l>Interstellar Graines - B.T. DrainePublished in \"Encyclopedia that Astronomy andAstrophysics\" (IOP Publishing and MacMillan), 1266-1273, 2001
B. T. Draine
Princeton UniversityTable of Contents1. OverviewSubmicron solid corpuscle are dispersed through interstellar gas.These interstellar seed absorb and also scatter light, thusshielding some areas from ultraviolet radiation, but additionally limitingour capability to recognize photons which have actually been emitted byastronomical objects.Dust seed reradiate took in energy in the infrared, therefore contributingto the all at once emission spectrum of expensive systems ranging fromdusty disks roughly stars come ultraluminous starburst galaxies.A naked-eye watch of the skies from a dark site on a clean summer night revealsdramatic dark spot in the Milky Way. These dark regions are no dueto a deficiency of stars - castle are rather the an outcome of obscurationby dust clouds interposed between the Earth and distant stars.The obscuration has tendency to be better at much shorter wavelengths; together a result,the light getting to us native distant, hidden stars is \"reddeinter-base.net\".This reddening through interstellar dust have the right to be interpreted as arising fromscattering and absorption by a populace of interstellar submicrondust grains.The grain population spans a range of sizes,from molecule (1)containing just tens of atom toparticles as huge as ~ 0.3 µm, containing ~ 1010atoms. Most of the serial mass shows up to bedue to two varieties of solid, in approximatelyequal amounts: (1) amorphous silicate mineral, and (2)carbonaceous material.A variety of elements - consisting of silicon and iron - are primarilyin solid kind in the interstellar medium.Approximately 2/3 that the interstellar carbon in diffuse clouds isin solid form (see INTERSTELLAR absorption LINES).It is the course essential to characterize the wavelength-dependentinterstellar extinction therefore that astronomical observations canbe \"corrected\" because that the obscuring results of dust.In addition, the infrared emissions from dust grainsprovides a an useful probe of dense regions, and the dust grainsthemselves play necessary roles in interstellar chemistry (shieldingfrom ultraviolet radiation, and also catalyzing the development of H2),interstellar gas dynamics (radiation pressure forces on dust grains,and coupling of fee dust grains to magnetic fields)and heating and cooling that interstellar gas.Dust grains are main to many problems in contemporary astrophysics.
2. Observational Evidence: SummaryThere are countless different astronomical phenomena i beg your pardon both disclose theexistence of interstellar dust grains, and administer information allowingus come infer the properties of this dust.Some the the information is quite direct, as result of absorption, scattering,or emissions of irradiate by the grains: Wavelength-dependent extinction - attenuation and \"reddening\" ofthe irradiate from distant stars early out tointervening dust(see number 1).
Figure 1. Wavelength-dependent extinction, normalized come the die out at ns = 900 nm, because that different varieties of clouds, figured out by the worth of RV AV/(AB - AV), where V = 550 nm and B = 440 nm. The average extinction because that diffuse clouds is identified by RV 3.1. Thick gas close to the surfaces of molecular clouds can have RV as huge as 5.5. The extinct at i is roughly proportional come NH = N(H) + 2N(H2) + N(H+), v AI / NH 2.6 × 10-26 m2 / H.
Spectroscopic features in the extinction.There space a number of extinction features,including: A strong and very broad die out \"bump\" in ~ 217.5nm(see number 1),probably because of carbonaceous material, maybe graphite. Infraredextinction attributes at 9.7µm and 18µm, almostcertainly as result of silicates. A number of weaker \"diffuse interstellar bands\" (seeFigure 2),the strongest of which areat 443 nm and 578 nm, and also which remain typically unidentified.
