Cosmic dust: Encyclopedia - Cosmic dust

Cosmic dust

Cosmic dust - Introductory Material

Cosmic Dust refers to particles in space which are assemblages of a few molecules to tenth-millimeter-sized grains. Cosmic dust can be further distinguished by its astronomical location; for example: interplanetary dust, interstellar dust, comet dust, circumplanetary dust. This article covers bulk and radiative properties of cosmic dust, the dust particles' origins, end-fates, and specific locations in space.

Historically, cosmic dust used to be an annoyance to astronomers because of the way that the dust obscures the object that they wish to observe. When the field of infrared astronomy began, those so-called annoying dust particles were observed to be significant constituents of the Universe and found to be vital components of astrophysical processes.

For example, the dust can drive the mass loss that occurs when a star is nearing the end of its life, those particles are an essential part of the early stages of star formation, and they form planets around other stars. In our own solar system, dust plays a major role in the zodiacal light, Saturn's B Ring spokes, the outer diffuse planetary rings at Jupiter, Saturn, Uranus and Neptune, the resonant dust ring at the Earth, and the overall behavior of comets.

The study of dust is a many-faceted research topic that brings together different scientific fields: physics (solid-state, electromagnetic theory, surface physics, statistical physics, thermal physics), math (fractal math), chemistry (chemical reactions on grain surfaces), meteoritics, as well as every branch of astronomy and astrophysics. While being multidisciplinary, the disparate research areas can be linked by the following theme: the cosmic dust particles evolve cyclically; chemically, physically and dynamically. The evolution of dust traces out paths in which the universe recycles material, in processes analogous to the daily recycling steps with which many people are familiar: production, storage, processing, collection, consumption, and discarding. Observations and measurements of cosmic dust in different regions provide an important insight into the universe's recycling processes; in the clouds of the diffuse interstellar medium, in molecular clouds, in the circumstellar dust of young stellar objects, and in planetary systems such as our own solar system, where astronomers consider dust as in its most recycled state. The astronomers accumulate observational ‘snapshots’ of dust at different stages of its life and, over time, form a more complete movie of the universe's complicated recycling steps.

The detection of cosmic dust points to another facet of cosmic dust research: dust acting as photons. Once cosmic dust is detected, the scientific problem to be solved is an inverse problem to determine what processes brought that encoded photon-like object (dust) to the detector. Parameters such the particle's initial motion, material properties, intervening plasma and magnetic field determined the dust particle's arrival at the dust detector. Slightly changing any of these parameters can give significantly different dust dynamical behavior. Therefore one can learn about where that object came from, and what is (in) the intervening medium.

Cosmic dust - Some bulk properties of cosmic dust

Cosmic dust is dust grains and agreggates of dust grains. These particles are irregularly-shaped with porosity ranging from fluffy to compact. The composition, size, and other properties depends on where the dust is found. General diffuse interstellar medium dust should be distinguished from dust grains in dense clouds, which should be distinguished from planetary rings dust, which should be distinguished from circumstellar dust, and so on. For example, grains in dense clouds have acquired a mantle of ice and the average dimensions are larger than those dust particles in the diffuse interstellar medium. Interplanetary dust particles (IDPs) are generally larger still.

Other dust composition variances are the following.

In circumstellar dust, astronomers have found signatures of CO, silicon carbide, amorphous silicate, polycyclic aromatic hydrocarbons, water ice, polyformaldehyde, just to name a few.

In the diffuse interstellar medium, there is a lot of evidence for silicate and carbon grains.

The elemental composition of IDPs (asteroidal and cometary) is one of three major types: chondritic, 60%, iron-sulfur-nickel, 30%, and mafic silicates, which are iron-magnesium-rich silicates, (i.e. olivine and pyroxene), 10% (Jessberger, 1992, Gruen, E. 1999).

Cometary dust is generally different (however some overlap exists) from asteroidal dust. Asteroidal dust resembles carbonaceous chondritic meteorites, and cometary dust resembles interstellar grains which can include elements silicates, polycyclic aromatic hydrocarbons, and water ice.

The densities of most stratospheric-captured IDPs range between 1 and 3 g/cm³, with an average density at about 2.0 g/cm³ (Love, Joswiak, and Brownlee, 1994).

Typical IDPs are fine-grained mixtures of thousands to millions of mineral grains and amorphous components. We can picture an IDP as a "matrix" of material with embedded elements which were formed at different times and places in the solar nebula and before our solar nebula's formation. Examples of embedded elements in cosmic dust are GEMS, chondrules, and CAIs.

In (Evans, 1994), the author makes a argument that the gas-to-dust ratio in the interstellar medium suggests that a large fraction of heavy elements (other then hydrogen and helium) must be tied up in dust grains, the assembled elements for the molecules most likely being carbon, nitrogen, oxygen, magnesium, silicon, sulphur, iron, and compounds of these.

