from three decades earlier: Spitzer and
IRAS fluxes at 12 um and 25 um are essentially equal for all but a minority of
the sample. For this minority, the IRAS to Spitzer flux ratio exceeds a factor
of three. The outliers are suggested to be stars where formation of a dust
cloud or dust puff is a rare event. A single puff ejected prior to the IRAS
observations may have been reobserved by Spitzer as a cooler puff at a greater
distance from the RCB. RCBs which experience more frequent optical declines
have, in general, a circumstellar environment containing puffs subtending a
larger solid angle at the star and a quasi-constant infrared flux. Yet, the
estimated subtended solid angles and the blackbody temperatures of the dust
show a systematic evolution to lower solid angles and cooler temperatures in
the interval between IRAS and Spitzer. Dust emission by these RCBs and those in
the LMC is similar in terms of total 24 um luminosity and [8.0]-[24.0] color
index.