Figure 2. A section of the die out curve mirroring some that the \"diffuse interstellar band\" die out features, labelled through their particular wavelengths. These diffuse bands might be due to impurities in grains, or come \"free-flying\" big molecules/ultrasmall grains.
an absorption feature at 3.4 µm, seen in diffuse clouds,presumably because of the C-H stretching mode in aliphatic hydrocarbons. A variety of absorptionfeatures, seen only in molecular clouds,due to ice mantles which supposedly coat the seed in this regions.The the strongest such function is a 3.1µm feature attributed toH2O ice. Polarization of starlight - preferential attenuation that onelinear polarization over another by aliginter-base.net interstellar dust grains,so that originally unpolarized irradiate from a star is partiallypolarized by the time it get the Earth(see POLARIZATION the STARLIGHT). Reflection nebulae - dust clouds which are reasonably close tobright stars, so that the starlight reflected by dust grains nearthe cloud surface makes the cloud visible(see have fun NEBULAE). X-ray haloes about X-ray point sources located behind interstellardust clouds. The haloes an outcome from small-angle scattering of X-raysby interstellar dust grains. Infrared emissions from dust grains heated by interstellarstarlight (see figure 3).
Figure 3. Infrared emissions from interstellar dust grains from dust in diffusive clouds, per H nucleon. Crosses indicate data native the InfraRed Astronomy Satellite (IRAS) at 100, 60, 25, and 12 µm. Squares suggest data from the FIRAS tool on the COsmic Background explorer (COBE). Diamonds indicate data native the DIRBE instrument on COBE. The hefty solid line at 12 - 5 µm and also 4.6 - 3 µm is the spectrum measure by the InfraRed Telescope in an are (IRTS).
Infrared emission functions indicative the aromatichydrocarbons.The strongest functions from the basic interstellar mediumare in ~ 11.3µm, and also 7.7µm (seeFigure 3). Emission attributes at 18µm and 9.7µm,generally assumed tobe due to silicates, indigenous dust in regionswith radiation areas 104 times more powerful than the median starlight background. There is solid evidence because that emission from interstellar grainsin the far-red, presumably fluorescence complying with absorption of a shorterwavelength photon. In the regional interstellar medium, the dust-to-gas ratio appearsto be approximately consistent - the dust complies with the gas.Although limited, we likewise have part quite direct evidence concerninginterstellar grains: Relic interstellar grains discovered in primitive \"carbonaceouschondrite\" meteorites. Results with interplanetary probesof interstellar seed passing with the solar system.In addition, there space a number of astrophysicalphenomena which provide indirectinformation about interstellar grains: Underabundances, family member to solar abundances, that certainelements in interstellar gas -including iron, silicon, magnesium, and carbon(see INTERSTELLAR absorption LINES).These aspects are presumed to it is in underabundant in the gas phase becausea large fraction of the atoms room locked increase in interstellar grains. The visibility in different parts that theinterstellar tool of molecular hydrogen (H2)with abundances far exceeding what could be producedby purely gas-phase processes. The forced rate ofof H2 catalysis top top interstellar grain surface providesinformation on interstellar grain properties, including complete surface area.3. Extinct by Interstellar Dust GrainsThe \"extinction curves\" displayed in number 1 representthe wavelength-dependent extinctionA atwavelength it was observed on different kinds the sightlines.Careful researches of the extinction on plenty of lines that sightreveals the the extinction curves deserve to be around described bya one parameter household of die out curves, and the curves shown inFigure 1 room obtaiinter-base.net from installation functionsoriginally emerged byCardelli, Clayton, & Mathis (1989)and others. It is practically to take it RV AV /(AB - AV)as the parameter, where V = 550 nm and B = 440 nm.Values the RV as small as 2.75, and as large as 5.5,are observed in various regions.Grains in diffuse regions tend to have RV 3.1, if largervalues of RV tend to be seen once studying theextinction bydust in dense clouds. Bigger values the RV -corresponding come \"greyer\" extinction - space indicative of bigger grains.The observed tendency for larger values the RV to befound indenser regions strongly argues that characteristic grain sizes arelarger in these regions.It can be displayed that the raised values that RV cannotbe explaiinter-base.netsimply by accretion the atoms and also molecules from the gas phase - becauseof the very big amount of surface ar area added by the smaller grains,accretion native the gas step would an outcome in only a very little increase