Cosmic dust - Radiative properties of cosmic dust

A dust particle interacts with electromagnetic radiation in a way that depends on its cross section, the wavelength of the electromagnetic radiation, and on the nature of the grain: its refractive index, size, etc. The radiation process for an individual grain is called its emissivity, dependent on the grain's efficiency factor. Furthermore, we have to specify whether the emissivity process is extinction, scattering, or absorption. In the radiation emission curves, several important signatures identify the composition of the emitting or absorbing dust particles.

Dust particles can scatter light nonuniformly. Forward-scattered light means that light is redirected slightly by diffraction off its path from the star/sunlight, and back-scattered light is reflected light.

The scattering and extinction ("dimming") of the radiation gives useful information about the dust grain sizes. For example, if the object(s) in one's data is many times brighter in forward-scattered visible light than in back-scattered visible light, then we know that a significant fraction of the particles are about a micrometer in diameter.

The scattering of light from dust grains in long exposure visible photographs is quite noticeable in reflection nebulas, and gives clues about the individual particle's light-scattering properties. In x-ray wavelengths, many scientists are investigating the scattering of x-rays by interstellar dust, and some have suggested that astronomical x-ray sources would possess diffuse haloes, due to the dust.

Cosmic dust - Dust grain formation

The large grains start with the silicate particles forming in the atmospheres of cool stars, and carbon grains in the atmospheres of cool carbon stars. Stars, which have evolved off the main sequence, and which have entered the giant phase of their evolution, are a major source of dust grains in galaxies.

How do astronomers know that that dust is formed in the envelopes of late-evolved stars? They know from their observations. An observed (infrared) 9.7 micrometre emission silicate signature for cool evolved (oxygen-rich giant) stars. And an observed (infrared) 11.5 micrometre emission silicon carbide signature for cool evolved (carbon-rich giant) stars. These help provide evidence that the small silicate particles in space came from the outer envelopes (ejecta) of these stars. (Humphreys, (1972) and (Evans, A., pg 164-167))

How do astronomers know that dust wasn't formed in interstellar space? They know because the conditions in interstellar space are generally not suitable for the formation of silicate cores. The arguments are that: given an observed typical grain diameter a, the time for a grain to attain a, and given the temperature of interstellar gas, it would take considerably longer than the age of universe for interstellar grains to form (Evans, E., pg 147-148). Furthermore, Evans states that grains are seen to form in the vicinity of nearby stars in real-time. "Real-time" meaning in a) nova and supernova ejecta, and b) R Coronae Borealis, which seem to eject discrete clouds containing both gas and dust.

Cosmic dust - Dust grain destruction

How are the interstellar grains destroyed? There are some ultraviolet processes which lead to grain "explosions" (d'Hendecourt et al., 1985; Greenberg, 1976). Evans (1994) also describes evaporation, sputtering (when an atom or ion strikes the surface of a solid with enough momentum to eject atoms from it), and grain-grain collisions, which have a major influence on the grain size distribution, as well.

These destructive processes happen in a variety of places. Some grains are destroyed in the supernovae/novae explosion (and then some grains form sometime afterwards). Some of the dust is ejected out of the protostellar disk in the strong stellar winds that occur during a protostar's active T Tauri phase. Plus there are some gas-phase processes in a dense cloud where ultraviolet photons eject energetic electrons from the grains into the gas.

Dust grains incorporated into stars are also destroyed, but only a relatively small fraction of the mass of a star-forming cloud actually ends up in stars. This means a typical grain goes through many molecular clouds and has mantles added and removed many times before the grain core is destroyed.

Cosmic dust - Some dusty clouds in the universe

Our solar sytem has its own Interplanetary dust cloud; extrasolar systems too.

There are different types of nebulae with different physical causes and processes. One might see the following classifications:

• diffuse nebula
• infrared (IR) reflection nebula
• supernova remnant
• molecular cloud
• HII regions
• photodissociation regions

Distinctions between those types of nebula are that different radiation processes are at work. For example, H II regions, like the Orion Nebula, where a lot of star-formation is taking place, are characterized as thermal emission nebulae. Supernova remnants, on the other hand, like the Crab Nebula, are characterized as nonthermal emission (synchrotron radiation).

Some of the better known dusty regions in the universe are the diffuse nebula in the Messier catalog: M1, M8, M16, M17, M20, M42, M43, for example. You can see these online at

SEDS Messier Catalog

Some larger 'dusty' catalogs that you can access from the NSSDC, CDS, and perhaps other places are:

• Sharpless (1959) A Catalogue of HII Regions
• Lynds (1965) Catalogue of Bright Nebulae
• Lunds (1962) Catalogue of Dark Nebulae
• van den Bergh (1966) Catalogue of Reflection Nebulae
• Green (1988) Rev. Reference Cat. of Galactic SNRs

at

• The National Space Sciences Data Center (NSSDC)
• CDS Online Catalogs

Adapted from the Wikipedia article "Cosmic dust", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki