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NASA NTRS1979

Iron and the formation of astrophysical dust grains

Condensation of metallic iron or the iron carbide cohenite Fe3C is proposed as the source of the approximately 1000 K dust condensate in nova shells, Wolf-Rayet stars, and other objects. Iron grains may serve as nucleation sites for carbon in carbon-star ejecta; however, nucleation of graphite at its equilibrium saturation temperature is almost certainly kinetically inhibited. A gas phase rich in CO, C2H2, and HCN is produced instead.

Lewis, J. S.↗
NASA NTRS1983

Formation of fine dust on Saturn's rings as suggested by the presence of spokes

The common interpretation of spokes on the B ring of Saturn is that they are the result of light scattered by electrostatically levitated micrometer- and submicrometer-size dust particles. The origin of this dust in terms of radiation-induced thermal fatigue and collisions between the particles of the ring as well as meteoritic bombardment is investigated.

Smoluchowski, R.↗
NASA NTRS2010

The Origin of Dust in the Early Universe: Probing the Star Formation History of Galaxies by Their Dust Content

Two distinct scenarios for the origin of the approximately 4 x 10(exp 8) Solar Mass of dust observed in the high-redshift (z = 6.4) quasar J1148+5251 have been proposed. The first assumes that this galaxy is much younger than the age of the universe at that epoch so that only supernovae, could have produced this dust. The second scenario assumes a significantly older galactic age, so that the dust could have formed in lower-mass AGB stars. Presenting new integral solutions for the chemical evolution of metals and dust in galaxies, we offer a critical evaluation of these two scenarios. ^N;"(,, show that the AGB scenario is sensitive to the details of the galaxy's star formation history (SFH), which must consist of an early intense starburst followed by a period of low stellar activity. The presence or absence of massive amounts of dust in high-redshift galaxies can therefore be used to infer their SFH. However, a problem with the AGB scenario is that it produces a stellar mass that is significantly larger than the inferred dynamical mass of J1148+5251, an yet unresolved discrepancy. If this problem persists, then additional sites for the growth or formation of dust, such as molecular clouds or dense clouds around active galactic nuclei, must be considered.

Dwek, Eli↗
NASA NTRS1993

Interstellar Dust: Physical Processes

Dust is formed in stellar environments, and destroyed by sputtering, shattering and vaporization in shock waves due to cloud-cloud collisions and supernova blast waves. Dust is also destroyed during star formation. We review the dust formation and destruction balance. The calculated destruction time-scale is less than or equal to one billion years and the star dust injection time-scale is approx. 2.5 billion years. Hence, the fractions of elemental carbon and silicon locked up in stardust are less than 0.3 and less than 0.15, respectively. An efficient ISM dust formation route is therefore implied. In particular, in dense clouds dust grows; through the processes of coagulation and the accretion of gas phase molecules e.g. H20, CO, CH4. These icy materials may then be photoprocessed to refractory materials in more diffuse regions. The resulting carbonaceous grain mantle may actually be the glue that holds the coagulated grains together.

Jones, A. P.↗
NASA NTRS2018

ALMA Spectral Line and Imaging Survey of a Low and a High Mass-Loss Rate AGB Star Between 335 and 362 GHz

