Interstellar Extinction Feature Research Articles

INTERSTELLAR ANALOGS FROM DEFECTIVE CARBON NANOSTRUCTURES ACCOUNT FOR INTERSTELLAR EXTINCTION

Because interstellar dust is closely related to the evolution of matter in the galactic environment and many other astrophysical phenomena, the laboratory synthesis of interstellar dust analogs has received significant attention over the past decade. To simulate the ultraviolet (UV) interstellar extinction feature at 217.5 nm originating from carbonaceous interstellar dust, many reports focused on the UV absorption properties of laboratory-synthesized interstellar dust analogs. However, no general relation has been established between UV interstellar extinction and artificial interstellar dust analogs. Here, we show that defective carbon nanostructures prepared by high-energy collisions exhibit a UV absorption feature at 220 nm which we suggest accounts for the UV interstellar extinction at 217.5 nm. The morphology of some carbon nanostructures is similar to that of nanocarbons discovered in the Allende meteorite. The similarity between the absorption feature of the defective carbon nanostructures and UV interstellar extinction indicates a strong correlation between the defective carbon nanostructures and interstellar dust.

NANODIAMONDS ENVELOPED IN GLASSY CARBON SHELLS AND THE ORIGIN OF THE 2175 Å OPTICAL EXTINCTION FEATURE

It is shown that small nanodiamond particles encapsulated into cages of glassy carbon may contribute to the 2175 A (5.67 eV) interstellar extinction feature, in the normalized interstellar extinction curve, for a ratio between the size of the diamond core to its overall exterior radius in the range 0.6-0.9. Results for optical absorption by diamond nanoclusters encased in multishell fullerenes are also presented. These are also possible candidates to explain the feature.

On buckyonions as an interstellar grain component

Abstract The carrier of the 2175 Å interstellar extinction feature remains unidentified since its first detection over 40 yr ago. In recent years, carbon buckyonions have been proposed as a carrier of this feature, based on the close similarity between the electronic transition spectra of buckyonions and the 2175 Å interstellar feature. We examine this hypothesis by modelling the interstellar extinction with buckyonions as a dust component. It is found that dust models containing buckyonions (in addition to amorphous silicates, polycyclic aromatic hydrocarbon molecules, graphite) can closely reproduce the observed interstellar extinction curve. To further test this hypothesis, we call for experimental measurements and/or theoretical calculations of the infrared vibrational spectra of hydrogenated buckyonions. By comparing the infrared emission spectra predicted for buckyonions vibrationally excited by the interstellar radiation with the observed emission spectra of the diffuse interstellar medium, we will be able to derive (or place an upper limit on) the abundance of interstellar buckyonions.

On buckyonions as a carrier of the 2175 Å interstellar extinction feature

AbstractIn recent years buckyonions have been suggested as a carrier of the 2175 Å interstellar extinction feature, based on the close similarity between the electronic transition spectra of buckyonions and the 2175 Å interstellar extinction feature. We examine this hypothesis by calculating the interstellar extinction with buckyonions as a dust component. It is found that dust models containing buckyonions (in addition to amorphous silicates, PAHs, graphite or amorphous carbon) can closely reproduce the observed interstellar extinction curve. However, a more severe challenge to the buckyonion hypothesis is provided by the non-detection of the ~7–8 μm C–H stretching bands expected from buckyonions in the diffuse interstellar medium. This will allow us to place an upper limit on the abundance of buckyonions.

Theoretical Support for Buckyonions as Carriers of the UV Interstellar Extinction Feature

Theoretically simulated UV-visible photoabsorption spectra of the buckyonions are compared with the most recent and complete experimental data. The very good agreement found provides support for our theoretical model of the buckyonions and allows for a detailed analysis of the correspondence between these spectra and the UV interstellar extinction feature. The excellent agreement found between the theoretical and observational features and the consistency of the former with the observational data constraints gives very strong support for the buckyonion origin of the UV interstellar spectrum.

Ultraviolet Absorption in Amorphous Carbons: Polycyclic Aromatic Hydrocarbons and the 2175 Å Extinction Feature

Amorphous carbons have been prepared by plasma deposition in a vacuum and in the presence of a variety of gases. On subsequent heating in a vacuum, ultraviolet absorption spectra of certain of these materials show the development of a feature centered at 2175 A (4.6 μm-1). We find that the spectrum of amorphous carbon prepared from a hydrogen plasma provides a good match to the interstellar spectrum. In addition, infrared spectra and high-resolution imaging of these deposits indicate the presence of polycyclic aromatic hydrocarbon (PAH) islands in these solids, suggesting that the UV absorption feature can be attributed to the π-π* plasmon resonance in PAH components having structures similar to coronene. An excellent fit to the laboratory data is obtained with the calculated π-π* plasmon in dehydrogenated coronene, C24. Our data suggest that relatively small dehydrogenated PAH molecules similar to molecular groups seen in amorphous carbon may be the origin of the interstellar extinction feature at 2175 A.

Laboratory investigation of the contribution of complex aromatic/aliphatic polycyclic hybrid molecular structures to interstellar ultraviolet extinction and infrared emission.

