Component Of Interstellar Dust Research Articles

The role of point defect reconstructions and polycyclic aromatic hydrocarbons in silicate dust preservation

ABSTRACT Forsterite is a primary constituent of interstellar dust and planetary systems. It is believed to originate from the outflows of oxygen-rich stars and undergo further processing within the interstellar medium through the action of cosmic rays and shocks. Under these harsh conditions, point defects may form, such as MgO Schottky vacancies. These vacancies can then undergo atom reconstruction as part of a chemical process to maintain the system’s crystalline structure. Polycyclic aromatic hydrocarbons (PAHs) are ubiquitously observed interstellar molecules and are thought to form through gas-phase reactions akin to sooting flames. However, their role and impact on dust stability remain unknown. In this study, we employ an atomistic artificial-intelligence-based method, surrogate machine learning trained directly by density functional theory. Specifically, we utilize gofee (global optimization with first-principles energy expressions) to predict possible reconstructions of MgO vacancies on a crystalline forsterite (010) surface as an important component of interstellar dust and planetary systems. We identify nine possible reconstructions involving the formation of unbound Si and O atoms. We investigate their energy stability and find that the reconstruction of Si–O atoms stabilizes the vacancy by about 0.54 eV. Additionally, if PAHs bind with the unbound O and Si atoms of the vacancy, the vacancy is stabilized by approximately 1.76 eV. We demonstrate that PAHs, along with the reconstruction of unbound atoms on the dust surface, affect the stability of the dust, which might open up avenues for diverse chemistry.

Processing of hydrocarbon dust in star-forming galaxies revealed with AKARI

Abstract Hydrocarbon dust is one of the dominant components of interstellar dust, which mainly consists of polycyclic aromatic hydrocarbons and aliphatic hydrocarbons. While hydrocarbon dust is thought to be processed in interstellar radiation fields or shocks, detailed processing mechanisms are not completely understood yet. We investigate the processing of hydrocarbon dust by analyzing the relation between the luminosities emitted by hydrocarbon dust and the total infrared luminosities $(L_{\mathrm{IR}})$ for 138 star-forming galaxies at redshift $z \lt 0.3$. Using near-infrared 2.5–5$\, \mu {\rm m}$ spectra obtained with AKARI, we derived the luminosities of the aromatic hydrocarbon feature at 3.3$\, \mu {\rm m}$ ($L_\mathrm{aromatic}$) and the aliphatic hydrocarbon feature at 3.4–3.6$\, \mu {\rm m}$ ($L_\mathrm{aliphatic}$). We also derived $L_\mathrm{IR}$ and the radiation field strength by modeling the spectral energy distributions of the 138 galaxies with AKARI, WISE, and IRAS photometry data. We find that galaxies with higher $L_\mathrm{IR}$ tend to exhibit lower $L_\mathrm{aliphatic}/L_\mathrm{aromatic}$ ratios. Furthermore, we find that there is an anti-correlation between $L_\mathrm{aliphatic}/L_\mathrm{aromatic}$ ratios and the radiation field strength, and also that the galaxies with low $L_\mathrm{aliphatic}/L_\mathrm{aromatic}$ ratios are dominated by merger galaxies. These results support the suggestion that hydrocarbon dust is processed through photodissociation in strong radiation fields and/or shocks during merging processes of galaxies; the $L_\mathrm{aliphatic}/L_\mathrm{aromatic}$ ratio is likely to decrease in such harsh interstellar conditions since the aliphatic bonds are known to be chemically weaker than the aromatic bonds.