Context. Low and intermediate mass stars are known to power strong stellar winds when evolving through the asymptotic giant branch (AGB) phase. Initial mass, luminosity, temperature, and composition determine the pulsation characteristics of the star and the dust species formed in the pulsating photospheric layers. Radiation pressure on these grains triggers the onset of a stellar wind. However, as of today, we still cannot predict the wind mass-loss rates and wind velocities from first principles neither do we know which species are the first to condense in the upper atmospheric regions. Aims. We aim to characterise the dominant physical, dynamical, and chemical processes in the inner wind region of two archetypical oxygen-rich (C/O<1) AGB stars, that is, the low mass-loss rate AGB star R Dor (M~ 1x10(exp 7) M☉/yr) and the high mass-loss rate AGB star IK Tau (M~5x10(exp 6) M☉/yr). The purpose of this study is to observe the key molecular species contributing to the formation of dust grains and to cross-link the observed line brightnesses of several species to the global and local properties of the star and its wind. Methods. A spectral line and imaging survey of IK Tau and R Dor was made with ALMA between 335 and 362 GHz (band 7) at a spatial resolution of ~150 mas, which corresponds to the locus of the main dust formation region of both targets. Results. Some two hundred spectral features from 15 molecules (and their isotopologues) were observed, including rotational lines in both the ground and vibrationally excited states (up to v=5 for SiO). Detected species include the gaseous precursors of dust grains such as SiO, AlO, AlOH, TiO, and TiO2.We present a spectral atlas for both stars and the parameters of all detected spectral features. A clear dichotomy for the sulphur chemistry is seen: while CS, SiS, SO, and SO2 are abundantly present in IK Tau, only SO and SO2 are detected in R Dor. Also other species such as NaCl, NS, AlO, and AlOH display a completely different behaviour. From some selected species, the minor isotopologues can be used to assess the isotopic ratios. The channel maps of many species prove that both large and small-scale inhomogeneities persist in the inner wind of both stars in the form of blobs, arcs, and/or a disk. The high sensitivity of ALMA allows us to spot the impact of these correlated density structures in the spectral line profiles. The spectral lines often display a half width at zero intensity much larger than expected from the terminal velocity, v1, previously derived for both objects (36 km/s versus v(∞)~17.7 km/s for IK Tau and 23 km/s versus v(∞)~5.5 km/s for R Dor). Both a more complex 3D morphology and a more forceful wind acceleration of the (underlying) isotropic wind can explain this trend. The formation of fractal grains in the region beyond _400 mas can potentially account for the latter scenario. From the continuum map, we deduce a dust mass of ~3.7 x 10(exp 7) M☉ and ~2 x 10(ex0 8) M☉ for IK Tau and R Dor, respectively. Conclusions. The observations presented here provide important constraints on the properties of these two oxygen-dominated AGB stellar winds. In particular, the ALMA data prove that both the dynamical and chemical properties are vastly different for this high mass-loss rate (IK Tau) and low mass-loss rate (R Dor) star.

Stars: AGB and post-AGB, Stars: mass loss, Stars: ↗
NASA NTRS2018

ALMA Spectral Line and Imaging Survey of a Low and a High Mass-Loss Rate AGB Star Between 335 and 362 GHz

Context. Low and intermediate mass stars are known to power strong stellar winds when evolving through the asymptotic giant branch (AGB) phase. Initial mass, luminosity, temperature, and composition determine the pulsation characteristics of the star and the dust species formed in the pulsating photospheric layers. Radiation pressure on these grains triggers the onset of a stellar wind. However, as of today, we still cannot predict the wind mass-loss rates and wind velocities from first principles neither do we know which species are the first to condense in the upper atmospheric regions. Aims. We aim to characterise the dominant physical, dynamical, and chemical processes in the inner wind region of two archetypical oxygen-rich (C/O < 1) AGB stars, that is, the low mass-loss rate AGB star R Dor (M’ ∼ 1 × 10(exp −7) M/yr) and the high mass-loss rate AGB star IK Tau (M’ ∼ 5 × 10(exp −6) M/yr). The purpose of this study is to observe the key molecular species contributing to the formation of dust grains and to cross-link the observed line brightnesses of several species to the global and local properties of the star and its wind. Methods. A spectral line and imaging survey of IK Tau and R Dor was made with ALMA between 335 and 362 GHz (band 7) at a spatial resolution of ∼150 mas, which corresponds to the locus of the main dust formation region of both targets. Results. Some two hundred spectral features from 15 molecules (and their isotopologues) were observed, including rotational lines in both the ground and vibrationally excited states (up to v = 5 for SiO). Detected species include the gaseous precursors of dust grains such as SiO, AlO, AlOH, TiO, and TiO2. We present a spectral atlas for both stars and the parameters of all detected spectral features. A clear dichotomy for the sulphur chemistry is seen: while CS, SiS, SO, and SO2 are abundantly present in IK Tau, only SO and SO2 are detected in R Dor. Also other species such as NaCl, NS, AlO, and AlOH display a completely different behaviour. From some selected species, the minor isotopologues can be used to assess the isotopic ratios. The channel maps of many species prove that both large and small-scale inhomogeneities persist in the inner wind of both stars in the form of blobs, arcs, and/or a disk. The high sensitivity of ALMA allows us to spot the impact of these correlated density structures in the spectral line profiles. The spectral lines often display a half width at zero intensity much larger than expected from the terminal velocity, v∞, previously derived for both objects (36 km/s versus v(∞) ∼ 17.7 km/s for IK Tau and 23 km/s versus v(∞) ∼ 5.5 km/s for R Dor). Both a more complex 3D morphology and a more forceful wind acceleration of the (underlying) isotropic wind can explain this trend. The formation of fractal grains in the region beyond ∼400 mas can potentially account for the latter scenario. From the continuum map, we deduce a dust mass of ∼3.7 × 10(exp −7) Mand ∼2 × 10(exp −8) M for IK Tau and R Dor, respectively. Conclusions. The observations presented here provide important constraints on the properties of these two oxygen-dominated AGB stellar winds. In particular, the ALMA data prove that both the dynamical and chemical properties are vastly different for this high mass-loss rate (IK Tau) and low mass-loss rate (R Dor) star.