a in the radii that allgrains, v minimal impact on the extinction at B and also V.Instead, tiny grains should coagulate withlarger grains to transform diffuse cloud dust to dense cloud dust.4. Extinction FeaturesThe the strongest spectroscopic function in the die out curve isa conspicuous \"bump\" in ~ 217.5 nm (see figure 1).This feature has around the wavelengthand width meant for tiny (radius a 15 nm)particles the graphite.While to know of the feature is tho notcertain, it appears highly most likely that the is due to tiny carbonaceousparticles v sp2 carbon-carbon bonds as ingraphite and polycyclic aromatic hydrocarbons.The toughness of the 217.5nm attribute requires that ~ 15% the thesolar abundance of carbon be current in small (a 5 nm) particles.Between 1.32 µm and also 400 nm there are countless weak diffuseextinction features, known as the \"diffuse interstellar bands\", or DIBs.Approximately 300 such attributes have been identified, withfull broad at half-maximum (FWHM) rangingfrom ~ 0.05 nm approximately ~ 4 nm. The strongest DIB is at 442.9 nm.The very first DIBs were well-known in 1934 by Merrill, yet they remainessentially unidentified to this date.They must be because of either \"impurities\" within dust grains, orto small free-flying molecule (i.e., ultrasmall dust grains).A few of the weaker functions have been established recentlyas electronic transitions that C7-, and it seemslikely that countless otherof the DIBs are additionally produced by tiny hydrocarbon molecules, either neutralor charged (either positively or negatively).A vast extinction attribute at 3.4 µm is attributed to the C-Hstretching mode in aliphatic (chainlike) hydrocarbons.This function is existing in the diffusive interstellar medium.There are solid infrared extinction attributes peaking at9.7 µm and also 18 µm which are practically certainlydue to amorphoussilicates with a ingredient approximating that of olivine(MgxFe2-xSiO4).In regions where the dust is warm (e.g., circumstellar dust, or the dust nearthe Trapezium in the Orion Nebula), this features show up in emission.In part circumstellar dust shells and disks (e.g., the dustydisk around the Herbig Ae/Be star HD 100546), spicy featurescharacteristic of crystalline silicates appear in emission, yet thesefeatures have actually not to be detected in either emission or absorption in theinterstellar medium, indicating the the bulk of interstellar silicatesare amorphous.In dark clouds, a number of additional features appear in the infraredextinction, presumably because of growth of molecular ice mantles onthe refractory dust grain cores.The strongest function is in ~ 3.08 µm and is because of amorphousH2O ice.Additional attributes have been established as frozen CH3OH(3.53 µm), CO (4.67 µm),CH4 (7.65 µm), andCO2 (15.2 µm).5. Alignment of Dust GrainsWhen originally unpolarizedstarlight passes with the dusty interstellar medium, the acquiresboth linear and also circular polarization.The straight polarization is as result of preferential attenuation that onelinear polarization end the other, because of a populace of nonsphericaldust grains which are somehow aliginter-base.net.The linear polarization peaks close to V = 550 nm, with optimal polarizationP 0.03AV. The one polarization, generally quite small,is as result of conversion of linearto circular polarization once the alignment direction that the dust grainsundergoes a twist along the line-of-sight to the source.Spinning dust grains have magnetic moments antiparallel to their angularvelocities, and also the angular momentum because of this precesses approximately thelocal galactic magnetic field.As a result, the observed direction of starlight direct polarization need to beeither parallel or perpendicular come the projection of the magnetic fieldon the sky. Theories of serial alignment lead us to expect the grainangular inert to often tend to align with the magnetic field, and also the \"long\"axis of the seed to tend to it is in perpendicular to the angular momentum.The long axis the the grain because of this tends to it is in perpendicular tothe magnetic field, and as a an outcome light becomes linearly polarizedparallel come the estimate of the magnetic field on the aircraft ofthe sky.Our knowledge of the physics the dust serial alignment is not yetcomplete, however it appears that the it was observed alignment in diffusive clouds isproduced largely bya combination of paramagnetic dissipation (originallyproposed through Davis and also Greenstein in 