These words seem a lugubrious choice forthe title of a Presidential Address, sowhy have I chosen them? The words are

frequently used in funeral services, emphasiz-ing the story in the Book of Genesis: we aremade of Earth’s dust and to dust we mustreturn. There is no escape from the logic ofthose words: we are part of a cycle of matterinto and out of the Earth.

However, these words remind me of a muchgrander astronomical cycle: the cycle of matterinto stars, in the process of star formation frominterstellar gas, followed by the ejection ofmatter, including the ashes of stellar nucle-osynthesis – often in the form of dust grains –back into the interstellar medium. As that cycleis repeated, so the interstellar medium isincreasingly enriched in the heavy elementsthat are the products of nucleosynthesis and inthe dust grains which those elements can form.

Of course, there is a link between the twocycles, the small-scale cycle of matter betweenhuman bodies and the Earth and the grand-scale cycle of matter into and out of stars andthat link is dust. Planet Earth is simply inter-stellar dust processed in the events that led tothe formation of the Sun some 4.6 billion yearsago. That interstellar dust began its journeyinside a star, was ejected into space and mixedwith the interstellar gas, and ultimately wasincorporated into the interstellar cloud thatcontracted to form the proto-solar nebula fromwhich the Sun and planets formed. Thus, dust isinvolved in both cycles. We shall see that in thegrand astronomical cycle, dust is no mere pas-senger, but an important and active component.

The relation between the solar system andcosmic dust is not simply that the interstellarmedium provided the building material for theSun and planets, though that is certainly thecase. The solar system is also continually bom-barded by interstellar matter, including inter-stellar dust. This was one of the remarkablediscoveries of the Ulysses spacecraft on itsJupiter flyby: high-velocity dust was enteringthe solar system (Grün et al. 1994) with a con-siderable flux, about 5 ´10–21 g cm–2 s–1, fordust grains with a radius of about 0.4 µm(comparable with size of grains that causeinterstellar extinction). A few million tonnes ofinterstellar dust enter the solar system withinJupiter’s orbit per day. Some of this will mixwith interplanetary dust, and about 100 tonnesof interplanetary dust arrives at Earth per day.

Although most of the discussion in this arti-cle will be concerned with interstellar matter inour galaxy, the Milky Way, this is a restrictionmerely of convenience. In fact, interstellar gasand dust is now detected even in very distantgalaxies, representing epochs when the uni-verse had evolved to less than 10% of its pre-sent age. In these objects, molecular gas anddust are clearly evident (Omont et al. 1996,

Ohta et al. 1996). The results of Omont et al.show both 1.3 mm continuum emission fromdust and CO (5–4) line emission from inter-stellar matter in a gravitationally lensed quasarat a redshift of z = 4.69. Evidently, star forma-tion in the early universe has been sufficientlyrapid to give a substantial abundance of theheavy elements that are required for moleculesand dust. The universe, it seems, is chemicallyricher and more dusty then we had believed,and the molecules and dust that are the conse-quences of that chemical richness provide newways of probing the distant universe.

The concept of dust as an important, activecomponent in the universe is relatively recent.Although the presence of interstellar dust hasbeen explicitly recognized for a century ormore, and was first indicated by the descrip-tion by William Herschel (first President of ourSociety) of “holes in the Heavens”, for manyastronomers dust has simply been an irritatingfog that prevents a clear optical view of starsand galaxies. The advent of infrared,millimetre-wave and submillimetre-waveastronomy have changed that concept for ever,and UK astronomers using telescopes such asUKIRT, AAT, and JCMT and space facilitiessuch as IRAS and ISO have made major con-tributions. Dust introduces the problems ofsolid-state and surface science into astronomy,and the work is supported by laboratory stud-ies of dust analogues, and by work of exquisiteprecision on the analysis of meteorites.

The theme of this article is straightforward.This is a dusty universe; we know fairly wellthe composition of the dust; we also now rec-ognize that dust has important and active rolesto play in the evolution of the universe. Itdeserves our study, and not merely because weand our planet are composed of star-dust.

In the first part of this article I shall describeour present understanding of the nature of cos-mic dust. In the second part, I shall describe afew important roles of dust.

Characterization of cosmic dustWe have a broad understanding of the natureof cosmic dust. We can observe on timescalesof weeks or months the consequences of dustformation in supernovae, novae, and stellarenvelopes and we can infer that these are themain sources of dust injection into the inter-stellar medium, in the ratio about 74:24:2(Dorschner and Henning 1996). We know thatsome of these objects are carbon-rich, andsome kind of sooty carbonaceous grains areassumed to arise in these cases (much as sootforms in a candle flame). In oxygen-richobjects, detection of infrared absorptions cor-responding to the Si–O bond stretching andbending modes indicates silicates. In darkerregions of interstellar space, various kinds ofmolecular ice, principally H2O-ice, are detect-

ed, again by infrared absorption. Thus, in gen-eral terms we know what the dust is made of,but if we wish to determine its detailed proper-ties we shall need to be considerably more pre-cise. For example, table 1 lists some proposalsthat have been made just for the nature of thecarbonaceous dust. While there is some basicsimilarity between all these materials, theirchemical and physical properties in the inter-stellar medium will be widely different.