L. Decin↗
NASA NTRS1994

Constraints on chondrule agglomeration from fine-grained chondrule rims

Fine-grained rims around chondrules, Ca,Al-rich inclusions, and other coarse-grained components occur in most types of unequilibrated chondrites, most prominently in carbonaceous chondrites of the CM group. Based on mineralogical and petrographic investigations, it was suggested that rim structures in unequilibrated ordinary chondrites could have formed in the solar nebula by accretion of dust on the surfaces of the chondrules. Dust mantles in CM chondrites seem to have formed by accretion of dust on the surfaces of chondrules and other components during their passage through dust-rich regions in the solar nebula. Concentric mantles with compositionally different layers prove the existence of various distinct dust reservoirs in the vicinity of the accreting parent body. Despite mineralogical and chemical differences, fine-grained rims from other chondrite groups principally show striking similarities to dust mantle textures in CM chondrite. This implies that the formation of dust mantles was a cosmically significant event like the chondrule formation itself. Dust mantles seem to have formed chronologically between chondrule-producing transient heating events and the agglomeration of chondritic parent bodies. For this reason the investigation of dust mantle structures may help to answer the question of how a dusty solar nebula was transformed into a planetary system.

Metzler, K.↗
NASA NTRS2015

Investigating Sensitivity to Saharan Dust in Tropical Cyclone Formation Using Nasa's Adjoint Model

As tropical cyclones develop from easterly waves coming of the coast of Africa they interact with dust from the Sahara desert. There is a long standing debate over whether this dust inhibits or advances the developing storm and how much influence it has. Dust can surround the storm and absorb incoming solar radiation, cooling the air below. As a result an energy source for the system is potentially diminished, inhibiting growth of the storm. Alternatively dust may interact with clouds through micro-physical processes, for example by causing more moisture to condense, potentially increasing the strength. As a result of climate change, concentrations and amount of dust in the atmosphere will likely change. It it is important to properly understand its effect on tropical storm formation. The adjoint of an atmospheric general circulation model provides a very powerful tool for investigating sensitivity to initial conditions. The National Aeronautics and Space Administration (NASA) has recently developed an adjoint version of the Goddard Earth Observing System version 5 (GEOS-5) dynamical core, convection scheme, cloud model and radiation schemes. This is extended so that the interaction between dust and radiation is also accounted for in the adjoint model. This provides a framework for examining the sensitivity to dust in the initial conditions. Specifically the set up allows for an investigation into the extent to which dust affects cyclone strength through absorption of radiation. In this work we investigate the validity of using an adjoint model for examining sensitivity to dust in hurricane formation. We present sensitivity results for a number of systems that developed during the Atlantic hurricane season of 2006. During this period there was a significant outbreak of Saharan dust and it is has been argued that this outbreak was responsible for the relatively calm season. This period was also covered by an extensive observation campaign. It is shown that the adjoint can provide insight into the sensitivity and reveals a relatively low sensitivity to dust compared to, for example, the thermodynamic variables. However a secondary sensitivity though moisture is seen. If dust dries the air it can significantly reduce the cyclone intensity through the moisture.

Tropical Cyclones↗
NASA NTRS1996

Imaging Polarized Dust Emission in Star Formation Regions with the OVRO MM Array

We present OVRO interferometric observations of linearly polarized emission from magnetically aligned dust grains which allow the magnetic field geometry in nearby star formation regions to be probed on scales ranging from 100 to 3000 AU. Current results include observations of the young stellar objects NGC1333/IRAS 4A, IRAS 16293-2422 and Orion IRc2-KL.