Grain Alignment in the Circumstellar Shell of IRC+10° 216

Dust-induced polarization in the interstellar medium (ISM) is due to asymmetric grains aligned with an external reference direction, usually the magnetic field. For both the leading alignment theories, the alignment of the grain’s angular momentum with one of its principal axes and the coupling with the magnetic field requires the grain to be paramagnetic. Of the two main components of interstellar dust, silicates are paramagnetic, while carbon dust is diamagnetic. Hence, carbon grains are not expected to align in the ISM. To probe the physics of carbon grain alignment, we have acquired Stratospheric Observatory for Infrared Astronomy/Higch-resolution Airborne Wideband Camera-plus far-infrared photometry and polarimetry of the carbon-rich circumstellar envelope (CSE) of the asymptotic giant branch star IRC+10° 216. The dust in such CSEs are fully carbonaceous and thus provide unique laboratories for probing carbon grain alignment. We find a centrosymmetric, radial, polarization pattern, where the polarization fraction is well correlated with the dust temperature. Together with estimates of a low fractional polarization from optical polarization of background stars, we interpret these results to be due to a second-order, direct radiative external alignment of grains without internal alignment. Our results indicate that (pure) carbon dust does not contribute significantly to the observed ISM polarization, consistent with the nondetection of polarization in the 3.4 μm feature due to aliphatic CH bonds on the grain surface.

Residue from vacuum ultraviolet irradiation of benzene ices: Insights into the physical structure of astrophysical dust.

We have irradiated benzene ices deposited at 4 K on a cold, interstellar dust analog with vacuum ultraviolet (9 eV) irradiation for periods lasting from several hours to nearly a day, after which the irradiated ice was warmed to room temperature. Vacuum ultraviolet photoabsorption spectra of the aromatic residue left at room temperature were recorded and showed the synthesis of benzene derivatives. The residue was also imaged using an electron microscope and revealed crystals of various sizes and shapes. The result of our experiments suggests such geometrically shaped dust particles may be a key component of interstellar dust.

Graphene oxide nanoparticles in the interstellar medium

ABSTRACT Dust particles play a major role in the formation, evolution and chemistry of interstellar clouds, stars, and planetary systems. Commonly identified forms include amorphous and crystalline carbon-rich particles and silicates. Also present in many astrophysical environments are polycyclic aromatic hydrocarbons (PAHs), detected through their infrared emission, and which are essentially small flakes of graphene. Astronomical observations over the past four decades have revealed a widespread unassigned ‘extended red emission’ (ERE) feature which is attributed to luminescence of dust grains. Numerous potential carriers for ERE have been proposed but none has gained general acceptance. In this Letter it is shown that there is a strong similarity between laboratory optical emission spectra of graphene oxide (GO) and ERE, leading to this proposal that emission from GO nanoparticles is the origin of ERE and that these are a significant component of interstellar dust. The proposal is supported by infrared emission features detected by the Infrared Space Observatory (ISO) and the Spitzer Space Telescope.

The essential elements of dust evolution

Context. There remain many open questions relating to the depletion of elements into dust, e.g., exactly how are C and O incorporated into dust in dense clouds and, in particular, what drives the disappearance of oxygen in the denser interstellar medium? Aims. This work is, in part, an attempt to explain the apparently anomalous incorporation of O atoms into dust in dense clouds. Methods. We re-visit the question of the depletion of the elements incorporated into the carbonaceous component of interstellar dust, i.e., C, H, O, N and S, in the light of recent analyses of the organics in comets, meteorites and interplanetary dust particles. Results. We find that oxygen could be combined with ≈10–20 % of the carbon in the dust in dense regions in the form of a difficult to observe, organic carbonate, (−O−O>C =O), which could explain the unaccounted for 170–255 ppm oxygen depletion. Conclusions. We conclude that, while C, O and N atoms are depleted into an amorphous a-C:H:O:N phase, we posit that a significant fraction of C and O atoms could be sequestered into an organic carbonate, which provides a viable solution to the oxygen depletion problem. Further, the thermal or photolytic decomposition of this carbonate may have a bearing on the formation of CO2 in the ISM.

Desorption of volatile molecules from the surface of interstellar carbonaceous dust analogs

AbstractThe desorption of volatile molecules from dust grains in cold dense clouds is crucial for the chemical inventory in the various stages of cloud collapse. In this work we investigate the desorption of N2, CO, CH4 and CO2 from surfaces of hydrogenated amorphous carbon (HAC), which, according to IR observations, is one of the main components of interstellar dust.