How can we find out about interstellar dust?The traditional source of information has beenthe variation of interstellar extinction withwavelength. In fact, the standard interstellarextinction is smooth and contains few features(figure 1) and is therefore rather easily “curve-fitted” by a wide range of models. There issurely more information to be obtained fromthe variety of interstellar extinction curves (fig-ure 2) once a plausible model has been devel-oped. But the interstellar extinction curve isnot the best starting point; its information con-tent is too low and non-specific. A comprehen-sive list of information sources is in table 2.

A good indication of the chemical nature ofinterstellar dust can be obtained from measuresof depletion for lines of sight through diffuse

Cosmic dust

3.8 June 2000 Vol 41

Ashes tdust t

An account of the RAS Presidential by David A Williams, Perren Professor of

Dust has long been recognized in

astronomy; it is only in recent

years, however, that it has been

seen as both important and active.

Infrared, millimetre and sub-

millimetre wave astronomy are now

unravelling the chemistry and

composition of interstellar dust,

showing that it includes silicate and

carbon grains, hydrocarbon

molecules, and ices. Far from just

obscuring the view, dust is providing

new ways to understand the

workings of the universe.

material towards bright stars. Compared to areference source such as HII regions aroundbright stars, one finds that many elements arevery underabundant relative to hydrogen. Forexample, more than 99% of Fe atoms (relativeto H) are missing from the gas along the line of

sight to the bright star z Oph and more than99.9% of Ca atoms (relative to H) are missing(see figure 3). About two thirds of the C and Oatoms expected on this line of sight, relative toH atoms, are missing. If we assume that theinterstellar medium is reasonably well mixed,then we have to account for these missingatoms. The inference is that they are locked upin dust, and we can then work out the gas:dustmass ratio (usually around 100) and a crudechemical composition (which supports the soot

and silicate picture previously indicated).

The ISO dataObservations of dust using the Infrared SpaceObservatory have made significant advancesbeyond the general picture described above.ISO gave continuous wavelength coveragefrom about 2 µm up to 200 µm, a spectralregion previously only selectively explored.Figure 4 (from Whittet et al. 1996) gives a typ-ical ISO infrared absorption spectrum of cold

Cosmic dust

3.9June 2000 Vol 41

Table 1: Species suggested ascomponents of carbonaceousdust (after Duley 1993)

Amorphous carbonBacteria (freeze-dried)CarbynesCoalDiamondDiamond-like carbon (DLC)FullerenesGlassy carbonGraphiteHydrogenated amorphous carbonPolycyclic aromatic hydrocarbons (PAHs)Pyrolytic graphiteQuasi-carbonaceous condensate (QCC)SootViruses“Yellow stuff”

o ashes,odustAddress of 11 February 2000, given Astronomy, University College London.

15

13

11

9

7

5

3

10

0 1 2 3 4 5 6 7 8 9 10l–1 (mm–1)

t

1: The “standard” interstellar extinction curve,presented in terms of the variation of opticaldepth, t with inverse wavelength l (in µm).

2: The variations in theinterstellar extinctioncurve for different lines ofsight (adapted from Witt etal. 1984). The curveconnecting the filledsquares is the “standard”interstellar extinctioncurve.

100%

10%

Kr N

C

1%

0.1%

0 200 400 600 800 1000 1200 1400

perc

enta

ge o

f ele

men

t in

gas

BZn

Ge

Ga

Cu Mn

P

Mg

Cr Fe

NiTi

Co

O

S

VHELIO = –15 km s–1

condensation temperature (K)

3: Depletions of the elementsin an interstellar cloudtowards z Oph (adapted fromFederman et al. 1993). Thefigure shows the percentageof the cosmic abundance ofvarious elements that are inthe gas phase, in thisparticular interstellar cloud.These data are shown as afunction of the condensationtemperature of eachelement.

weak UV rise

“average”

weak bump

strong bump

strong UV rise

2 3 4 5 6 7 8 91/l (µm–1)

10

8

6

4

2

0

0

0

0

0

0

0

10

8

6

4

2

0

E(l–

V) /

E(B–

V)

HD167838

HD166734

HD170740

BD+23°3745

HD190603

HD194153

BD+41°3807

HD197512

HD204827

BD+63°1964

dust towards a high-mass embedded stellarobject NGC 7538:IRS9. The spectrum showsnot only silicate features at wavelengths near10 µm and 20 µm arising from Si–O stretchingand bending modes from the underlying sili-cate grains, but also the chemical variety of icymantles on the silicates. The very strong H2O-ice absorption at 3 µm (O–H stretch) is wellknown, and here is accompanied by a featureat 6.3 µm (H2O bending); the strong CO2

bands at 4.3 µm (stretching) and 15.3 µm(bending) were unexpected. Solid CO at4.7 µm was previously well studied, but for thefirst time we see that CO2 may be more impor-tant. There are small amounts of other species(formic acid HCOOH and methane CH4) but –so far, at least – the great richness of gas phasechemistry (over 100 different varieties of mole-cule detected) is not reflected in detections ofices on dust in cold dark clouds. It is likely,however, that the chemical richness is also pre-sent in the ice, but at currently undetectablelevels. All these identifications rest on somesuperb laboratory work on ices (Schutte 1999,Ehrenfreund 1999, and references therein).

