Dust Around R Coronae Borealis Stars. II. Infrared Emission Features In An H-Poor Environment

Residual Spitzer/Infrared Spectrograph spectra for a sample of 31 R Coronae Borealis (RCB) stars are presented and discussed in terms of narrow emission features superimposed on the quasi-blackbody continuous infrared emission. A broad similar to 6-10 mu m dust emission complex is seen in the RCBs showing an extreme H-deficiency. A secondary and much weaker similar to 11.5-15 mu m broad emission feature is detected in a few RCBs with the strongest similar to 6-10 mu m dust complex. The Spitzer infrared spectra reveal for the first time the structure within the similar to 6-10 mu m dust complex, showing the presence of strong C-C stretching modes at similar to 6.3 and 8.1 mu m as well as of other dust features at similar to 5.9, 6.9, and 7.3 mu m, which are attributable to amorphous carbonaceous solids with little or no hydrogen. The few RCBs with only moderate H-deficiencies display the classical "unidentified infrared bands (UIRs)" and mid-infrared features from fullerene-related molecules. In general, the characteristics of the RCB infrared emission features are not correlated with the stellar and circumstellar properties, suggesting that the RCB dust features may not be dependent on the present physical conditions around RCB stars. The only exception seems to be the central wavelength of the 6.3 mu m feature, which is blueshifted in those RCBs showing also the UIRs, i.e., the RCBs with the smallest H deficiency.