Copyright Amara Graps 1996-7. All rights reserved.
This item posted to sci.astro on Nov 4, 1996. Amara says that you can find a copy of the text on her Web site at http://www.amara.com/ftpstuff/dustevolve.txt
This essay is about the evolution of dust in the Universe. In particular, I describe some ways that dust forms in dusty clouds, cycles through solar systems (even comets), through a star's late evolution and back into a nebula.
Dust particles are vital components of astrophysical processes and significant constituents of the Universe. For example, they can drive the mass loss that occurs when a star is nearing the end of its life, they are essential parts 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 one of those many-faceted research topics that brings together different scientific fields. In this case, 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.
As you can tell, I think dust and its evolutionary processes are pretty fascinating.
Some distinctions between those types of nebula are that very different radiation processes are at work. For example, HII 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).
The processing of dust in the Galaxy is a wonderfully rich study. The dust grains evolve cyclically- chemically, physically and dynamically. There are at least four kinds of dust populations (Dorschner, J., (1996), pg. 487-8)):
The scenario that I describe here originated with J. Mayo Greenberg and has been expanded upon by many people since then. It is called the grain "core-mantle" model. It has some pretty good observational and theoretical arguments supporting it.
The dust processing cycle, in brief, looks like the following (Dorschner, J., (1996), pg. 487-8). From the stellar winds of evolved stars, new dust is formed and is injected into interstellar space. Young stardust is mixed with old, heavily-processed, diffuse ISM dust, and is subject to passing supernova shocks and ultraviolet radiation. Dusty clouds form. The protostar environment is a fertile ground for solids on all size scales from dust grains to planets (Hanner, M.S. 1995) to form. Star formation in cool molecular clouds become both a *sink* of old dust and well as a *source* of new dust.
Now in more detail.
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 have entired the giant phase of their evolution, are a major source of dust grains in the Galaxy.
How do we know that that dust is formed in the envelopes of late-evolved stars? We have some good observations. An observed (infrared) 9.7 micron emission silicate signature for cool evolved (oxygen-rich giant) stars. And an observed (infrared) 11.5 micron 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. (See Humphreys et al, (1972) ApJ 172, 75, and Evans, A. The Dusty Universe, pg 164-167)
How do we know that dust wasn't formed in interstellar space? We know because the conditions in interstellar space are 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 (see pg 147-148 of Evans' book). Evans says in his book that grains are seen to form in the vicinity of nearby stars in real time. "Real time" meaning a) nova and supernova ejecta, and b) R Coronae Borealis, which seems to eject discrete clouds containing both gas and dust.
Then supernovae/novae explosions of the star eject that material out into space. The dust that you see in supernovae remnants (like from M1, the Crab nebula) was mostly _not_ produced in the supernovae. It was present in the star's outer envelope before the supernovae explosion. The explosion just pushed that material outwards.
Note that new grains _can_ form in the debris of the supernovae explosion (even easier in the debris of a novae explosion) but that material has to be quite cool, so it takes some time (say, on the order of a year) after an explosion for the debris to be cool enough for grains to form. It's a race between dropping temperature, which favors grain formation, and dropping density, which opposes it.
When the grains have cooled down to about 15K, those silicates are the cores for the growth of mantles of ices. The mantles are formed by accretion of (gas phase) atoms and molecules of oxygen, carbon, nitrogen, sulfur, along with hydrogen. The grains are nonspherical, and various sticking processes cause other simple molecules (H2O, CO, H2S, CH3OH, OCS, OCN- etc.) to stick. The mantle ices are always being photoprocessed by ultraviolet (UV) radiation from either distant stars or by UV created by cosmic rays or from local hot stars and/or stellar winds. The photoprocessing changes the basic composition of the ices and causes some complex organic refractory residues to form.
So now the heavily photoprocessed material is out in a diffuse environment (diffuse cloud). That cloud may eventually become a denser molecular cloud. In the denser clouds, more organic mantle formation occurs, layering the silicate core- like the rings in a tree trunk. The innermost layers of the dust grain have been the most irradiated, and the outermost layers are the result of the most recent processes.
Then, within those molecular clouds, critical densities lead to star formation, and that dust gets captured and becomes part of the new star, and the cycle stars all over again. A typical grain anywhere in space will have undergone at least 20 cycles (And remember that, in addition, new grains are formed.)