Dust close to a hot star may be heated suffi-ciently to emit in the infrared region. An exam-ple of the emission spectrum of warm dust isshown in figure 5; this is the ISO spectrum ofthe reflection nebula NGC 7023. Such spectrahave been well studied earlier but the ISO spec-trum is more precise. The general interpreta-tion given to emission spectra of this type wasmade first by Sellgren (1984); there are dustgrains sufficiently small to be heated signifi-cantly by the absorption of single photons ofvisible or UV radiation. The heated grain thencools rapidly by radiating in the infrared; it isthis cooling radiation that is detected. Thespectrum illustrated in figure 5 has featuresthat are generally similar to those of polycyclicaromatic hydrocarbons (PAHs). For example,the aromatic C–H stretching and bendingmode wavelengths are near 3.3 and 11.3 µm.The observed spectrum is related in some wayto PAHs, but no specific identifications haveyet been made. This uncertainty has led to theinvention of many names for this spectrum: theUnidentified Infrared Bands (UIBs); the Aro-matic Infrared Bands (AIBs); the Infrared Emis-sion Features (IEFs); as well as the PAH bands.

In spite of the lack of precise identification,there seems little uncertainty about the exis-tence of many small grains or large moleculesin the vicinity of hot stars. One possibility isthat the PAHs are not themselves well defined,but simply fragments of the erosion of largercarbon particles. It is unclear to what extentthe general interstellar medium is populatedwith these fragments. Large molecules will besubject to chemical attack and photoprocess-ing, so a continual supply would be needed.There is a general all-sky emission discovered

by the InfraRed Astronomical Satellite (IRAS)which suggests that very small grains are ubiq-uitous. Further support for this point of viewcomes from the study of (optical) Diffuse Inter-stellar Bands (DIBs). High-resolution spec-troscopy of several bands shows structure thatmay be the unresolved rotational envelope oflarge molecules (Hibbins et al., see reviews byHerbig 1995 and Williams 1996).

One remarkable feature of the UIBs is thatthey are stable in wavelength when the intensi-ty of radiation varies by more than a factor of104. Can PAH molecules be sufficiently photo-resistant, or is there another carrier of theUIBs? We return to this point below.

ISO has also given new information aboutthe silicate component of interstellar dust. Ithas been assumed that interstellar silicates areamorphous, because the absorption near10 µm is smooth and well fitted by laboratorydata for amorphous silicates. However, theISO emission spectra for warm dust aroundstars (Waters 1999) is in part crystalline (figure6). The individual features in the emissionspectrum allow a detailed mineralogical studyof circumstellar dust. Bands at 10.2, 11.4, 16.5,19.8, 23.8, 27.9, 33.7 and 69 µm indicate thepresence of pure forsterite, i.e. an Fe-poorolivine. Pyroxene is less abundant, and thecrystalline:amorphous ratio is less than 10%. It

appears that though the amorphous silicate isdominant, individual silicate types can now beidentified. No evidence for crystalline compo-nents in the interstellar absorption feature hasbeen found, but it may contain crystalline com-ponents whose spectra are smoothed in a com-posite of many silicate types, and varyingFe/Mg composition (Bowey and Adamson2000). Studies of these trends will reveal muchabout the nature of the annealing process inthe interstellar medium. It seems clear thateven silicates must evolve due to shocks.

A comparison of ISO data for emission fromdust around a young star and dust of cometHale-Bopp (Malfait et al. 1998) shows strongsimilarity, with crystalline forsterite in both.This supports the idea that some crystalline sil-icates can survive the journey from circumstel-lar origin, through the interstellar medium,into the proto-solar nebula.

The interplanetary–interstellarconnectionThe study of interplanetary dust and of mete-orites is far more advanced than the study ofinterstellar or circumstellar dust. Circumstellarand interstellar dust can only be examined byindirect means (see table 2). By contrast, solarsystem materials can be collected and exam-ined in the laboratory, and detailed physical

Cosmic dust

3.10 June 2000 Vol 41

3 ´ 10–16

10–16

3 ´ 10–17

10–17

3 ´ 10–18

3 5 7 10 20

HCOOH

CH4??

H2O13CO2

CO

CO2

??

OCN–?

CO2

silicates

silicates

NGC7538:IRS9

H2O

F (W

cm

–2 m

m)

l (mm)

1 ´ 10–12

8 ´ 10–13

6 ´ 10–13

4 ´ 10–13

2 ´ 10–13

0

2.5 5 10 25wavelength (mm)

W m

–2 m

m –

1

6.2

mm 7.

7 m

m8.

6 m

m

11.0

mm

11.3

mm

12.7

mm

13.6

mm 16

.4 m

m

3.3

mm

4: The ISO spectrumof the embeddedstellar objectNGC7538:IRS9(Whittet et al. 1996).The spectrum showsabsorption featuresarising from variousmolecular species inthe ice mantles ondust grains.

5: The ISOspectrum of thereflection nebulaNGC7023 (Moutouet al. 1999).

and chemical properties determined. Since allthe interplanetary material was originallyinterstellar, then – making allowance for pro-cessing within the proto-solar nebula – we maybe able to use interplanetary material as aguide to the nature of interstellar dust.

One of the most significant of recent resultsemphasizes this interstellar–interplanetary con-nection. Bradley et al. (1999) have identified inthe matrices of chondritic porous interplane-tary dust particles, embedded in a layer ofamorphous carbon, “GEMS”, or Glass withEmbedded Metals and Sulphides. These GEMShave a typical size of about 0.5 µm. The GEMSare silicates, and show absorption featuresnear 9 and 18 µm. They contain FeNi and FeSnanometre-sized inclusions that are superpara-magnetic. The GEMS are Mg-rich and Fe-poor.

