From Where Does All that Dust Come ?

[dougettinger]

Observing the Sombero Galaxy and the recent APOD of the Center of Centaurus A one cannot help but see the dark or opaque dust clouds and lanes. And in so-called star nurseries, dust is amply available for protostar formation. Where does this dust come from ? I presume this dust is made of materials other than helium and hydrogen because of its opaqueness (?) Does this dust come strictly from supernovae that are able to produce heavier metals than H and He ? Do novae also contribute to this dust or do they only produce higher isotopes of H and He that are not opaque after the red giant phase ?

Why do we see this prolific dark dust in spiral galaxies, but never in eliptical galaxies and in closed clusters surrounding galaxies ?

And if there was no dust in the beginning after the Big Bang phase change of atoms what created the very first stars ?

Doug Ettinger
Pittburgh, PA

[bystander]

Cosmic dust
Interstellar medium
List of molecules in interstellar space

Stellar Birth

[neufer]

Observing the Sombero Galaxy and the recent APOD of the Center of Centaurus A one cannot help but see the dark or opaque dust clouds and lanes. And in so-called star nurseries, dust is amply available for protostar formation. Where does this dust come from ? I presume this dust is made of materials other than helium and hydrogen because of its opaqueness (?) Does this dust come strictly from supernovae that are able to produce heavier metals than H and He ? Do novae also contribute to this dust or do they only produce higher isotopes of H and He that are not opaque after the red giant phase ?

Why do we see this prolific dark dust in spiral galaxies, but never in elliptical galaxies and in closed clusters surrounding galaxies ?

And if there was no dust in the beginning after the Big Bang phase change of atoms what created the very first stars ?

This subject is new to me but maybe we can figure out your answers after a little reading:

<<Stardust grains are contained within meteorites, from which they are extracted in terrestrial laboratories. So-called carbonaceous chondrites are especially fertile reservoirs of stardust. Each stardust grain existed before the earth was formed. The meteorites have preserved the previously interstellar stardust grains since that time. Stardust is a scientific term; not just a poetic one, referring to refractory dust grains that condensed from cooling ejected gases from individual presolar stars. Many different types of stardust have been identified by laboratory measurements of the highly unusual isotopic composition of the chemical elements that comprise each stardust grain. Many new aspects of nucleosynthesis have been discovered from those isotopic ratios.

The following website http://www.dtm.ciw.edu/users/nittler/psg_main.html contains
an excellent introduction to, and photographs of, many differing types of stardust.

An important property of stardust is the hard, refractory, high-temperature nature of the grains. Prominent are silicon carbide, graphite, aluminum oxide, aluminum spinel, and other such grains that would condense at high temperature from a cooling gas, such as in stellar winds or in the decompression of the inside of a supernova. They differ greatly from the solids formed at low temperature within the interstellar medium. Also important are their extreme isotopic compositions, which are expected to exist nowhere in the interstellar medium. This also suggests that the stardust condensed from the gases of individual stars before the isotopes could be diluted by mixing with the interstellar medium. These allow the source stars to be identified. For example, the heavy elements within the SiC grains are almost pure s process isotopes, fitting their condensation within AGB star red giant winds inasmuch as the AGB stars are the main source of s process nucleosynthesis and have atmospheres observed by astronomers to be highly enriched in dredged-up s process elements. Another dramatic example comes from the supernova condensates, usually shortened by acronym to SUNOCON to distinguish them from other stardust condensed within stellar atmospheres. SUNOCONs show evidence that they condensed containing abundant radioactive ⁴⁴Ti, which has a 65 yr halflife. It was thus still alive when the SUNOCON condensed within the expanding supernova interior but would have been extinct after mixing with the interstellar gas. Its discovery proved the prediction from 1975 to identify SUNOCONs in this way. But SiC SUNOCONs are only about 1% as numerous as are SiC stardust.

Exciting as stardust is, it is but a modest fraction of the condensed cosmic dust. It seems that stardust is less than 0.1% of the mass of total interstellar solids. Its interest lies in the new information that it has brought to the sciences of stellar evolution and nucleosynthesis.

A fascinating aspect to human culture is the study within terrestrial laboratories of solids that existed before the earth existed. This was once thought impossible, especially in the decades when cosmochemists were confident that the solar system began as a hot gas virtually devoid of any remaining solids, which would have been vaporized by high temperature. The very existence of stardust shows that this historic picture was incorrect.