1951) and also radiativetorques top top irregulardust grains because of anisotropic starlight.In some situations the alignment may result from drift the the dust grainsthrough the gas cloud.6. Scattering of irradiate by Dust GrainsReflection nebulae such together NGC2023 or the beautiful filamentary structuresnear the Pleaides present that dust grainsscatter starlight.While many conspicuous as soon as a bright star is located near the surfaceof a thick clouds, reflected starlight additionally manifests itself asthe \"diffuse galactic light\" - reflect starlight checked out in alldirections in the sky where over there is dust.Measurements that the surface brightness of reflection nebulae, or ofthe diffuse galactic light, administer constraints ~ above the scattering propertiesof interstellar dust grains - both the full \"albedo\" = Csca /Cext,and the angular dependence of the scattering, oftencharacterized by g , where
is the scattering angle.At visible wavelengths, the albedo 0.5 and thegrains room moderately forward-scattering, g 0.5, constant withcurrent models for interstellar seed which reproduce thewavelength-dependentextinction and also the infrared emissions from dust.Scattering by dust seed can also be observed at X-ray wavelengths.A grain is basically transparent toh 0.5 keV X-ray photons.Since the refractive table of contents of the grain product is very close to 1, thescattering can be calculated in the \"Rayleigh-Gans\" approximation, andone find that only small-angle scattering is expected.X-ray halos roughly compact X-ray sources have actually been imaged bythe Einstein and ROSAT observatories; the observedX-ray halos appear to be roughly consistent with the scatteringexpected for a serial model arisen to account because that the optical-UVextinction curve.7. Dust serial LuminescenceObservations of reflection nebulae (e.g., NGC 7023)as well together the basic interstellar diffuse clouds or\"cirrus\" show up to show evidence the far-red continually radiation in excess ofwhat is supposed from simple scattering that starlight through dust grains.The far-red emission peaks near ~ 700 nm, and has a spectrum resemblingthe luminescence from hydrogenated amorphous carbon illuminated by 550 nm radiation.This says that few of the grain material may resemblehydrogenated amorphous carbon, return ultrasmall silicon seed havealso to be proposed as the source of the emission.Even if it is assumed the hydrogenated amorphous carbon is responsible forthe it was observed emission, an accurate estimate of the amount of materialrequired is not possible, since luminescence efficiencies of hydrogenatedamorphous carbon in the activities dependon the ready of the sample.8. Infrared emission from Dust GrainsStarlight is in component absorbed by dust grains, and also the absorbed energy isreradiated through dust grains in the infrared.The observed emission spectrum for interstellar dust is presented inFigure 3.It consists of far-infrared emissions peaking in ~ 140 µm,plus considerable emission at much shorter wavelengths.The emission at µm showsconspicuous emission functions at 11.3,8.6, 7.7, 6.2, and 3.3 µm; emission peaks at this samewavelengthshave likewise been observed from have fun nebulae, planetary nebulae,HII regions, and circumstellar dust.The emission features havebeen determined as characteristics of polycyclic fragrant hydrocarbons(PAHs): C-H large (3.3 µm), C-C big (6.2 and 7.7µm),in-plane C-H bend (8.6 µm) and also out-of-plane C-H bending (11.3µm).Variations in the relative strengths and precise wavelengthsof these attributes from one objectto another may it is in due to transforms in the PAH mixture, including changesin the fraction which space positively or negatively charged.The \"classical\" grains with radii 0.01 - 0.3 µm radiate aspectrum characteristics of heat emission at the (steady) temperatureof the grain, ~ 15-20 K because that grains in the diffuse interstellar medium.This accounts for nearly all of the emission at > 60 µm.For very little grains, however, absorb of a single starlight photoncan appreciably change the serial temperature.For example, a single photon of energyh = 10 eV have the right to heat a230 atom graphite grain to a optimal temperature T 300 K.At this temperature the grain deserve to radiate efficiently at wavelengthsas brief as 8 µm.The observed emission attribute near 7.7 µm could thus be dueto heat emission indigenous grains with ~ 100 - 300 atoms.The relatively huge amount of strength radiated at µmrequires the the grains with 300 atom account foran appreciablefraction ( 15%) that thetotal absorb of starlight by interstellar grains.9. Microwave emissions from Dust GrainsSensitive