The list of determined properties is almostexactly what one would write down as desir-able properties for interstellar dust. First, theparticles are silicates of about the size rangeneeded to cause interstellar extinction at visualwavelengths. Secondly, they show infraredabsorption features that match the interstellarfeatures at 9 and 18 µm, corresponding to the

Si–O stretching and bending motions withinsilicates. Thirdly, the superparamagnetic inclu-sions are precisely what had been predicted byMathis (1986) as necessary to produce the par-tial alignment of large dust grains by the inter-stellar magnetic field; this alignment leads topartial polarization of starlight within theinterstellar medium. Interstellar magnetic fieldsare too weak, and silicates of too low a mag-netic susceptibility to provide this alignmentwithout superparamagnetic inclusions in thedust grains. Finally, the sulphur, magnesiumand iron content is as predicted from depletionmeasurements. Thus, these scientifically pre-cious GEMS may be providing us with a sam-ple of largely unprocessed interstellar dust.

Meteorites are another source of largelyunprocessed interstellar material. Primitivemeteorites contain interstellar grains of variouskinds, including diamond, graphite, silicon car-bide, and aluminium oxide. The isotopic com-positions of these grains can be determined,and reflect the type of star in which the grainswere formed; the isotopic abundances of thegrains are unlikely to change during the pas-sage of the grains through the interstellar medi-um. It is evident from these studies that theinterstellar cloud that generated the proto-solar nebula had received dust contributionsfrom many types of star. Therefore, mixing inthe interstellar medium is efficient.

The carbonaceous componentWhile it is clear that a significant component ofinterstellar dust is carbonaceous, a glance attable 2 indicates that its nature is still con-tentious. There has been some recent workconcerning the carbonaceous component thatsuggests a resolution of several problems.

Jones and d’Hendecourt (2000) pointed outthat nanometre-sized diamonds (as in mete-orites) will be carriers of C–H sp2 (aromatic)

bonds of the kind found in PAHs, and arepotential carriers of the UIBs. They point outthat nanodiamonds are robust against pho-todestruction, and would provide stable UIBsin a wide range of physical conditions.

Duley and collaborators (Duley and Seahra1998, 1999, Seahra and Duley 1999) have pro-posed the same origin for three key featuresoften attributed in a loose way to carbona-ceous dust. These features are the well known“bump” in extinction at 2175 nm (see figures 1and 2); the Extended Red Emission or ERE(Gordon et al. 1998), a broad band featureusually centred around 600 nm and more than100 nm wide, and the weak 3.4 µm absorptiondetected on long low-density paths in the inter-stellar medium. The bump is conventionallyattributed to graphite particles of a restrictedsize range. The ERE is usually assigned toband-gap emission in an amorphous material(amorphous carbon, Duley 1985) or nano-particles of silicon (Witt et al. 1998, Ledoux etal. 1998). The 3.4 µm absorption is assigned toaliphatic (sp3) C–H on dust grains. The workof Duley and Seahra shows that all these fea-tures can be accounted for by a carrier that isrepresented as a stack of PAH molecules with arange of hydrogenation; the smaller PAHs aredehydrogenated while the larger moleculesremain hydrogenated. It is important to findstructures of carbonaceous material that canprovide several of the observed interstellar fea-tures, as there is limited carbon available.Snow and Witt (1996) revised downwards thecosmic abundance of carbon, and this has putpressure on several grain models.

Carbonaceous material evolves in the inter-stellar medium, as UV or H-insertion affect thesp2/sp3 balance. Cecchi-Pestellini and Williams(1998) show how this may lead to an extinctioncurve that is time dependent. Scott et al. (1997)have shown that UV irradiation of hydrogenat-ed amorphous carbon releases a variety ofproducts, including alkane chains, alkenechains, small PAHs, and carbon clusters (up to~200 atoms). The carbon clusters show absorp-tions at some of the wavelengths of the UIBs.

Towards a comprehensive grain modelIt has been a long struggle to identify the solidcomponent in the interstellar medium, but Ifeel that the final result is almost within ourgrasp. The contribution from solar systemmaterial, and some inspired laboratory workmake possible a plausible dust model whichcan account for all observed features withoutcontravening abundance constraints. My per-sonal choice is given in table 3.

This table needs to be accompanied by arecognition that interstellar dust is notimmutable; it responds to its environment.Interstellar shocks can change the nature ofdust, both chemically and physically (though

Cosmic dust

3.11June 2000 Vol 41

Table 2: Information sources forinterstellar dust

Elemental depletionsInterstellar extinctionInterstellar polarizationScattering of starlightDiffuse interstellar bandsExtended red emissionIR absorption featuresIR emission featuresIR continuum emissionInterplanetary dust particles

6: The ISO spectrum ofwarm silicate dustnear the star HD100546; wavelengthsin micrometres areindicated on the x-axis, and intensity inJanskys on the y-axis(after Waters et al.1999). The contributionto the emissioncrystalline silicates isindicated.

200

100

0

–10010 100

HD 100546cold continuumcrystalline forsteriteamorophous olivineFeOTotal

Jones et al. 1997 estimate that ~10% of grainsare never shocked). Stellar UV and H atom flux-es alter the chemical and optical properties ofcarbonaceous material, and ices composed ofsimple molecules H2O, CO, NH3 etc, may beprocessed by UV or cosmic rays to more com-plex molecules. It seems clear that methanol(CH3OH) which is abundant in certain regionsnear hot stars, is formed in this way.

