The Chemistry of Cosmic Dust Analogues from C, C2, and C2H2 in C-Rich Circumstellar Envelopes

Interstellar carbonaceous dust is mainly formed in the innermost regions of circumstellar envelopes around carbon-rich asymptotic giant branch (AGB) stars. In these highly chemically stratified regions, atomic and diatomic carbon, along with acetylene are the most abundant species after H2 and CO. In a previous study, we addressed the chemistry of carbon (C and C2) with H2 showing that acetylene and aliphatic species form efficiently in the dust formation region of carbon-rich AGBs whereas aromatics do not. Still, acetylene is known to be a key ingredient in the formation of linear polyacetylenic chains, benzene and polycyclic aromatic hydrocarbons (PAHs), as shown by previous experiments. However, these experiments have not considered the chemistry of carbon (C and C2) with C2H2. In this work, by employing a sufficient amount of acetylene, we investigate its gas-phase interaction with atomic and diatomic carbon. We show that the chemistry involved produces linear polyacetylenic chains, benzene and other PAHs, which are observed with high abundances in the early evolutionary phase of planetary nebulae. More importantly, we have found a non-negligible amount of pure and hydrogenated carbon clusters as well as aromatics with aliphatic substitutions, both being a direct consequence of the addition of atomic carbon. The incorporation of alkyl substituents into aromatics can be rationalized by a mechanism involving hydrogen abstraction followed by methyl addition. All the species detected in gas phase are incorporated into the nanometric sized dust analogues, which consist of a complex mixture of sp, sp2 and sp3 hydrocarbons with amorphous morphology.

How much graphene in space?

The possible presence of graphene in the interstellar medium (ISM) is examined by comparing the interstellar extinction curve with the ultraviolet absorption of graphene calculated from its dielectric functions experimentally obtained with the electron energy loss spectroscopy (EELS) method. Based on the absence in the interstellar extinction curve of the [Formula: see text] π-π* electronic interband transition of graphene, we place an upper limit of [Formula: see text] of C/H on the interstellar graphene abundance, exceeding the previous estimate by a factor of [Formula: see text]3 which made use of the dielectric functions measured with the spectroscopic ellipsometry (SE) method. Compared with the SE method which measures graphene in air (and hence its surface is contaminated) in a limited energy range of [Formula: see text]0.7-5 [Formula: see text], the EELS probes a much wider energy range of [Formula: see text]0-50 [Formula: see text] and is free of contamination. The fact that the EELS dielectric functions are substantially smaller than that of SE naturally explains why a higher upper limit on the graphene abundance is derived with EELS. Inspired by the possible detection of C24, a planar graphene sheet, in several Galactic and extragalactic planetary nebulae, we also examine the possible presence of C24 in the diffuse ISM by comparing the model IR emission of C24 with the observed IR emission of the Galactic cirrus and the diffuse ISM towards l = 44°20' and b = -0°20'. An upper limit of [Formula: see text] on C24 is also derived from the absence of the characteristic vibrational bands of C24 at [Formula: see text]6.6, 9.8, and 20 [Formula: see text] in the observed IR emission.

η CARINAE'S DUSTY HOMUNCULUS NEBULA FROM NEAR-INFRARED TO SUBMILLIMETER WAVELENGTHS: MASS, COMPOSITION, AND EVIDENCE FOR FADING OPACITY

Infrared observations of the dusty, massive Homunculus Nebula around the luminous blue variable η Carinae are crucial to characterize the mass-loss history and help constrain the mechanisms leading to the Great Eruption. We present the 2.4 - 670 μm spectral energy distribution, constructed from legacy ISO observations and new spectroscopy obtained with the Herschel Space Observatory. Using radiative transfer modeling, we find that the two best-fit dust models yield compositions which are consistent with CNO-processed material, with iron, pyroxene and other metal-rich silicates, corundum, and magnesium-iron sulfide in common. Spherical corundum grains are supported by the good match to a narrow 20.2 μm feature. Our preferred model contains nitrides AlN and Si₃N₄ in low abundances. Dust masses range from 0.25 to 0.44 M _ʘ but M _tot ≥ 45 M _ʘ in both cases due to an expected high Fe gas-to-dust ratio. The bulk of dust is within a 5″ × 7″ central region. An additional compact feature is detected at 390 μm. We obtain L _IR = 2.96 × 10⁶ L _ʘ, a 25% decline from an average of mid-IR photometric levels observed in 1971-1977. This indicates a reduction in circumstellar extinction in conjunction with an increase in visual brightness, allowing 25-40% of optical and UV radiation to escape from the central source. We also present an analysis of ¹²CO and ¹³CO J = 5 - 4 through 9 - 8 lines, showing that the abundances are consistent with expectations for CNO-processed material. The [¹²C II] line is detected in absorption, which we suspect originates in foreground material at very low excitation temperatures.

High energy electron irradiation of interstellar carbonaceous dust analogs: Cosmic ray effects on the carriers of the 3.4 µm absorption band

The effects of cosmic rays on the carriers of the interstellar 3.4 μm absorption band have been investigated in the laboratory. This band is attributed to stretching vibrations of CH₃ and CH₂ in carbonaceous dust. It is widely observed in the diffuse interstellar medium (ISM), but disappears in dense clouds. Destruction of CH₃ and CH₂ by cosmic rays could become relevant in dense clouds, shielded from the external ultraviolet field. For the simulations, samples of hydrogenated amorphous carbon (a-C:H) have been irradiated with 5 keV electrons. The decay of the band intensity vs electron fluence reflects a-C:H dehydrogenation, which is well described by a model assuming that H₂ molecules, formed by the recombination of H atoms liberated through CH bond breaking, diffuse out of the sample. The CH bond destruction rates derived from the present experiments are in good accordance with those from previous ion irradiation experiments of HAC. The experimental simplicity of electron bombardment has allowed the use of higher energy doses than in the ion experiments. The effects of cosmic rays on the aliphatic components of cosmic dust are found to be small. The estimated cosmic ray destruction times for the 3.4 μm band carriers lie in the 10⁸ yr range and cannot account for the disappearance of this band in dense clouds, which have characteristic lifetimes of 3 × 10⁷ yr. The results invite a more detailed investigation of the mechanisms of CH bond formation and breaking in the intermediate region between diffuse and dense clouds.