monitorings of the cosmic microwave lift radiation haverevealed 10-60 GHz emissions from interstellar issue withintensities substantially exceeding whatwould be meant from an extrapolation the the heat far-infraredemission to these reduced frequencies.It shows up likely that the observed 10-60 GHz emissions is largelyrotational electrical dipole emissions from very rapidly rotating ultrasmallgrains, although a fraction of the radiation might be thermal emissionfrom seed containing products that room ferromagnetic(e.g., metallic Fe inclusions) or ferrimagnetic (e.g., magnetiteFe3O4).10. Interstellar Dust seed in the Solar SystemThe solar mechanism was developed out of interstellar gas and also dustapproximately 4.5 billion years ago.The formation of planetesimals and planets to be accompanied in manycases by high temperatures and also violent conditions, and most interstellardust particles were destroyed. However, the class of meteorites knownas carbonaceous chondrites (see METEORITES)contain smallparticles with unusual isotopic ratios (see ISOTOPICANOMALIES) which indicate that they walk notform in the solar nebula, yet rather must have been developed in a an ar withan anomalous ingredient (e.g., outflow from an evolved star) longbefore the development of the solar system.Therefore this particles must have actually been part of the interstellar grainpopulation prior to the development of the solar nebula.To date, 5 different kind of presolar grains have actually been secluded andidentified(see Table 1).The evidence that the grains listed in Table 1were truly interstellar is compelling, but it is vital to realize thatthey apparently perform not include common interstellar grains, for thesimple reason that theprocedures offered to isolation interstellar grains in meteorites aredesiginter-base.net come deliberatelydestroy silicate material (which comprises the bulk of the carbonaceouschondrite meteorite \"matrix\").These laboratory measures are because of this not going come findinterstellar silicate grains also if they room present.
Detectors plank the Ulysses and Galileo probes measured effects byinterplanetary dust particles, but likewise observed impactsattributed to theflux the interstellar grains expecteddue to the 20 km s-1 motion of the sunlight relativeto the neighborhood interstellar medium.The detectors space sensitive only to collisions through the large-size endof the interstellar serial distribution, butthe observeddistribution of impact energies shows up to be consistent with both theoverall numbers and size circulation expected based on studies ofinterstellar extinction.The lines of evidence detailed above serve to strong constrain theoreticalmodels because that the interstellar grain population.Unfortunately, interstellar grain researchers have actually thus far not beenable to uniquelydetermine the grain model from the available observational constraints,so discussions continue concerning the details.Nevertheless, over there is broad consensus top top a number of properties ofinterstellar grains: The grain populace must have a vast size distribution,with grain radii (4)covering the range 0.5nm - 300nm - a element ofat least 108 in grain mass. Most of the serial mass is in particles through radii50 a 300 nm. Most of the grain area is in particles with radiia 50 nm. Approximately 50% that the serial mass in diffusive clouds iscontributed by amorphous silicate material, accountancy for the broad9.7µm and 18µm silicate features.Most that the silicon, iron, and magnesium variety is in hard form. Approximately 50% of the grain mass in diffuse clouds iscontributed through carbonaceous material. About 2/3 that the full carbonabundance is in hard form. The 217.5nm feature is probablydue to some form of carbonaceous material, possibly graphitic.11. Graphite-Silicate serial ModelIt is not feasible to invert the monitorings to attain a uniquegrain model. Instead, one makes some assumptions concerning the graincomposition andthe type of the size distribution, and also then attempts to readjust the modelto accomplish a good match come the it was observed interstellar extinction,infrared emission, and other border such as observed gas phaseabundances.One grain model which has proven reasonably successful in conforming toobservations consists of a mixture the carbon seed andsilicate grains.The carbon grain product is taken to have actually the optical constants ofcrystalline graphite.A an excellent fit to the extinction deserve to be obtaiinter-base.net if both graphite andsilicate grains have actually size distributions which are approximatelya power-law, dn / da a-3.5,truncated atamin 5nm and also amax 250 nm. The 217.5 