With table 3 indicating the likely nature ofinterstellar dust, and recognizing that dustresponds to its environment, we are now readyto examine the active role of dust.

Roles of dust in star-forming regionsDust grains can play important roles in manyregions of interstellar and circumstellar space.Most obviously, dust shields the interiors ofclouds from starlight. Less obviously, perhaps,grains provide the main heating mechanism forregions of space pervaded by starlight, throughthe photoelectric effect which effectively cou-ples the energy of the stars to the gas. The sur-faces of dust grains are important, too. Het-erogeneous catalysis provides the mosteffective formation route for H2; other mole-cules, too, are almost certainly formed in suchreactions. The grains provide substrates onwhich ices can form, thus removing molecules(especially coolants H2O, CO etc) from the gasphase and enhancing the depletion of elementsalready locked up in grains. These ices maysubsequently be converted by UV or cosmic-ray fluxes into more complex chemicals whichmay be returned to the gas at some later stage.Dust grains tend to carry electric charges andcan in some circumstances become the maincharge carrier; when this occurs, dust providesthe main coupling between magnetic fields andthe largely neutral gas. Thus, dust grains affectthe thermal and dynamic behaviour of the gas.

For a given gas:dust ratio, the rate of colli-sions between atoms and molecules in the gaswith dust grains increases rapidly with density,and therefore many regions of space wheredust grains have a significant effect lie in star-forming regions at relatively high density. Ishall describe several scenarios within the gen-eral topic of star-formation in which dustgrains play an important role.

The initial stages of star formationMost of the star formation in the galaxy occursin Giant Molecular Clouds, a class of objectswhose masses range up to ~106 M( (makingthem the largest non-stellar objects in thegalaxy). A detailed study in emission from 13COof one of them, the Rosette Molecular Cloud,by Williams, Blitz and Stark (1995) showedthat it was clumpy, with clumps ranging inmass between a few tens and a few thousandsof solar masses. The authors noted that if thecolumn density of 13CO molecules through a

clump was above a criti-cal value, then it waslikely to have formed astar, otherwise not. Whyshould the star formationprocess depend on the13CO column density?

In fact, 13CO in theseobservations is simply ameasure of the amountof gas or indeed dust, ina column through theclump. The critical den-sity of 13CO may there-fore be interpreted as avisual optical depth, t,due to dust extinction,and corresponds totcrit = 2.5 along a radiusof a clump. For clumpswith t > tcrit , we expectto find embedded stars.

Rewriting the observational criterion for starformation in these terms suggests that themicroscopic properties of the cloud, sensitive tothe visual extinction, may control the onset ofstar formation. Ruffle et al. (1998) showed thatthe fractional ionization is very sensitive to tand falls rather abruptly, by a factor of about100, as t increases, near to the special valuetcrit = 2.5. This result encourages the notion thatmagnetic effects are important for cloud sup-port. Magnetic fields can support gas againstgravity if there are enough ions and electrons inthe gas: the ions and electrons are “tied” to thefield lines, and also coupled to the neutralatoms and molecules by collisions. This mag-netic support consists of magnetic pressureacross the field lines, and turbulent support bymagnetic waves along the field lines. However,if the ions and electrons are too low in abun-dance the ion-neutral friction is too weak to

prevent the neutral gas sliding past the fieldlines as it moves under its own gravity.

Why does the fractional ionization fall sosteeply near the tcrit? The high fractional ioniza-tion at the edge of a cloud is caused by the ambi-ent UV interstellar radiation field, but the inten-sity of this field is heavily extinguished by dust.At a depth equivalent to tcrit , the UV intensity isreduced to about 10–3 of its value at the surface.Thus, many fewer ions are being created, andthose that are present are recombining on thesurface of dust grains, and also in the gas.

Therefore, it seems likely that the presence ofdust grains in Giant Molecular Clouds controlsthe structure of those clouds by modulating themagnetic support available to a clump. Suffi-ciently large clumps cannot be magneticallysupported, and proceed to collapse under theirown weight and ultimately to make stars. Fig-ure 7 shows a false colour image, taken at a

Cosmic dust

3.12 June 2000 Vol 41

Table 3: Dust components responsible for dust features

Phenomenon CarrierExtinction

UV rise small (~nm) silicate and carbon grains217.5 nm bump carbons (stacked PAHs)visible GEMS

crystalline silicatescarbon grains/coatings

IR carbon grains; band gapERE carbon grains, band gapUIBs PAHs

small (~nm) carbon(diamond) grains

IR absorptions GEMScrystalline silicatesices (H2O, CO, CO2, CH3OH,...)saturated hydrocarbons

7: Part of the dark cloudLynds 183, imaged at850 µm with the SCUBAcamera on the JCMT(Ward-Thompson et al.2000). The false-colourimage indicates thedensest region in white,through yellow, red andmauve, to black for theleast dense gas. Theimage is roughly 0.2 pcin extent. The vectorsindicate the magneticfield directions asdeduced from thedetection of polarizedemission from aligneddust grains.

wavelength of 850 µm, of a dark cloud that isprobably about to collapse to make a star. Thecloud contains a magnetic field which is help-ing to support the cloud. However, the mag-netic field in the cloud is becoming weaker andthe cloud will eventually collapse.