Akeson, Rachel↗
NASA NTRS1996

Formation of Cosmic Carbon Dust Analogs in Partially Hydrogenated Atmospheres

We present here UV-visible spectra of carbon grains produced by direct condensation of the carbon vapors in partially hydrogenated atmospheres. The freshly formed grains exhibit an extinction peak in the wavelength range 200-240 nm whose exact position depends on the hydrogen abundance. The results are discussed in terms of formation and evolution of circumstellar and interstellar cosmic dust.

Blanco, A.↗
NASA NTRS1976

Dust evolution from comets

The studies of the evolution of cometary debris are reviewed. The subject is divided into three major sections: (1) the developments in the immediate vicinity of the cometary nucleus, which is the source of the dust; (2) the formation of the dust tail; and (3) the blending of the debris with the dust component of interplanetary matter. The importance of the physical theory of comets is emphasized for the understanding of the early phase of evolution. A physico-dynamical model designed to analyze the particle-emission mechanism from the distribution of light in the dust tail is described and the results are presented. Increased attention is paid to large particles because of their importance for the evolution of the zodiacal cloud. Finally, implications are discussed for the future in situ investigations of comets.

Sekanina, Z.↗
NASA NTRS1977

Dust evolution from comets

The studies of the evolution of cometary debris are reviewed. The subject is divided into three major sections: (1) the developments in the immediate vicinity of the cometary nucleus, which is the source of the dust; (2) the formation of the dust tail; and (3) the blending of the debris with the dust component of interplanetary matter. The importance of the physical theory of comets is emphasized for the understanding of the early phase of the evolution of cometary dust. A physico-dynamical model designed to analyze the particle-emission mechanism from the distribution of light in the dust tails is described and the results are presented. Increased attention is paid to large particles because of their importance for the evolution of the zodiacal cloud. Finally, implications are discussed for the future in situ investigations of comets.

Sekanina, Z.↗
NASA NTRS2004

Granular Material Flows with Interstitial Fluid Effects

The research focused on experimental measurements of the rheological properties of liquid-solid and granular flows. In these flows, the viscous effects of the interstitial fluid, the inertia of the fluid and particles, and the collisional interactions of the particles may all contribute to the flow mechanics. These multiphase flows include industrial problems such as coal slurry pipelines, hydraulic fracturing processes, fluidized beds, mining and milling operation, abrasive water jet machining, and polishing and surface erosion technologies. In addition, there are a wide range of geophysical flows such as debris flows, landslides and sediment transport. In extraterrestrial applications, the study of transport of particulate materials is fundamental to the mining and processing of lunar and Martian soils and the transport of atmospheric dust (National Research Council 2000). The recent images from Mars Global Surveyor spacecraft dramatically depict the complex sand and dust flows on Mars, including dune formation and dust avalanches on the slip-face of dune surfaces. These Aeolian features involve a complex interaction of the prevailing winds and deposition or erosion of the sediment layer; these features make a good test bed for the verification of global circulation models of the Martian atmosphere.

Hunt, Melany L.↗
NASA NTRS2016

Formation of Cosmic Carbon Dust Analogues in Plasma Reactors

Cosmic carbon dust analogs are produced, processed and analyzed in the laboratory using NASA's COSmIC (COSmIC Simulation Chamber) Facility. These experiments can be used to derive information on the most efficient molecular precursors in the chemical pathways that eventually lead to the formation of carbonaceous grains in the stellar envelopes of carbon stars.

Astrophysics↗
NASA NTRS2001

A Long-Term Space Astrophysics Research Program: The Evolution of the Quasar Continuum

