The mid-infrared aliphatic bands associated with complex hydrocarbons

Abstract

Context. The mid-infrared emission features commonly attributed to polycyclic aromatic hydrocarbons (PAHs) vary in profile and peak position. These profile variations form the basis of their classification: Classes A, B, C reflect profiles with increasing central wavelength while Class D has similar central wavelength as Class B but a similar broad shape as Class C. A well-known empirical relationship exists between the central wavelength of these emission features in circumstellar environments and the effective temperature of their central stars. One posited explanation is that the presence of aliphatic hydrocarbons contributes to the variations in the shapes and positions of the features. Aims. We aim to test this hypothesis by characterising the aliphatic emission bands at 6.9 and 7.25 µm and identifying relationships between these aliphatic bands and the aromatic features. Methods. We have examined 5–12 µm spectra of 63 astronomical sources exhibiting hydrocarbon emission which have been observed by ISO/SWS, Spitzer/IRS, and SOFIA/FORCAST. We measured the intensities and central wavelengths of the relevant features and classified the objects based on their 7–9 µm emission complex. We examined correlations between the intensities and central wavelengths of the features, both aliphatic and aromatic, and investigated the behaviour of the aliphatic features based on the object type and hydrocarbon emission class. Results. The presence of the 6.9 and 7.25 µm aliphatic bands depends on (aromatic) profile class, with aliphatic features detected in all Class D sources, 26% of the Class B sources, and no Class C sources. The peak position of the aliphatic features varies, with more variability seen in Class B sources than Class D sources, mimicking the degree of variability of the aromatic features in these classes. Variations are observed within Class D 6–9 µm profiles, but are significantly smaller than those in Class B. While a linear combination of Classes B and C emission can reproduce the Class D emission features at 6.2 and 7.7–8.6 µm, it cannot reproduce the aliphatic bands or the 11–14 µm hydrocarbon features. A correlation is found between the intensities of the two aliphatic bands at 6.9 and 7.25 µm, and between these aliphatic features and the 11.2 µm feature, indicating that conditions required for a population of neutral hydrocarbon particles are favourable for the presence of aliphatic material. A comparison with experimental data suggests a different assignment for the aliphatic 6.9 µm band in Class D and (some) Class B environments. Finally, we discuss evolutionary scenarios between the different classes.

Structure and evolution of interstellar carbonaceous dust. Insights from the laboratory

“…Class D spectra are only observed in carbon-rich post-AGB objects. The bands from interstellar sources (class A) are highly aromatic, whereas circumstellar sources, especially those of class D, present a higher aliphatic component (Jensen et al, 2022). Class A emission is ubiquitous and is attributed to the fluorescence mechanism commented on above, which is independent of the photon flux and, thus, of the distance to the photon source.…”

“…Band profile modifications are especially prominent in the 6-9 μm region. The sources are classified into A, B, C and D (Peeters et al, 2002;van Diedenhoven et al, 2004;Matsuura et al, 2014;Jensen et al, 2022). Examples of these classes are shown in Figure 4.…”

Photocleavage of Aliphatic C–C Bonds in the Interstellar Medium

“…On the other hand, the UIEs are accompanied by IR emission bands at 3.4, 6.85, and 7.25 μm, which are due to aliphatic hydrocarbons (Pinho & Duley 1995;Yang et al 2013;Jensen et al 2022;Yang & Li 2023), and other carriers different from free PAHs have been proposed for the UIEs. These are usually comprised of a mixture of aromatic and aliphatic hydrocarbons and include mixed aromatic and aliphatic organic molecules (Kwok & Zhang 2011, 2013 as well as hydrogenated amorphous carbon (HAC) nanoparticles (Duley & Williams 1988;Jones et al 1990;Jones 2012c;Jones & Habart 2015;Jones & Ysard 2022).…”