Modelling the properties of interstellar dust using the Si K-edge

AbstractThe properties of interstellar dust (ID) can be studied in great detail by making use of X-ray spectroscopy techniques. The radiation of X-rays sources is scattered and absorbed by dust grains in the interstellar medium. The X-ray band is especially suitable to study silicates - one of the main components of ID -since it contains the absorption edges of Si, Mg, O and Fe. In the Galaxy, we can use absorption features in the spectra of X-ray binaries to study the size distribution, composition and crystalline structure of grains. In order to derive these properties, it is necessary to acquire a database of detailed extinction cross sections models, that reflects the composition of the dust in the interstellar medium. We present the extinction profiles of a set of newly acquired measurements of 14 dust analogues at the Soleil Synchrotron facility in Paris, where we focus on silicates and the Si-K edge in particular, which is modelled with unprecedented accuracy. These models are used to analyse ID in the dense environments of the Galaxy.

Carbonaceous nano-dust emission in proto-planetary discs: the aliphatic-aromatic components

Context. In the interstellar medium, carbon (nano-)grains are a major component of interstellar dust. This solid phase is more vulnerable to processing and destruction than its silicate counterpart. It exhibits a complex, size-dependent evolution that is due to interactions within different radiative and dynamical environments. Infrared signatures of these carbon nano-grains are seen in a large number of discs around Herbig HAeBe stars. Aims. We probe the composition and evolution of carbon nano-grains at the surface of (pre-)transitional proto-planetary discs around Herbig stars. Methods. We present spatially resolved infrared emission spectra obtained with the Nasmyth Adaptive Optics System (NAOS) Near-Infrared Imager and Spectrograph (CONICA) at the Very Large Telescope (VLT) in the 3–4 μm range with a spatial resolution of 0.1′′, which allowed us to trace aromatic, olefinic, and aliphatic bands that are attributed to sub-nanometer hydrocarbon grains. We applied a Gaussian fitting to analyse the observed spectral signatures. Finally, we propose an interpretation in the framework of the The Heterogeneous dust Evolution Model of Interstellar Solids (THEMIS). Results. We show the presence of several spatially extended spectral features that are related to aromatic and aliphatic hydrocarbon material in discs around Herbig stars, from ~10 to 50–100 au, and even in inner gaps that are devoid of large grains. The correlation and constant intensity ratios between aliphatic and aromatic CH stretching bands suggests a common nature of the carriers. Given their expected high destruction rates through UV photons, our observations suggest that they are continuously replenished at the disc surfaces.

Aliphatic hydrocarbon content of interstellar dust

In the interstellar medium, carbon is distributed between the gas and solid phases. However, while about half of the expected carbon abundance can be accounted for in the gas phase, there is considerable uncertainty as to the amount incorporated in interstellar dust. The aliphatic component of the carbonaceous dust is of particular interest because it produces a significant 3.4 $\mu$m absorption feature when viewed against a background radiation source. The optical depth of the 3.4 $\mu$m absorption feature is related to the number of aliphatic carbon C-H bonds along the line of sight. It is possible to estimate the column density of carbon locked up in the aliphatic hydrocarbon component of interstellar dust from quantitative analysis of the 3.4 $\mu$m interstellar absorption feature providing that the absorption coefficient of aliphatic hydrocarbons incorporated in the interstellar dust is known. We generated laboratory analogues of interstellar dust by experimentally mimicking interstellar/circumstellar conditions. The resultant spectra of these dust analogues closely match those from astronomical observations. The measurements of the absorption coefficient of aliphatic hydrocarbons incorporated in the analogues were carried out by a procedure which combined FTIR and $^{13}$C NMR spectroscopies. The absorption coefficients obtained for both interstellar analogues were found to be in close agreement (4.76(8) $\times$ 10$^{-18}$ cm group$^{-1}$ and 4.69(14) $\times$ 10$^{-18}$ cm group$^{-1}$), less than half those obtained in studies using small aliphatic molecules. The results thus obtained permit direct calibration of the astronomical observations, providing rigorous estimates of the amount of aliphatic carbon in the interstellar medium.

ORGANIC DUST IN THE INTERSTELLAR MEDIUM

The traditional view of dust in the interstellar medium is that it is made of graphite and silicates. In this paper, we discuss the evidence for complex organics being a major component of interstellar dust. Comparison between astronomical infrared spectra and laboratory spectra of amorphous carbonaceous materials suggests that organics of mixed aromatic-aliphatic structures are widely present in circumstellar, interstellar, and galactic environments. Scenarios for the synthesis of these compounds in the late stages of stellar evolution are presented.

