Dust Around R Coronae Borealis Stars. Ii. Infrared Emission Features in an H-Poor Environment

Abstract

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 ∼6-10 μm dust emission complex is seen in the RCBs showing an extreme H-deficiency. A secondary and much weaker ∼11.5-15 μm broad emission feature is detected in a few RCBs with the strongest ∼6-10 μm dust complex. The Spitzer infrared spectra reveal for the first time the structure within the ∼6-10 μm dust complex, showing the presence of strong C-C stretching modes at ∼6.3 and 8.1 μm as well as of other dust features at ∼5.9, 6.9, and 7.3 μm, 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 μm feature, which is blueshifted in those RCBs showing also the UIRs, i.e., the RCBs with the smallest H deficiency.

Carbon dust in the evolved born-again planetary nebulae A30 and A78

“…The Spitzer IRS spectra of A30 and A78 are almost identical, which reflects the similar evolutionary stage and born-again origin discussed in previous works (e.g., Fang et al 2014;Toalá et al 2015). The low-resolution IRS spectra of A 30 and A 78 presented in the top left panel of Figure 5 show a number of broad features, which were thoroughly discussed by García-Hernández et al (2013). Those authors compared the spectra of A 30 and A 78 with those of RCBs.…”

“…All IRS observations were processed using the CUbe Builder for IRS Spectra Maps (Smith et al 2007). We note that the low-resolution spectra of A 30 and A 78 were analyzed by García-Hernández et al (2013) in comparison with the extremely H-deficient R Coronae Borealis (RCB) stars to discuss the C-rich origins of these sources. Those authors were mainly interested in the broad dust features below 10 𝜇m (the RCB stars generally display featureless spectra at longer wavelengths) and did not analyze the high-resolution Spitzer spectra.…”

“…Those authors compared the spectra of A 30 and A 78 with those of RCBs. After an appropriate continuum subtraction technique, García-Hernández et al ( 2013) demonstrated that RCB objects exhibited the presence of broad features at 5.9, 6.4, 7.3 and 8.0 𝜇m whilst the bornagain PNe only present the latter three broad features (see Figure 13 in García-Hernández et al 2013). These authors concluded that the presence of such broad features can be attributed to amorphous carbon formed in H-deficient environments.…”

Carbon-Rich Dust Past the Asymptotic Giant Branch: Aliphatics, Aromatics, and Fullerenes in the Magellanic Clouds

“…Their spectra typically resemble a Planck curve of a few hundred K, due to emission from warm amorphous carbon dust (see Kraemer et al 2005). The recent work by García-Hernández et al (2013) reveals PAH-related structures in the spectra of several Galactic R CrB stars observed by the IRS.…”

The infrared view of dust and molecules around V4334 Sgr (Sakurai's Object): a 20-year retrospective

“…The observational (García-Hernández et al 2013) and laboratory evidence (Grishko & Duley 2002) seem therefore to point to the existence of a cooler, hydrogen-rich carbon dust shell outside the dust shell that is prominent in the near-and mid-IR. Evans et al (2006) reported the detection of a number of small hydrocarbon molecules in the Spitzer spectrum of Sakurai; these are listed in Table 4 and illustrated in Fig.…”