There is an interesting corollary to thisapproach. As the clump-collapse envisagedabove proceeds, gas phase atoms and dustgrains interact more frequently. Ruffle et al.(1999) have suggested that this may accountfor the long-standing problem concerning theabundance of interstellar sulphur. In low-den-sity clouds, swept by stellar UV radiation, sul-phur is present as S+ ions and its abundancerelative to hydrogen is essentially that in theSun. In dense dark clouds sulphur cannot bedetected directly, but chemical models suggestthat nearly all of it should be in CS molecules.These are detected, but to account for theirabundance then 99% of the sulphur must beremoved, i.e. sulphur is heavily depleted. How-ever, oxygen and carbon are only mildlydepleted. What is so special about sulphur?

Ruffle et al. propose that sulphur is depletedinto dust grains at a stage in the collapsingcloud when the sulphur is present as S+ andthat it combines with dust grains that are neg-

atively charged. This is the phase at which thefractional ionization falls rapidly with depth,i.e. near tcrit . This picture requires that sulphuris incorporated in the grains in a permanentway: could this be the source of the FeS inclu-sions in GEMS? Further study is needed of thisidea. By contrast, the interaction of neutralcarbon and oxygen species with dust grains ismuch slower, and by the time sulphur is deplet-ed by a factor of 102 or more, oxygen and car-bon are depleted by a factor of less than 5.

Chemistry in infalling clumpsIn the previous section I indicated that dustgrains may influence the formation withininterstellar clouds of structures (of size equiva-lent to tcrit) that may collapse to form stars. Letus now consider in more detail the next stageof the star formation process. Dust grains alsohave important roles at this phase of theprocess. First, any molecules that stick to grainsurfaces no longer radiate, and so the build upof molecular ices on dust reduces the ability ofa clump to cool. If the gravitational potentialenergy of a collapsing clump cannot be radiat-ed away, then the collapse will be inhibited bya temperature and pressure rise. Secondly, theloss of molecules from the gas phase increases

the importance of atomic ions in the recombi-nation of electrons. Since recombination on toatomic ions is much slower than on to molecu-lar ions, the fractional ionization tends to rise.As we have seen, this influences the magneticsupport. Therefore, the gas–dust interactiondirectly affects the dynamics of the collapse.

The gas–dust interaction also affects in a sig-nificant way the nature of the chemistry occur-ring in the collapsing cloud. Consider the stagein the collapse where much of the oxygen, inthe form of H2O, is already frozen out on thedust. We may expect some CO to be frozen outtoo, but CO has a much higher vapour pres-sure than H2O so there will be much more COin the gas. Cosmic rays ionize gas-phase CO togive C+ ions. Normally, C+ ions react with H2Oand ultimately re-form CO

H2O eC+ ® HCO+ ® CO

but when the abundance of gas-phase H2O isreduced, then C+ reacts in a much slower chan-nel with the major molecule, H2

H2

C+ ® CH2+ ® products

to make a variety of products, including varioushydrocarbons. Therefore, the molecular tracersof the infalling gas change as the collapse pro-ceeds, and in attempting to observe the collapseof a dense core one must select an appropriatespecies. Searches for signatures of collapse inthe line profiles of ammonia (Myers and Benson1983) failed because ammonia is depleted earlyin the collapse, and the ammonia line profileshows little deviation from that expected froma static core with mainly thermal excitation.Menten et al. (1984) suggested that the NH3

molecules in the inner and faster-movingregions were simply frozen-out, and did notcontribute to the expected line profiles. Rawl-ings et al. (1992) confirmed this and indicatedthat other species should exhibit signatures ofinfall. For an optically thick line, the signatureis a broad double-peaked profile with the short-er-wavelength peak stronger. An example ofsuch a profile is given in figure 8; the line detect-ed is of 13CO in the collapsing core B335. Pre-dicted line profiles for HCO+ in the same core,and their sensitivity to parameter choices, areshown in figure 9.

Of course, dust is also a tracer of collapsingregions, and submillimetre-wave data fromSCUBA on JCMT can be used to trace theclump density with radius. But the essentialinformation in star-forming regions concernsinfall rates, and for velocity data molecularspectra are required. The ultimate aim of thiswork is to determine how much mass is deposit-ed in the star; is the accretion rate constant ordoes it vary in time? There is some evidence thatit varies (Brown and Chandler 1999).

This story is still being told; but it is evidentthat the gas–dust interaction determines the

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2

0

2 4 6 8 10 12 14

T A (K)

velocity/(km/s) LSR frame

8: The 13CO 3–2 emission line from B335, antenna temperature as a function of LSR velocity. This linedisplays the characteristic profile associated with infall (Rawlings and Evans 2000).

9: Calculated line profiles for B335observed with JCMT, for the HCO+

4–3 line (Rawlings and Yates 2000).These curves illustrate the sensitivityto model parameters. Regarding thetop curve as “standard”, then thesecond curve from the top illustratesthe effect of reducing the cosmic-rayflux, the third shows the effect ofincreasing the probability of stickingof molecules to grain surfaces, andthe lowest curve shows the effect ofhalting the infall and pausing as astatic core.

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0–1.5 –1.0 –0.5 0 0.5 1.0 1.5

velocity (kms–1)

T(B)

effective tracers of the star-formation process.It also determines the cooling efficiency, andthe level of magnetic support.