Planckintermediate results. XXII. Frequency dependence of thermal emission from Galactic dust in intensity and polarization

Planck has mapped the intensity and polarization of the sky at microwave frequencies with unprecedented sensitivity. We use these data to characterize the frequency dependence of dust emission. We make use of the Planck 353 GHz I, Q, and U Stokes maps as dust templates, and cross-correlate them with the Planck and WMAP data at 12 frequencies from 23 to 353 GHz, over circular patches with 10° radius. The cross-correlation analysis is performed for both intensity and polarization data in a consistent manner. The results are corrected for the chance correlation between the templates and the anisotropies of the cosmic microwave background. We use a mask that focuses our analysis on the diffuse interstellar medium at intermediate Galactic latitudes. We determine the spectral indices of dust emission in intensity and polarization between 100 and 353 GHz, for each sky patch. Both indices are found to be remarkably constant over the sky. The mean values, 1.59 ± 0.02 for polarization and 1.51 ± 0.01 for intensity, for a mean dust temperature of 19.6 K, are close, but significantly different (3.6σ). We determine the mean spectral energy distribution (SED) of the microwave emission, correlated with the 353 GHz dust templates, by averaging the results of the correlation over all sky patches. We find that the mean SED increases for decreasing frequencies at ν< 60 GHz for both intensity and polarization. The rise of the polarization SED towards low frequencies may be accounted for by a synchrotron component correlated with dust, with no need for any polarization of the anomalous microwave emission. We use a spectral model to separate the synchrotron and dust polarization and to characterize the spectral dependence of the dust polarization fraction. The polarization fraction (p) of the dust emission decreases by (21 ± 6)% from 353 to 70 GHz. We discuss this result within the context of existing dust models. The decrease in p could indicate differences in polarization efficiency among components of interstellar dust (e.g., carbon versus silicate grains). Our observational results provide inputs to quantify and optimize the separation between Galactic and cosmological polarization.

The multiwavelength polarization of Cygnus X–1

Polarization measurements of the microquasar Cygnus X–1 exist at γ-ray, X-ray, ultraviolet, optical and radio frequencies. The γ-ray emission has been shown to be highly linearly polarized. Here, we present new infrared polarimetric data of Cygnus X–1 taken with the 10.4 m Gran Telescopio Canarias and the 4.2 m William Herschel Telescope. We show that the broad-band, radio-to-γ-ray flux spectrum and polarization spectrum in the hard state are largely consistent with a simple phenomenological model of a strongly polarized synchrotron jet, an unpolarized Comptonized corona and a moderately polarized interstellar dust component. In this model, the origin of the γ-ray, X-ray and some of the infrared polarization is the optically thin synchrotron power law from the inner regions of the jet. The model requires the magnetic field in this region to be highly ordered and perpendicular to the axis of the resolved radio jet. This differs from studies of some other X-ray binaries, in which the magnetic field is turbulent, variable and aligned with the jet axis. The model is able to explain the approximate polarization strength and position angle at all wavelengths including the detected X-ray (3–5 keV) polarization, except the observed position angle of the γ-ray polarization, which differs from the model by ∼60°. Past numerical modelling has shown that a curved synchrotron spectrum can produce a shift in position angle by ∼60°, which may account for this.

Amorphous organic solids as a component of interstellar dust

The discovery of rapid synthesis of complex organic solids in the late stages of stellar evolution has led to a new realization that carbonaceous compounds can be a major significant component of interstellar dust. Signatures of aromatic and aliphatic solids are seen in interstellar clouds as well as the diffuse interstellar medium. Similar features are also seen in the integrated spectrum of galaxies. This has raised the possibilities that many of the unidentified astronomical phenomena such as the diffuse interstellar bands, the 217 nm feature, the extended red emission, the 21 and 30 μm emission features, could also arise from complex organics. In this paper, we discuss the possible chemical structures of these organic solids and the relationships between circumsmtellar and interstellar dust with the organics found in meteorites, asteroids, comets and planetary satellites. The possibility that all these organics share a common origin is discussed.