Dust is an important driving force in the development of a protostar's accretion disk. In particular, it has a large effect on the turbulence and convection in the disk. Before the dust grains settle into a midplane region, (and before the T-Tauri phase of the protostar) the homogenized distribution of the grains are a primary cause of opacity, leading to steep temperature gradients and convection and turbulence. (Other driving forces for turbulence in this early stage may be the angular momentum in the Solar Nebula by infalling matter, and the thermal energy liberated by gravitational collapse.)
How are the grains destroyed? There are some ultraviolet processes which lead to grain "explosions" (d'Hendecourt et al., 1985 Astr Ap, 152, 130; Greenberg, J.M., 1976, Ap. Space Sci, 39, 9). Evans' book 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 pretty complicated 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.
I said previously that a typical grain anywhere in space will have undergone at least 20 cycles. How does this work, since a typical star lives for 5+ billion years, and the time since the Big Bang is only 15 billion years (say)?
Mayo Greenberg explains (in Greenberg's chapter of the IAU Symposium #135: Interstellar Dust book) : "The mean star production rate of 12 solar masses per year implies an interstellar medium turnover time of ~5*10^9 years, so that this is the absolute maximum lifetime of a dust particle no matter how resistant to destruction. If we use a mean molecular cloud-diffuse cloud period of 2*10^8 years (10^8 years in each), then a typical grain anywhere in space will have undergone at least 20 cycles so that, for example, the typical diffuse cloud dust particle age is greater than 10^9 years and consists of a mix of particles which have undergone a wide variety of photoprocessing."
So the grain recycling works through molecular clouds with the formation and destruction of grain mantles. One time in 20 or so, the grain core gets incorporated into a star, and is destroyed. The other times, the grain gets ejected, and only the mantle is destroyed.
I found an interesting table in Gehrz's chapter (pg 447) of the IAU #135 Interstellar Dust book: "Types of Dust Grains in Stellar Outflows".
Stellar Type Input to Interstellar Medium, Relative to all Stars M Stars (Miras) 35% RLOH/IR stars 32% Carbon stars 20% Supernovae 8% M supergiants 4% Wolf-Rayet stars 0.5% Planetary Nebulae 0.2% Novae 0.1% RV Tauri stars 0.02% O,B stars 0
Gehrz concludes in his last section titled: "The Ecology of Stardust in the Galaxy" that:
1. M stars, RLOH/IR stars and M supergiants are the primary sources of silicates, while carbon stars, WR stars and novae produce most of the carbon and SiC. Novae, supernovae, and WR stars may be responsible for most of the grains with chemical anomalies.
2. The current star formation rate implies that star formation is depleting the interstellar medium (ISM) gas by some 3 to 10 solar masses per year.
3. There is a deficit in stardust production/grain destruction. Supernovae shock waves destroy ISM grains on very short time-scales (Seab, 1987, _Interstellar Processes_, Hollenbach and Thronson ed. Reidel) processing 10-30 solar masses per year and destroying 0.1-0.3 solar masses per year in dust. Gehrz estimates that 0.01-0.08 solar masses per year of dust is returned to the ISM by stars. He feels that grain growth in dark clouds is an attractive mechanism to make up the dust deficit.
Studies of the local interstellar medium (LISM) during the last 20 years have yielded a picture of the Sun located in a cool (~7000 K according to Axford), low-density region (Ferlet, R., et al. 1991) flowing past our Solar System, this region being immersed in an irregular, very hot (~million degree K), and tenuous (~0.05 per cubic center), larger, local bubble of tens to hundreds of parsecs in size. (Axford and Suess, 1995). The larger local bubble is thought to have been produced by a combination of one or more supernovae and stellar winds associated with a group of O and B stars.
One technique to learn about the dusty cloud environment out of which our Solar System formed is by examining some of the primitive meteorites that have reached Earth. Primitive meteorites contain a few parts per million of pristine interstellar grains that provide information on nuclear and chemical processes in stars (this information is from Anders and Zinner, 1993). The meteorites' grains' interstellar origin is shown by isotope ratios that are highly anomalous to isotope ratios of the same material on Earth. Microdiamonds, of average size 10 angstroms, contain anomalous noble gases which shows the signature of the nuclear r- and p processes. Silicon carbide, of grain size ~1 micron, shows the signature of the s-process, and most likely comes from red giant carbon (AGB) stars of 1-3 solar masses. Graphite spheres, of size ~1 micron, contain highly anomalous carbon and noble gases, as well as large amounts of fossil Mg-26 from the decay of Al-26, which seem to come from at least three sources: AGB stars, novae and Wolf-Rayet stars.