nm attribute is then reproducedby the a 5 nmgraphite grains.This grain model - the combination of graphite and also silicate grains,and the dn / da a-3.5 power-law - was an initial put forward in 1977by Mathis, Rumpl, and Nordsieck, and also is often referred toas the \"MRN\" model.The MRN version achieves a good fit to the RV = 3.1extinction curvefor diffuse clouds with essentially all of the Mg, Si, and also Fe inthe silicate grains, and approximately 2/3 the the C in the graphitegrains, in reasonable agreement with it was observed depletions.The grains are heated to temperatures ~ 18 K by starlight,and the resulting thermal emission is approximately consistent withthe observed far-infrared emissions at > 60 µm.12. Ultrasmall Dust GrainsUnfortunately, the initial graphite-silicate grain model explained abovefails come reproduce the µm infrared emissionshown in number 3.The model lacks the ultrasmall grain componentrequired to describe the observed 3 50 µminfrared emission from interstellar dust.The simplest change is come allowthe carbon grain distribution to prolong down to verysmall sizes, v the the smallest grains presume to have actually infrared opticalproperties suitable to describe the emission features at3.3, 6.2, 7.6, 8.6, and also 11.2 µm when heated to the appropriatetemperatures by absorb of solitary starlight photons.In order come have adequate numbers of ultrasmall grains, if stillreproducing the die out curve, the size circulation of in ~ leastthe carbonaceous grains deserve to no longer be approximated by a singlepower-law.13. Other Grain ModelsOther serial models, with various grain geometries and/or compositions,have to be proposed to account for the observedinterstellar extinction and infrared emission. Mathis and also Whiffen propose a grain model wherein the bigger grainsare porous aggregates of little graphite and also silicate particles. Part authors have favored metal oxides (MgO, SiO, FeO) either inaddition to, or in place of, silicates. Part models assume the silicate grains in diffuse cloudsto it is in coated through a carbonaceous \"mantle\" material, which mightbe hydrogenated amorphous carbon. Metallic Fe and FeS incorporate significant fractions of the Fe insome serial models. Tiny particles of crystalline silicon, v hydrogenated or oxidizedsurfaces, have actually been proposed as an explanation for the observedluminescence close to ~ $700nm.Grain models generally tend to havethe bulk of the Fe, Mg, Si in dust, approximately2/3 the the C, and around 20% of the O, bring about dust-to-gas massratios of ~ 0.007.14. Dynamics of Interstellar GrainsInterstellar grains are acted on through forces because of a number ofdistinct physical processes, including: Gas drag forces when the grain velocity differs from that of the gas.In addition to direct collisions through atoms and ions, over there isalso a \"plasma drag\" pressure on charged grains due to momentumtransfer v ions which execute not in reality collide with the grain.For subsonic motion, the plasma drag force is typically a factor~ 20 - 30 larger than the drag due to direct collisions through ions. Electromagnetic forces on fee grains. Since there is usuallya \"plasma\"reference structure in which the electrical field is very little (interstellarplasma being a very great conductor),the electromagneticforce have the right to be attributed come the Lorentz forceF = Q(v / c) × B Lorentz force,where Q is the grain charge, B is the magnetic ar andv is the serial velocity relative to the plasma. Scattering and also absorption of photons(\"radiation pressure\") once the serial is illuminated by ananisotropic radiation field. Poynting-Robertson drag, as once a serial is relocating perpendicularto a directional radiation field.While no normally vital in theinterstellar medium, Poynting-Robertson drag can be an extremely important fordust grains orbiting stars (see INTERPLANETARY DUST). Recoil forces when photoelectrons or photodesorbed moleculesare emitted anistropically from a serial which is illuminated byan anisotropic ultraviolet radiation field. Gravitational force. In clouds sustained by gas pressure, dustgrains will have tendency to \"sediment\" towards the minimum of the gravitationalpotential.Because the gas and also grains room subject to different forces, the dust grainsgenerally have a drift velocity loved one to the gas.In common diffuse clouds, anisotropic starlight can an outcome in driftvelocities of bespeak ~ 0.1 km s-1. In regionswith strong anisotropic ultraviolet radiation fields(such as photodissociation regions) the drift velocities can be larger.The rotational dynamics that interstellar grains are additionally of good