Silicon depletion in damped Ly αsystems

Silicates are an important component of interstellar dust that has been poorly investigated in high redshift galaxies. As a preliminary step to studying silicates at high redshift, we survey silicon depletions in damped Ly α (DLA) systems. Silicon depletion is mild in the Galactic interstellar medium (ISM) and is expected to be weaker in most DLA systems, so we introduce a method for improving the accuracy of DLA depletion measurements. We compare abundance ratios measured in the gas with calculations of total abundance ratios of gas and dust predicted by models of galactic chemical evolution tailored for DLA systems. To tune the model parameters, we use the dust-free observational diagram S/Zn versus Zn/H, and we also compare the look back time estimated from the absorption redshift with the evolutionary time predicted by the model. By applying our method to a large set of DLA column densities, we succeeded in measuring the depletion of silicon in 74 systems. For comparison, we also measure iron and magnesium depletions (105 and 10 systems, respectively) with the same method. The mean depletion of silicon that we derive, ⟨ δSi ⟩ ≃ −0.27 ± 0.16 dex, is surprisingly close to that of iron, ⟨ δFe ⟩ ≃ −0.42 ± 0.28 dex, despite iron being much more depleted than silicon in the Galactic ISM. Silicon depletion in DLA systems does not correlate with metallicity, at variance with iron depletion, for which we confirm a rise with [Fe/H] found in previous work. Magnesium depletion seems to behave more in accordance with silicon than with iron. The different behaviors of the silicon and iron depletions suggests a complex history of dust production at the early stages of galactic chemical evolution.

Interstellar dust within the life cycle of the interstellar medium

Cosmic dust is omnipresent in the Universe. Its presence influences the evolution of the astronomical objects which in turn modify its physical and chemical properties. The nature of cosmic dust, its intimate coupling with its environment, constitute a rich field of research based on observations, modelling and experimental work. This review presents the observations of the different components of interstellar dust and discusses their evolution during the life cycle of the interstellar medium. Interstellar dust grains are found everywhere in the Universe: in the Solar System, around stars at all evolutionary stages, in interstellar clouds of all kind, in galaxies and in the intergalactic medium. Cosmic dust is intimately mixed with the gas-phase and represents about 1% of the gas (in mass) in our Galaxy. The interstellar extinction and the emission of diffuse interstellar clouds is reproduced by three dust components: a population of large grains, the BGs (Big Grains, ∼10-500nm) made of silicate and a refractory mantle, a population of carbonaceous nanograins, the VSGs (Very Small Grains, 1-10nm) and a population of macro-molecules the PAHs (Polycyclic Aromatic Hydrocarbons) (1). These three components are more or less abundant in the diverse astrophysical environments reflecting the coupling of dust with the environment and its evolution according to the physical and dynamical conditions. The life cycle of cosmic matter is closely related to the cycle of the interstellar medium (ISM) and of our Galaxy. Dust is mainly form within the atmosphere and circumstellar shells surrounding dying stars. It is expelled in the ISM by stellar outflows and transit in the different phases of the ISM where it is submitted to various physical and chemical processes that either destroy it or change its nature (size, structure, composition, porosity...). When a molecular cloud collapse to form new stars, the interstellar dust grains are incorporated into the protostellar object in which the survival grains pursue their evolution, into proto-planetary disks and stellar systems. At their turn, these stars, at the end of their life will produce new dust grains. Interstellar dust was extensively studied via the observations from the instruments on-board the space-missions IRAS (InfraRed Astronomical Satellite, 1983), ISO (Infrared Space Observatory, 1995-1998) and Spitzer (2003-2005), free from the atmosphere opacity in the infrared, which gave tremendous amount of data. While these missions were observing in the mid- and far-infrared spectral range (3-200 m), the satellite Herschel, launched in May 2009, opens the still unexplored spectral range between infrared and ground-based radio observations (60-672 m, 5THz-480GHz). The spectrophotometers on-board these satellites allow to map with an increasing angular resolution the spatial distribution of the different dust components, at the Galaxy scale, but also in specific objects.