Stellar jets and molecular cloudsStellar jets are a remarkable phenomenon thathave been studied intensively since their dis-covery (Bachiller 1996 and references therein).The origin of these jets is unclear, but the inter-action of the jets with the ambient medium iswell studied both observationally and theoreti-cally. Jets are observed with a range of sizesfrom ~0.1 pc up to several parsecs. Jets mayeventually impact on dense molecular clouds,and in situations of that kind a remarkableobservation has frequently been made: associ-ated with the shocked region at the head of thejet (the so-called Herbig-Haro object) may befound a clump of intense molecular emission(e.g. Girart et al. 1994). These emission clumpswere first observed in lines of HCO+ and NH3;the clumps are small (dimension ~0.1 pc), andare cool and quiescent. They do not show evi-dence of being shocked, and they share thevelocity of the molecular cloud, not of the Her-big-Haro (H-H) object. Sufficient numbers ofsuch emission clumps have been detected nearH-H objects to indicate that this is not somechance association, but is a real phenomenon.The model that has been proposed (Girart etal. 1994) to account for these objects is aninteresting one: although it is clear that theH-H shock has not directly affected the emis-sion clump, it is possible that radiation fromthe H-H object could affect nearby clumps inthe molecular cloud. Therefore, it is supposedthat the jet provides at its head (the H-Hobject) a source of radiation that triggers theobserved response. Radiation evaporates icesdeposited on dust in some denser part of themolecular cloud; this releases H2O, CO, NH3

etc into the gas. The CO is photodissociatedand the C atoms ionized

hnCO ® C + O

hnC ® C+ + e

and the C+ ions react with H2O to form thedetected HCO+

C+ + H2O ® HCO+ + HIn effect, the model is of shock-generated

photochemistry in a clump of the molecularcloud that happens to be near the H-H object.The jet, therefore, is a probe of the interior ofa molecular cloud, and tells us about theclumpiness within that cloud.

The model has been explored with computa-tions by Taylor and Williams (1996) and amuch extended chemistry by Viti and Williams(1999a). Their prediction of a rather extensivechemistry in the nearby clump has been con-firmed by detections of many of the predictedmolecules, including CH3OH, HCO+, NH3,

H2CO, CS, HCN and SO (Girart et al. 2000).The implications of this model seem impor-

tant for molecular cloud dynamics. The obser-vations suggest that molecular clouds containmany clumps, but the model implies that theseclumps are transient on a timescale of ~106 yr.

If the clumps lasted much longer, then all gasphase molecules would have frozen out on tothe dust. With an age of a million years or sothen there has been some but not a total freeze-out. Then, the H-H radiation releases H2O andother species from the grains. This is necessaryas the H2O does not form effectively in the gasphase (Jones and Williams 1984, O’neill andWilliams 1999). We are led to a picture of amolecular cloud filled with transient clumps,and this kind of picture is now suggested by

recent millimetre-wave array observations of“core-D” in the much-studied source TMC-1(Peng et al. 1998). The picture does not showwhat is driving the dynamics. Perhaps it is mag-netic in origin. However, it is easy to imaginethat occasionally one of the transient clumps ismassive enough to become gravitationally-bound, and star formation begins.

There is an interesting counterpart to theshock-induced photochemistry occurring nearthe H-H object. Consider what is happeningnear the origin of the jet. A sufficiently youngstar will still be embedded in the dense corefrom which it was formed. The star is likely tohave established a powerful wind, even in itspre-Main Sequence phase. The wind is proba-bly confined into a bipolar outflow, as is com-monly observed, by a dense disk. The interac-tion of the wind and the disk is a region ofcomplex shocks and interfaces between hottenuous wind gas and cold dense core gas. Theshocks may be expected to generate local radi-ation fields similar to but more intense thanthose associated with the H-H object at the farend of the jet. The dense core material is coldand should contain icy matter. Therefore, thesituation is essentially similar to that in tran-sient clumps near H-H objects, and we mayexpect high molecular abundances to arise inthe wind/core interface as a consequence ofshock-generated photochemistry. In fact, highabundances of HCO+ – larger than expected byat least one order of magnitude – have indeedbeen seen in B335, a near-spherical star-form-ing dense core. Taylor et al. (1999) have evalu-ated the HCO+ abundance arising in the fastwind/dense core interaction and have shown

that it can easily account for the anomalouslyhigh HCO+ abundance observed. However, theline profile for the HCO+ emission from B335has yet to be determined, and until this is doneand compared with the observed double-peaked profile, then this assignment must beregarded as tentative.

With high angular resolution, as can beobtained from millimetre-wave arrays, theHCO+ emission should be seen to arise fromthe wind/core interface and not from the entirecore. Just such a situation has been found byHogerheide et al. (1998) using the Owens Val-ley mm-wave array for the dense core L1527.

Hot molecular coresIn studying star formation, an important aim isto understand why collapse occurred in a par-ticular region of a cloud, and why a star of aparticular mass was formed. To answer thesequestions we really need to have informationabout the physical conditions in the immedi-ately pre-stellar phase. For massive stars, par-ticularly, this information is hard to obtain,because they rapidly disrupt their birthplacesthrough their powerful winds and radiation.

Fortunately, there are remnants of the pre-existing cloud to be found near some veryyoung stars, and by studying them we can hopeto learn something about the star-formationprocess. These remnants are the so-called “HotMolecular Cores”. The adjective hot is subjec-tive, but used here in a relative sense. Theseobjects have temperatures typically of a fewhundred Kelvins, considerably warmer thancold clouds. They are dense, with more than amillion H2 molecules per cm3, as might beexpected in a collapsing core, and they aresmall, less than a tenth of a parsec across.