interest,in link with both the problem of serial alignment (which requiresalignment the the serial angular momentum vector with the regional magneticfield) and with electric dipole radiation which will be radiated byvery rapidly-rotating ultrasmall grains.The serial angular inert is influenced by: Collisions v gas atoms and molecules. Recoil associated with photoelectric emission. Recoil linked with H2 formation on the serial surface. Absorption and scattering of starlight by one asymmetric grain. Thermal infrared emission. The interstellar magnetic ar acting top top the magnetic dipolemoment resulting from the Barnett result in a rotate grain. The interstellar magnetic field acting on the magnetizationinduced in the spinning serial by the interstellar magnetic field(the \"Davis-Greenstein\" torque connected with paramagneticdissipation).Because the interstellar medium is much from thermodynamic equilibrium, someof these torques deserve to act systematically.Grains can be pushed to rotationalkinetic energies much bigger than kTgas (whereTgasis the gas temperature) by collisions v gas atom andmolecules (because the grain and also gas temperature differ), byphotoelectric emission or through absorption and scattering the starlight(because the starlight radiation ar is notin thermodynamic equilibrium through the serial temperature), and byH2 development (because the H2 abundance and gas and also grain temperaturesare not in thermodynamic equilibrium).Gradual alignment that the grain angular momentum v the magnetic fieldcan be produced by paramagnetic dissipation (because the serial rotationalkinetic energy is no in thermodynamic equilibrium v the\"vibrational\" temperature that the grain).15. Results of Interstellar Grains15.1. Photoelectric HeatingPerhaps the most important result of interstellar dust grains is theirrole in heater the gas via photoelectric emission.When a dust grain absorbs an ultraviolet photon the energyh , thereis a probability Y(h)- regularly referred to together the \"yield\" -that a photoelectron will escape native thegrain surface and thermalize through the regional gas.When this happens, the kinetic power of the emitted electron(at \"infinity\", since thegrain that is escaping from might be charged)acts to warmth the interstellar medium.On average, seed must catch electrons as rapidly asthey are ejected, and also the net heating rate then is same to thedifference in between the mean energy of the emitted electrons and thecaptured electrons.Since photoelectric yields Y might be of stimulate ~ 10%, and also sincethe kinetic power of the electron at infinity might be of order~ 1 eV (if the grain is not extremely positively charged)the photoelectric heating system - at ideal - converts ~ couple of % ofthe absorbed ultraviolet starlight power into heater of the gas.While this ~ few % conversion effectiveness may it seems ~ small, theavailability of energy in starlight is so great that photoelectric heatingis normally the leading heating system for diffusive interstellar gas.15.2. Radiative pressures on Dust GrainsIn astrophysics, anisotropic radiation fields are the rule, quite than theexception, and these anisotropic radiation fields can have actually dynamicalconsequences because that dust and also gas.Circumstellar grains space of course subject to very anisotropicradiation native the star.At a typical point in the interstellar medium, the nonuniformdistribution of stars in the galaxy, along with patchy obscurationby interstellar dust, results in starlight v appreciable anisotropy.The dipole ingredient of the starlight anisotropy might be typically~ 10%.When the radiation field is anisotropic, scattering and also absorption through thegrain produces a net force on the grain, generally referred to as the\"radiation pressure\" force.If the grain is coupled collisionally to the gas by gas drag and also perhapsmagnetic fields, then the force exerted ~ above the grains istransfer tothe gas.These radiation pressure pressures can overcome the dynamics the gas incertain regions, such together dust-forming winds from cool stars.15.3. H2 development on GrainsInterstellar chemistry mainly starts with the catalysis of H2on interstellar grain surfaces(see INTERSTELLAR CHEMISTRY).The inferred manufacturing rate of H2 in the interstellar mediumrequiresthat one appreciable portion of the H atoms arriving at interstellargrains surfaces must leave the grains as component of H2 molecules.While the details the the kinetics stay uncertain, the overall snapshot isbroadly together follows: once an H atom in a diffuse cloudarrives in ~ a serial surface, it has actually a high probability the sticking.The H atom then