AHerschelPACS and SPIRE study of the dust content of the Cassiopeia A supernova remnant

Using the 3.5-m Herschel Space Observatory, imaging photometry of Cas A has been obtained in six bands between 70 um and 500 um with the PACS and SPIRE instruments, with angular resolutions ranging from 6 to 37". In the outer regions of the remnant the 70-um PACS image resembles the 24-um image Spitzer image, with the emission attributed to the same warm dust component, located in the reverse shock region. At longer wavelengths, the three SPIRE bands are increasingly dominated by emission from cold interstellar dust knots and filaments, particularly across the central, western and southern parts of the remnant. Nonthermal emission from the northern part of the remnant becomes prominent at 500 um. We have estimated and subtracted the contributions from the nonthermal, warm dust and cold interstellar dust components. We confirm and resolve for the first time a cool (~35 K) dust component, emitting at 70-160 um, that is located interior to the reverse shock region, with an estimated mass of 0.075 Msun.

Polycyclic aromatic hydrocarbon processing in interstellar shocks

Context: PAHs appear to be an ubiquitous interstellar dust component but the effects of shocks waves upon them have never been fully investigated. Aims: To study the effects of energetic (~0.01-1 keV) ion (H, He and C) and electron collisions on PAHs in interstellar shock waves.Methods: We calculate the ion-PAH and electron-PAH nuclear and electronic interactions, above the threshold for carbon atom loss from a PAH, in 50-200 km/s shock waves in the warm intercloud medium. Results: Interstellar PAHs (Nc = 50) do not survive in shocks with velocities greater than 100 km/s and larger PAHs (Nc = 200) are destroyed for shocks with velocities greater/equal to 125 km/s. For shocks in the ~75 - 100 km/s range, where destruction is not complete, the PAH structure is likely to be severely denatured by the loss of an important fraction (20-40%) of the carbon atoms. We derive typical PAH lifetimes of the order of a few x10^8 yr for the Galaxy. These results are robust and independent of the uncertainties in some key parameters that have yet to be well-determined experimentally. Conclusions: The observation of PAH emission in shock regions implies that that emission either arises outside the shocked region or that those regions entrain denser clumps that, unless they are completely ablated and eroded in the shocked gas, allow dust and PAHs to survive in extreme environments.

Criteria for mixing rules application for inhomogeneous astrophysical grains

The analysis presented in this paper verifies which of the mixing rules are best for real components of interstellar dust in possible wide range of wavelengths. The Discrete Dipole Approximation (DDA) method with elements of different components with various volume fractions has been used. We have considered six materials: ice, amorphous carbon, graphite, SiC, silicates and iron; and the following mixing rules: Maxwell-Garnett, Bruggeman, Looyenga, Hanay and Lichtenecker, which must satisfy rigorous bounds. The porous materials have also been considered. We have assumed simplified spatial distribution, shape and size of inclusions. The criteria given by Draine have been used to determine the range of wavelengths for the considered mixtures in order to calculate the Qext using the DDA. From all the chosen mixing rules for the examined materials in majority of cases (13 out of 20), the best results have been obtained using the Lichtenecker mixing rule. In five cases, this rule is better for some volume fraction of inclusions.

Extended Red Emission in High Galactic Latitude Interstellar Clouds

We report initial results from an optical imaging survey of optically thin high Galactic latitude clouds, which is designed to study the surface brightness, structure, and spectral energy distribution of these objects. The primary aim of this paper is to study the extended red emission (ERE) that has been reported at high Galactic latitudes in earlier investigations and which is attributed to ultraviolet-excited photoluminescence of an as yet unidentified component of interstellar dust. We conduct this ongoing survey with remotely operated, fast, short focal length (0.5 m) telescopes equipped with absolutely calibrated CCD cameras yielding a field of view of 2° × 3°. The telescopes are located at New Mexico Skies at 7300 ft (2225 m) altitude near Mayhill, New Mexico. The optical surface brightness of our objects is typically a few percent of the brightness of the dark night sky, implying that the cloud SEDs must be deduced from differential surface brightness photometry in different filter bands. We find strong evidence for dust emission in the form of a broad (1000 A FWHM) ERE band with peak emission near 600 nm wavelength and peak intensity of ~5 × 10−9 ergs cm−2 s−1 A−1 sr−1 in optically thin clouds. This amounts to about 30% of the total optical surface brightness of these clouds, the remainder being consistent with expectations for dust-scattered light.