The Dust Properties of Two Hot R Coronae Borealis Stars and a Wolf-Rayet Central Star of a Planetary Nebula: In Search of a Possible Link

We present new Spitzer /IRS spectra of two hot R Coronae Borealis (RCB) stars, one in the Galaxy, V348 Sgr, and one lying in the Large Magellanic Cloud, HV 2671. These two objects may constitute a link between the RCB stars and the late Wolf–Rayet ([WCL]) class of central stars of planetary nebulae (CSPNe), such as CPD −56° 8032, that has little or no hydrogen in their atmospheres. HV 2671 and V348 Sgr are members of a rare subclass that has significantly higher effective temperatures than most RCB stars, but shares the traits of hydrogen deficiency and dust formation that define the cooler RCB stars. The [WC] CSPN star, CPD −56° 8032, displays evidence of dual-dust chemistry showing both polycyclic aromatic hydrocarbons (PAHs) and crystalline silicates in its mid-IR spectrum. HV 2671 shows strong PAH emission but no sign of having crystalline silicates. The spectrum of V348 Sgr is very different from that of CPD −56° 8032 and HV 2671. The PAH emission seen strongly in the other two stars is not present. Instead, the spectrum is dominated by a broad emission centered at about 8.2 μm. This feature is not identified with either PAHs or silicates. Several other cool RCB stars, novae, and post-asymptotic giant branch stars show similar features in their IR spectra. The mid-IR spectrum of CPD −56° 8032 shows emission features that may be associated with C 60 . The other two stars do not show evidence of C 60 . The different nature of the dust around these stars does not help us in establishing further links that may indicate a common origin. HV 2671 has also been detected by Herschel /PACS and SPIRE. V348 Sgr and CPD −56° 8032 have been detected by AKARI/Far-Infrared Surveyor. These data were combined with Spitzer , IRAS , Two Micron All Sky Survey, and other photometry to produce their spectral energy distributions (SEDs) from the visible to the far-IR. Monte Carlo radiative transfer modeling was used to study the circumstellar dust around these stars. HV 2671 and CPD −56° 8032 require both a flared inner disk with warm dust and an extended diffuse envelope with cold dust to fit their SEDs. The SED of V348 Sgr can be fit with a much smaller disk and envelope. The cold dust in the extended diffuse envelopes inferred around HV 2671 and CPD −56° 8032 may consist of interstellar medium swept up during mass-loss episodes.