explores some fraction of the grain surface byeither thermal diffusion or quantum tunnelling, till it eitherfinds another H atom with which it can recombine to kind H2,or it arrives at a place on the grain surface where thebinding by one of two people van der Waals pressures (\"physisorption\") or development of achemical bond (\"chemisorption\") is solid enough the it becomes trapped.If such trapping occurs, then the surface ar coverage the trappedH atom builds up till newly-arrived H atoms have an appreciableprobability that reacting through a previously-trapped H atom fairly thanbecoming trapped themselves, or the adsorbed H atoms room removedby some other procedure (e.g., photodesorption or thermal desorption).15.4. Coupling Magnetic fields to Neutral GasDynamically vital magnetic areas are generally present in interstellargas (see INTERSTELLAR MAGNETIC FIELDS).In dense molecular gas with an extremely low fountain ionization,a large portion of the \"free\" charge lives on positively and negativelycharged dust grains.Under these problems - i beg your pardon prevail in high density regions inmolecular clouds, and presumably in protostellar disks -dust grains dominate the coupling the magnetic fields to the gas, due to the fact that theneutral gas atoms and also molecules us arenot straight coupled come the magnetic field.The fee dust grains are coupled both come the magnetic ar (byLorentz forces) and to the neutral gas (by collisional drag).As a result, the grains will certainly drift with a velocity i beg your pardon is intermediatebetween the velocity of the neutral gas and also the velocity through which themagnetic ar lines (and plasma) \"slip\" through the neutral gas.Thus the dust grains identify the rate of \"ambipolar diffusion\" ofmagnetic field lines in dense molecular regions.Charged dust seed play a similar role in the dynamics ofmagnetohydrodynamicshock waves in gas of low fractional ionization however dynamicallysignificant magnetic fields.16. Formation and also Destruction that Interstellar GrainsWhile interstellar grains are observed to it is in ubiquitous, the is notobvious that this need to be so.In the violent interstellar medium(see INTERSTELLAR MATTER, SUPERNOVA REMNANTS),grains can be ruined when the gas i beg your pardon they are in is overtakenby a shock wave v shock rate vs 200 kms-1, together is intended in supernova blastwaves.Estimates the the rate of event of supernovae, togetherwith models because that the result blastwaves, lead one to estimate thatthe fixed in interstellar grains will be returinter-base.net come the gas phaseon a timescale of ~ 5 × 108 yr, brief compared tothe period of the Galaxy and also the interstellar medium.We understand that grains space injected into the interstellar medium inoutflows indigenous cool redgiants and also supergiants, and even in supernova ejecta, butif there to be no conversion of gaseous atomsback to solid form in the interstellar medium, we would certainly expect an extremely littlegrain material to be current at any time.In particular, also for facets like Fe we would certainly expect many of theatoms to be in the gas phase.The fact that this is no so - that many of the interstellar Fe is infact missing from the gas step - requires that there be efficientrecondensation of Fe, Si, Ca, and other elements earlier into heavy formin the interstellar medium.The \"mineralogy\" that interstellar grains must thus largely reflectthe surface ar chemistry which will occur on the surface of interstellargrains.
BibliographyDust in the Galactic Environment, byD.C.B. Whittet (London: IOP Publishing; 1992), gives anexcellent all at once description the interstellar dust grains.The observed optical nature of dust have been freshly reviewedby J.S. Mathis(1990), Ann. Rev. Astr. Astrophys., 28,37.The ultrasmall grain populace is questioned by Puget, J.L., & Leger, A.(1989), Ann. Rev. Astr. Astrophys., 27,161.An in its entirety review that interstellar and circumstellar dust is provided byDorschner, J., & Henning, T.(1995), Astr. Astrophys. Rev., 6, 271.Microwave emission from dust seed is the evaluation byDraine, B.T., & Lazarian, A.(1999), inMicrowave Foregrounds,ed. A. De Oliveira-Costa & M. Tegmark(San Francisco: Astron. Soc. Pacific), p. 1331 because all grains are \"molecules\", that isnatural to consider small molecules together the small-size finish of the overallgrain population.Back.2 fixed fractionin \"primitive\" carbonaceous chondrite meteorites.Back.3 SiC and graphite seed sometimescontain very tiny TiC, ZrC, and also MoC inclusions.Back.4 The particles are of course no spherical.By grain \"radius\" we describe the radius of a ball of same volume.Back.