Hot molecular cores are mapped in rotation-al emission lines of molecules, and reveal achemistry that is distinct from that found incold molecular clouds. Hot cores show anabundance of saturated molecules, whereas theremarkable feature of normal cold molecularclouds is the richness of unsaturated speciesthat arise even in the hydrogen-rich environ-ment of interstellar clouds.

The Hot Core G29.96-002 is illustrated infigure 10. Here, contours of 1.3 cm continuumemission from the ultracompact HII region areshown, together with contours of CH3CN(6–5) emission, delineating the hot core. Sever-al OH masers are found within this core. SinceOH masers are regarded as a signature of mas-sive star formation, it seems likely that the hotcore is warmed by a massive star within.

The origin of hot cores is now generallyaccepted to be as follows: during the cloud col-lapse that led to star formation, densities in thecentral regions of the cloud become very high,and most molecules other than H2 becomeincorporated in icy mantles on dust grains, and

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“The observations suggest thatmolecular clouds contain manyclumps, but the model impliesthat these clumps are transient

on a timescale of ~106 yr.”

hydrogenation of unsaturated species occurred.When the star had formed, rapid warming ledto evaporation of the icy mantles. Most of themolecules detected within the hot core arethought to have originated in the ice. Aftersome 104 years (if nothing else intervenes)chemical processes driven by cosmic rays willtend to return the chemistry to the more normalinterstellar form. Thus, hot cores are chemical-ly transient; they may also be dynamical tran-sients, as winds from the new star and radiationpressure from its ultracompact HII region erodeor disrupt the core. The timescale for disruptionis probably also of the order of 104 years. Anintense theoretical and experimental researchprogramme has established the validity of thisgeneral picture (Millar 1993, Walmsley andSchilke 1993, Millar and Hatchell 1998).

Thus, where we can observe hot cores, themolecules reflect the pre-stellar chemistry inthe cloud, literally frozen in time, as mantles ofvarious ices on interstellar dust grains. Themolecules are, therefore, a “fossil” record ofconditions within the collapsing cloud, though– like the true fossils found on Earth – theyhave undergone subsequent processing. Unrav-elling the consequences of this processing cantell us about the pre-stellar conditions in thecollapsing cloud. Here, I’ll mention briefly twouses of hot core observations as probes of starformation.

How fast do hot stars “turn-on”? Is theprocess instantaneous, or does it occur over atimescale that may be comparable with the ageof a hot core? The usual assumption is that thestar heats up very rapidly, and the dust iswarmed and total evaporation of ices occursinstantaneously. It may be, however, that thestar warms up relatively slowly, and the temper-ature of the dust grains rises slowly enough sothat weakly bound species appear first, andmore strongly bound species later on. If thisturned out to be the case, then by identifying thechemistry in a cloud we should be able to deter-mine the turn-on time for the associated star and

an age for the hot core (Viti and Williams1999b). These ideas are still in early develop-ment. However, it may be that the relative posi-tions and sizes of CH3CN and CH3OH emissioncontours in G29.96-002 could be accounted forin this way (Macdonald et al. 2000).

When do stellar winds begin? We observethat nearly all young stars have powerfulwinds, and that such winds arise early in thelife of the star. In principle, it should be possi-ble to use hot core chemistry to help define theonset of stellar winds, because these windsmust send a shock through a core, modifyingthe chemistry. For example, Charnley andKaufman (2000) argue that the low gas-phaseCO2 abundance in a hot core compared to thehigh phase CO2 solid phase abundance impliesthat shocks have destroyed the gas phase CO2.Lower velocity shocks may still cause chemicaldifferences which may tell us whether theshock passes through a hot core before or afterevaporation of the ices (Viti et al. 2000).

ConclusionCosmic dust is now fairly well characterized,and we can reasonably hope to understand itschemical and physical properties in the inter-stellar medium. Dust exerts several importantroles, especially in regions of higher densitywhere the gas–dust interactions are frequent.Therefore, dust affects the formation of starsand planets, and controls the means by whichwe can monitor these processes.

Dust can no longer be considered as a merefog preventing detailed study of stars andgalaxies. Rather, it is an essential component ofgalaxies, affecting the formation and providingthe source material for planets. ●

David A Williams, Perren Professor of Astronomy,University College London.Acknowledgements: I am grateful to many peoplefor encouragement and enjoyment in studying areasof research discussed in this article, including WaltDuley, Tom Hartquist, Ant Jones, Andy Lim, Tom

Millar, Geoff Macdonald, Jonathan Rawlings, SteveTaylor, and Serena Viti. I also acknowledge with thanks the generous sup-port of PPARC for my research.

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10 The Hot Molecular Core G29.96-0.02 (Kurtz et al. 2000). The dottedcontours are of 1.3 cm continuousemission from the ultracompact HIIregion around the young star. Thecontinuous contours are of CH3CN6–5 line emission, and these showthe position of the hot core relativeto the UCHII region. Within the hotcore are OH masers, indicated hereby filled triangles. The maserssuggest that there is a high-massstar within the core.

–2°42'30"

–2°42'35"

–2°42'40"

18h43m27.5s 18h43m27.0s

a (1950)

d (1

950)