The debris disk around the Vega-type star HD 34700 is detected in dust thermal emission from the near infrared (IR) to millimeter (mm) and submm wavelength range. Also detected is a distinct set of emission features at 3.3, 6.2, 7.7, 8.6, 11.3 and 12.7 µm, which are commonly attributed to polycyclic aromatic hydrocarbon (PAH) molecules. We model the observed dust IR spectral energy distribution (SED) and PAH emission features of the HD 34700 disk in terms of porous dust and astronomical-PAHs. Porous dust together with a mixture of neutral and ionized PAHs closely explains the dust IR SED and PAH emission features observed in the HD 34700 disk. Due to the stellar radiation pressure and Poynting-Robertson drag together with the photodissociation of PAHs, substantial removal of dust and PAHs has occurred in the disk, and continuous replenishment of these materials is required to maintain their current abundances. This implies that these materials are not primitive but secondary products probably originating from mutual collisions among planetesimals, asteroids, and comets.

Titanium carbide (TiC) nanocrystals were recently proposed as the carrier of the mysterious 21 µm emission feature observed in post-asymptotic giant branch stars, based on their close spectral match and the presolar nature of meteoritic TiC nanograins (which reveals their stellar ejecta origin). But we show in this Letter that the Kramers-Kronig dispersion relations, which relate the wavelength-integrated extinction cross section to the total dust mass, would impose a lower bound on the TiC mass. This Kramers-Kronig lower limit exceeds the maximum available TiC mass by a factor of at least ∼ 50, independent of the absolute value of the ultraviolet/visible absorptivity of nano TiC. The TiC model is therefore readily ruled out by the Kramers-Kronig physical principle.

Context. As the 4th most abundant element in the universe, carbon (C) is widespread in the interstellar medium (ISM) in various allotropic forms (e.g., fullerenes have been identified unambiguously in many astronomical environments, the presence of polycyclic aromatic hydrocarbon molecules in space has been commonly admitted, and presolar graphite as well as nanodiamonds have been identified in meteorites). As stable allotropes of these species, whether carbon nanotubes (CNTs) and their hydrogenated counterparts are also present in the ISM or not is unknown. Aims. We explore the possible routes for the formation of CNTs in the ISM and calculate their fingerprint vibrational spectral features in the infrared (IR). Methods. We study the hydrogen-abstraction/acetylene-addition (HACA) mechanism and investigate the synthesis of nanotubes using density functional theory (DFT). The IR vibrational spectra of CNTs and hydrogenated nanotubes (HNTs), as well as their cations, have also been obtained with DFT. Results. We find that CNTs could be synthesized in space through a feasible formation pathway. CNTs and cationic CNTs, as well as their hydrogenated counterparts, exhibit intense vibrational transitions in the IR. Their possible presence in the ISM could be investigated by comparing the calculated vibrational spectra with astronomical observations made by the Infrared Space Observatory, Spitzer Space Telescope, and particularly the upcoming James Webb Space Telescope.

Polycyclic aromatic hydrocarbon (PAH) molecules, ubiquitously seen in the interstellar medium (ISM) of our own and external galaxies, might have been incorporated into comets if they are formed from relatively unprocessed interstellar matter. The detection of PAHs in comets would be an important link between the ISM and comets. This review compiles our current knowledge on cometary PAHs, based on ground-based and space-borne observations of infrared vibrational and ultraviolet fluorescence spectra of comets, and laboratory analysis of interplanetary dust particles possibly of cometary origin and cometary samples returned to Earth by the Stardust spacecraft. The latter provided the most unambiguous evidence for the presence of PAHs in cometary nuclei.Comment: 15 pages, 8 figures; invited review article, in: "Deep Impact as a World Observatory Event -- Synergies in Space, Time, and Wavelength", Kaufl, H.U., & Sterken, C. (eds.), ESO Astrophys. Symp., in pres

Context. The 3.3 μm aromatic C–H stretching band of polycyclic aromatic hydrocarbon (PAH) molecules seen in a wide variety of astrophysical regions is often accompanied by a series of weak satellite bands at ∼3.4–3.6 μm. One of these sources, IRAS 21282+5050, a planetary nebula, also exhibits a weak band at ∼1.68 μm. While the satellite features at ∼3.4–3.6 μm are often attributed to the anharmonicities of PAHs, it is not clear whether overtones or combination bands dominate the 1.68 μm feature. Aims. In this work, we examine the anharmonic spectra of eight PAH molecules, including anthracene, tetracene, pentacene, phenanthrene, chrysene, benz[a]anthracene, pyrene, and perylene, to explore the origin of the infrared bands in the 1.6–1.7 μm wavelength region. Methods. Density functional theory (DFT) in combination with the vibrational second-order perturbation theory (VPT2) was used to compute the anharmonic spectra of PAHs. To simulate the vibrational excitation process of PAHs, the Wang–Landau random walk technique was employed. Results. All the dominant bands in the 1.6–1.7 μm wavelength range and in the 3.1–3.5 μm C–H stretching region are calculated and tabulated. It is demonstrated that combination bands dominate the 1.6–1.7 μm region, while overtones are rare and weak in this region. We also calculate the intensity ratios of the 3.1–3.5 μm C–H stretching features to the bands in the 1.6–1.7 μm region, I3.1 − 3.5/I1.6 − 1.7, for both ground and vibrationally excited states. On average, we obtain ⟨I3.1 − 3.5/I1.6 − 1.7⟩≈12.6 and ⟨I3.1 − 3.5/I1.6 − 1.7⟩≈17.6 for PAHs at ground states and at vibrationally excited states, respectively.