Closer to home, the interplanetary dust cloud has been studied for many years in order to understand its nature, origin, and relationship to solar systems (our own, as well as extrasolar systems).
The interplanetary dust particles (IDP) not only scatter solar light (called the "zodiacal light", which is confined to the ecliptic plane), the IDP also produce thermal emission, which is the most prominent feature of the night sky light in the 5-50 micron wavelength domain (Levasseur-Regourd, A.C. 1996) The grains characterizing the infrared emission near the earth's orbit have typical sizes of 10-100 microns (Backman, D., 1997). The surface area corresponds to a mass of roughly 10^{18}--10^{19} grams for a density of 3 grams-cm^-3, equivalent to a single solid body 5-10 kilometers in radius.
The sources of IDP include at least: asteroid collisions, cometary activity and collisions in the inner solar system, Kuiper Belt collisions, and ISM grains. (Backman, D., 1997)
The main physical processes "affecting" (destruction or expulsion mechanims) IDP are: expulsion by radiation pressure, inward Poynting-Robertson (PR) radiation drag, solar wind pressure (with significant electromagnetic effects), sublimation, mutual collisions, and the dynamical effects of planets. (Backman, D., 1997)
The lifetimes of these dust particles are very short compared to the lifetime of the Sun. If one finds grains around a star that is older than 10^8 years, then the grains *must* have been from recently released fragments of larger objects, i.e. they cannot be leftover grains from the protoplanetary nebula. (Backman, private communication) Therefore, the grains would be "second-generation" dust. The zodiacal dust in our Solar Sytem is 99.9% second-generation dust, 0.1% intruding ISM dust, and 0% primodial grains from the Solar Sytem's formation. (Primordial grains can now only be found embedded in unaltered meteorites.)
The interplanetary dust cloud has a complex structure (Reach, W., 1997). It has:
The isotope ratios of comet and interstellar dust are very similar, indicating a common origin (see R.F. Knacke chapter: "Comet Dust" in IAU Symposium No. 135, _Interstellar Dust_ ed. Allamandola and Tielens, publ. by Kluwer, 1989). Interplanetary dust particle isotope ratios are quite different.
In this section I expand on the dusty link between the molecular clouds and comets in a solar system.
Long-period comets are often thought to be pristine, unaltered relics since their formation. They may be the best probes of ancient Solar System (and pre-solar) processes available to us. (However, some experts with good observational evidence are challenging this view. See recent work by S.A. Stern)
A long-period comet in a highly eccentric orbit with a perihelion distance of only a few solar radii will experience a range of diverse conditions as it traverses its orbit (Lewis, 1995). Almost all of the time it will be so far from the Sun that it will be too cold for evaporation of ices to occur. When it passes through the terrestrial planet region, evaporation will be rapid enough to blow away small grains, but the largest grains may resist entrainment and stay behind on the comet nucleus, beginning the formation of a dust layer. Near perihelion, the heating and evaporation rate will be so great, that no dust can be retained.
Therefore, the thickness of dust layers covering the nuclei of a comet can indicate how closely and how often a comet's perihelion travels are to the Sun. If a comet has an accumulation of thick volatile-depleted dust layers, it may have frequent perihelion passages that don't approach the Sun too closely.
This thick accumulation of dust layers is actually a good description of virtually all of the periodic *short-period* comets. The general conclusion of studies by David Brin (UCSD), Harry Houpis (UCSD), Asoka Mendis (UCSD), Fraser Fanale (U HI), James Salvail (U HI), Paul Weissman (JPL), Hugh Keiffer (USGS), and others has been that dust layers with thicknesses of order meters may accumulate on the surfaces of short-period comet nuclei.
Note that the accumulation of dust layers over time would change the physical character of the short-period comet. A dust layer both inhibits the heating of the cometary ices by the Sun (the dust is very opaque and a poor conductor of heat), and slows the loss of gases from below. A comet nucleus in an orbit typical of short period comets would quickly decrease its evaporation rate to the point that neither a coma or tail would be detectable. Such a body may be described by terrestrial astronomers as a low-albedo near-Earth asteroid. It would be able to retain much of its ices over its entire dynamical lifetime (of ~100 M yr).
Comet dust can provide clues to comets' orgin, and the formation of our solar system. There are two main models about where comets came from: 1) the interstellar model and 2) the solar system model. The remaining portion of this essay came from information in Science News 149, June 1, 1996, pg. 346-347.