Four papers have been written. One reports on the major study funded by this grant: a pan-chromatic study of the quasar continuum at redshift 3. Two others make use of the quasar continuum shapes to find the minimum total accretion luminosity of the Universe, and hence the efficiency and spin of supermassive black holes; the second shows that the reemission of absorbed quasar radiation alleviates a major problem with galaxy formation and the FIR background. The last paper recognizes the role quasars may play in the initial formation of dust in the early Universe. The major study of a sample of z=3 and its comparison with a sample of z=0.l quasars across the whole X-ray to radio spectrum was completed and accepted for publication in ApJ Supplements. This study comprises the thesis work of Olga Kuhn. The two samples are matched in evolved luminosity, and so should be sampling the same black hole population at different z, and in different accretion states. Despite this no strong differences were found between the samples, except in the 'small bump' region of the optical/UV. This region is dominated by FeII emission, and may indicate abundance evolution in quasars. The lack of overall spectral changes argues strongly against a single population of quasars fading over cosmic time, and for a multiple generation, or multiple outburst model for quasars. A study of the total luminosity absorbed from quasars and re-emitted in the infrared produced two results (reported in two papers): The minimum intrinsic luminosity/Gpc(3) from AGN compared with the measured mass density in supermassive black holes [Gpc(-3)] requires a conversion efficiency of accreted mass into luminosity of greater than 15%. Non-rotating black holes cannot exceed 5% efficiency, while rapidly rotating black holes can reach 47%. Hence our result requires that most supermassive black holes must be rapidly rotating. The second result comes from considering the contribution that the re-radiated quasar radiation makes to the far infrared background (FIRB). The effective temperature of the re radiation is tightly constrained, but the detailed shape (e.g. line emission, range of temperature) is only of second order importance. At least 15perhaps 20-25% of the FIR background must come from AGN. This contribution significantly relieves problems in galaxy evolution that come from trying to use only starlight to make the FIRB. The third paper addresses the origin of the dust obscuration that is so widespread in AGN. The standard assumption is that the dust comes from the normal star-formation processes in galaxies and is drawn close to the nucleus along with the gas that powers the accretion. In complete contrast, using a wind outflow model for the broad emission line (BEL) region (Elvis 2000) as a basis, we show that BEL clouds will expand, cool and form dust as they flow outward, in strict analogy to the stellar winds of red supergiants.

Elvis, M.↗
NASA NTRS1988

Experiments on the formation, properties and processing of cosmic dust

The results from various experiments on the formation, properties, and processing of potential silicate components of cosmic dust are reviewed. Consideration is given to the role of evaporation and condensation in grain formation, nucleation experiments using volatile compounds and refractory species, and efforts to produce particles that might represent cosmic grains. Other studies include particle scattering experiments, studies on the growth of clusters, the sputtering process, the impact of temperature on sputtering yield and optical properties, and experimental and theoretical studies of the structure of particulate aggregates.

Donn, B. D.↗
NASA NTRS2004

Formation and Destruction Processes of Interstellar Dust: From Organic Molecules to carbonaceous Grains

The study of the formation and destruction processes of cosmic dust is essential to understand and to quantify the budget of extraterrestrial organic molecules. interstellar dust presents a continuous size distribution from large molecules, radicals and ions to nanometer-sized particles to micron-sized grains. The lower end of the dust size distribution is thought to be responsible for the ubiquitous spectral features that are seen in emission in the IR (UIBs) and in absorption in the visible (DIBs). The higher end of the dust-size distribution is thought to be responsible for the continuum emission plateau that is seen in the IR and for the strong absorption seen in the interstellar UV extinction curve. All these spectral signatures are characteristic of cosmic organic materials that are ubiquitous and present in various forms from gas-phase molecules to solid-state grains. Although dust with all its components plays an important role in the evolution of interstellar chemistry and in the formation of organic molecules, little is known on the formation and destruction processes of dust. Recent space observations in the UV (HST) and in the IR (ISO) help place size constraints on the molecular component of carbonaceous IS dust and indicate that small (ie., subnanometer) PAHs cannot contribute significantly to the IS features in the UV and in the IR. Studies of large molecular and nano-sized IS dust analogs formed from PAH precursors have been performed in our laboratory under conditions that simulate diffuse ISM environments (the particles are cold -100 K vibrational energy, isolated in the gas phase and exposed to a high-energy discharge environment in a cold plasma). The species (molecules, molecular fragments, ions, nanoparticles, etc) formed in the pulsed discharge nozzle (PDN) plasma source are detected with a high-sensitivity cavity ring-down spectrometer (CRDS). We will present new experimental results that indicate that nanoparticles are generated in the plasma. From these unique measurements, we derive information on the nature, the size and the structure of interstellar dust particles, the growth and the destruction processes of IS dust and the resulting budget of extraterrestrial organic molecules.

Salama, F.↗