We have demonstrated by experiment that, in an energetic environment, a simple polycyclic aromatic hydrocarbon (PAH) such as naphthalene will undergo chemical reactions that produce a wide array of more complex species (an aggregate). For a stellar wind of a highly evolved star (post-asymptotic giant branch [post-AGB]), this process would be in addition to what is expected from reactions occurring under thermodynamic equilibrium. A surprising result of that work was that produced in substantial abundance are hydrogenated forms that are hybrids of polycyclic aromatic and polycyclic alkanes. Infrared spectroscopy described here reveals a spectral character for these materials that has much in common with that observed for the constituents of circumstellar clouds of post-AGB stars. It can be demonstrated that a methylene (-CH2-) substructure, as in cycloalkanes, is the likely carrier of the 6.9 microns band emission of dust that has recently been formed around IRAS 22272+5433, NGC 7027, and CPD -56 8032. Ultraviolet spectroscopy previously done with a lower limit of 190 nm had revealed that this molecular aggregate can contribute to the interstellar extinction feature at 2175 angstroms. We have now extended our UV spectroscopy of these materials to 110 nm by a vacuum ultraviolet technique. That work, described here, reveals new spectral characteristics and describes how material newly formed during the late stages of stellar evolution could have produced an extinction feature claimed to exist at 1700 angstroms in the spectrum of HD 145502 and also how the newly formed hydrocarbon material would be transformed/aged in the general interstellar environment. The contribution of this molecular aggregate to the rise in interstellar extinction at wavelengths below 1500 angstroms is also examined. The panspectral measurements of the materials produced in the laboratory, using plasmas of H, He, N, and O to convert the simple PAH naphthalene to an aggregate of complex species, provide insight into possible molecular structure details of newly formed hydrocarbon-rich interstellar dust and its transformation into aged material that becomes resident in the interstellar medium. Specifically the presence of naphthalene-like and butadiene-like conjugated structures as chromophores for the 2175 angstroms ultraviolet extinction feature is indicated.

Experimental indication of a naphthalene-base molecular aggregate for the carrier of the 2175 angstroms interstellar extinction feature.

Experiments where the simple polycyclic aromatic hydrocarbon (PAH) naphthalene (C10H8) is subjected to the energetic environment of a plasma have resulted in the synthesis of a molecular aggregate that has ultraviolet spectral characteristics that suggest it provides insight into the nature of the carrier of the 2175 angstroms interstellar extinction feature and may be a laboratory analog. Ultraviolet, visible, infrared, and mass spectroscopy, along with gas chromatography, indicate that it is a molecular aggregate in which an aromatic double ring ("naphthalene") structural base serves as the electron "box" chromophore that gives rise to the envelope of the 2175 angstroms feature. This chromophore can also provide the peak of the feature or function as a mantle in concert with another peak provider such as graphite. The molecular base/chromophore manifests itself both as a structural component of an alkyl-aromatic polymer and as a substructure of hydrogenated PAH species. Its spectral and molecular characteristics are consistent with what is generally expected for a complex molecular aggregate that has a role as an interstellar constituent.

The 2175 Å interstellar extinction feature

Abstract The study of cosmic dust has become a central problem in modern astrophysics, its presence being recognized as a controlling feature in many diverse astrophysical processes. Yet, for over 20 years, a major piece of spectroscopic evidence relating to the physical nature and composition of interstellar dust has defied explanation, remaining a matter of extreme controversy, standing in the way of a full understanding of the nature of these small grains of cosmic matter. Here I review the observational properties of the ultraviolet 2175 A interstellar extinction feature, the constraints that these place upon its physical interpretation, and the various extinction mechanisms and carrier materials that seek to explain it. The solution to the problem of the 2175 A feature challenges both solid-state physics and astronomy, representing a unique opportunity for collaboration between these two sciences.

A unified model for the 3.28 ? emission and the ?2200 � interstellar extinction feature

The 3.28 μm feature observed in the diffuse galactic emission and the λ2200 A interstellar extinction feature must have a common source. This requirement rules out graphitic-type PAH molecules such as coronone and tends to favour naturally occurring bi-cyclic ring structures typified by quinoline and its derivatives.

A physical model for the 2175 A interstellar extinction feature

Recent IUE observations have shown that the 2175 A interstellar extinction feature is constant in wavelength but varies in width. A model has been constructed to explain these results. It is proposed that the 2175 A feature will only be seen when there is extinction due to carbon grains which have lost their hydrogen. In particular, the feature is caused by a separate population of small (less than 50 A radius), hydrogen-free carbon grains. The variations in width would be due to differences in either their temperature, size distribution, or impurity content. All other carbon grains retain hydrogen, which causes the feature to be suppressed. If this model is correct, then it implies that the grains responsible for the unidentified IR emission features would not generally cause the 2175 A feature. 53 references.

The 2200 � extinction feature and the shape-distribution of graphite grains

The observed profile of the 2200 A interstellar extinction feature is consistent with extinction by polydispersions in shape of small graphite spheroids.