The interstellar model is the following. It says that ices formed on dust grains in the dense cloud that preceded the Sun. The mix of ice and dust then gathers together into a comet without appreciable chemical modification. J. Mayo Greenberg first proposed this idea in 1986.
In the solar system model, the ices that formed in the interstellar cloud didn't initially gather together, but vaporized as the ices to become part of the accretion disk of gas and dust around the protosun. The vaporized ices later resolidified and assembled into comets. So the comets in this model would have a little bit different composition than those comets that were made directly from interstellar ice.
Comet Hyakutake data has been very good so far for providing some clues to the above.
Michael Mumma's et al's IR observations from the IRTF detected ethane in the comet. He thought at first that it showed that the comet originated near Jupiter's orbit, which had a variety of hydrocarbons. Now he thinks that the comet originated directly from a chilly part of the interstellar cloud (which can't exceed 20K). He has some confirming evidence for this theory from the fact that Hyakutake's nucleus has very little carbon monoxide, which makes chemical sense if the carbon is tied up in heavier molecules like ethane.
See, I'll bet you didn't know before that those annoying little dust particles are significant constituents of the Universe and vital components of astrophysical processes.
I condensed parts of this essay from three or four shorter dust essays and one long thread titled "Supernovae Distribution" running on sci.astro in January 1996. I want to thank Steve Willner in particular for his comments on that sci.astro thread.
The interplanetary dust sections were derived, in part, from reports given at the Exozody Workshop, NASA-Ames Research Center, Mountain View, California, October 23-25, 1997, and from discussions with the participants before, during, and after the Workshop. In particular, I want to thank Dana Backman for many of our discussions.
Allamandola, Lou and A.G.G.M Tielens eds., _IAU Symposium 135: Interstellar Dust_, 1989, Kluwer Press. (I used Mayo Greenberg's chapter: "The Core-Mantle Model of Interstellar Grains and the Cosmic Dust Connection" as a source for the sections of "Dust Grain Formation" and "Dust Grain Recycling" above.)
Anders, Edward, and Ernst Zinner, (1993), "Interstellar grains in primitive meteorites- diamond, silicon carbide, and graphite," in _Meteoritics_, 28, 490-514.
Axford, W.I. and S.T. Suess, "The Heliosphere," http://web.mit.edu/afs/athena.mit.edu/org/s/space/www/helio.review/ axford.suess.html
Backman, Dana, Extrasolar Zodiacal Emission - NASA Study Panel Report, 1997 for Exozody Workshop, NASA-Ames, October 23-25, 1997. http://astrobiology.arc.nasa.gov/workshops/zodiac/backman/backman.txt
Dermott, S.F. Jayaraman, S., Xu, Y.L., Gustafson, A.A.S., Liou, J.C., (1994), "A circumsolar ring of asteroid dust in resonant lock with the Earth," Nature 369, June 30, 1994, pg. 79.
Dermott, S.F., in talk titled "Signatures of Planets in Zodiacal Light," Exozody Workshop, NASA-Ames, October 23, 1997.
Dorschner, Johann, "Properties of Interstellar Dust," _Physics, Chemistry and Dynamics of Interplanetary Dust_, Gustafson, Bo A.S. and Martha Hanner, ed., , ASP Conference series, Vol 104, 1996, p 487-506.
Evans, Aneurin, _The Dusty Universe_ , Ellis Horwood, 1994. (excellent reference)
Ferlet, R., A. Vidal-Madjar, et al. (1991). Structure of the local interstellar medium. (Astrophysics at FUV and EUV wavelengths; Proceedings of the Topical Meeting of the Interdisciplinary Scientific Commission E /Meeting E3/ of the COSPAR 28th Plenary Meeting, The Hague, Netherlands, June 25-July 6, 1990. A92-19151 06-90) Advances in Space Research (ISSN 0273-1177) 11(11, 1991): 81-86.
Greenberg, J. Mayo and J.I. Hage, (1990) "From Interstellar Dust to Comets: A Unification of Observational Constraints," ApJ 361, 260-274.
Hanner, M.S. 1995, Highlights Astron. 10, 351.
Levasseur-Regourd, A.C., 1996, "Optical and Thermal Properties of Zodiacal Dust," _Physics, Chemistry and Dynamics of Interplanetary Dust_, ASP Conference series, Vol 104, 1996., p. 301.
John S. Lewis, _Physics and Chemistry of the Solar System_, Academic Press, 1995, pp. 285, 292-293
Reach, W., in talk titled "General Structure of the Zodiacal Dust Cloud," Exozody Workshop, NASA-Ames, October 23, 1997.