Cosmic dust is made of dust grains and aggregates 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, and conversely, a compositional analysis of a dust particle can reveal much about the dust particle's origin. General diffuse interstellar medium dust, dust grains in dense clouds, planetary rings dust, and circumstellar dust, are each different in their characteristics. For example, grains in dense clouds have acquired a mantle of ice and on average are larger than dust particles in the diffuse interstellar medium. Interplanetary dust particles (IDPs) are generally larger still. Most of the influx of extraterrestrial matter that falls onto the Earth is dominated by meteoroids with diameters in the range 50 to 500 micrometers, of average density 2.0 g/cm³ (with porosity about 40%). The densities of most IDPs captured in the Earth's stratosphere range between 1 and 3 g/cm³, with an average density at about 2.0 g/cm³.

Other specific dust properties:

• * In circumstellar dust, astronomers have found molecular signatures of CO, silicon carbide, amorphous silicate, polycyclic aromatic hydrocarbons, water ice, and polyformaldehyde, among others. (In the diffuse interstellar medium, there is evidence for silicate and carbon grains.)
  
  * Cometary dust is generally different (with overlap) from asteroidal dust. Asteroidal dust resembles carbonaceous chondritic meteorites, and cometary dust resembles interstellar grains, which can include the elements: silicates, polycyclic aromatic hydrocarbons, and water ice.

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 that have evolved off the main sequence and have entered the giant phase of their evolution are a major source of dust grains in galaxies. Star dust, sung and written in the popular media, is a colloquial term referring to the birthplace of most dust grains in the Universe. If one indeed traces the origin of the elements out of which human bodies are made, they are star dust.

Astronomers know that the dust is formed in the envelopes of late-evolved stars from specific observational signatures. An (infrared) 9.7 micrometre emission silicate signature is observed for cool evolved (oxygen-rich giant) stars. And an (infrared) 11.5 micrometre emission silicon carbide signature is observed 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.

It is believed that 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 the universe for interstellar grains to form. Furthermore, grains are seen to form in the vicinity of nearby stars in 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.

Most dust in our solar system is highly processed dust, recycled from the material out of which our solar system formed and subsequently collected in the planetesimals, and leftover solid material (for example: comets and asteroids), and reformed in each of those bodies' collisional lifetimes. During our solar system's formation history, the most abundant element was (and still is) H₂. The metallic elements: magnesium, silicon, and iron, which are the principal ingredients of rocky planets, condensed into solids at the highest temperatures. The range of elements of the solar nebula between H₂ and (Mg, Si, Fe) is not known well (Wood, J., 1999). Some molecules such as CO, N₂, NH₃, and free oxygen, existed in a gas phase. Some molecules, for example, graphite (C) and SiC condensed into solid grains. Some molecules also formed complex organic compounds and some molecules formed frozen ice mantles, of which either could coat the "refractory" (Mg, Si, Fe) grain cores.

The formation of these molecules was determined, in large part, by the temperature of the solar nebula. Since the temperature of the solar nebula decreased with heliocentric distance, scientists can infer a dust grain's origin(s) with knowledge of the grain's materials. Some materials could only have been formed at high temperatures, while other grain materials could only have been formed at much lower temperatures. The materials in a single interplanetary dust particle often show that the grain elements formed in different locations and at different times in the solar nebula. Most of the matter present in the original solar nebula has since disappeared; drawn into the Sun, expelled into interstellar space, or reprocessed, for example, as part of the planets, asteroids or comets.>>

[dougettinger]

Art, thanks for you prompt and excellent reply. Apparently, both nova and supernova are responsible for the dust in spiral galaxies. I am still pondering why there are no dust lanes or opaqueness due to dust found for eliptical galaxies ?

Doug Ettinger
Pittsburgh, PA

[dougettinger]

I will try to answer my own question simply by using common sense. Currently, I do have any good references to answer this question directly.

Typically, stars found in eliptical galaxies have low metal content; therefore, these stars must be consistently of a certain size that are long-lived and have not evolved in phases via novae and supernovae such as occurs very frequently in spiral galaxies. The continued disturbances in spiral galaxies, that seem not to occur in eliptical galaxies, create more massive stars, that in turn create novae and supernovae, that in turn create the higher metal stars and higher dust concentrations.

If someone wishes to refute or enhance this concept, please do so. Thank you.

Doug Ettinger
Pittsburgh, PA