l>Interstellar grains - The 75th Anniversary - A. Li


Our expertise of interstellar dust regarding its size, shape and also composition is mainly acquired from its communication with electromagnetic radiation: attenuation (absorption and also scattering)and polarization of starlight, and emission that IR and far-IR radiation. Presolar grains established inmeteorites and interplanetary dust corpuscle (IDPs)of cometary origin also contain advantageous informationregarding the nature the interstellar grains.The principal observational keys, both directand indirect, provided to constrain the properties of dust weresummarized in recent reviews ofDraine (2003) andLi (2004b).(1) grain Sizes.

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indigenous thewavelength-dependent interstellar extinction and polarization curves and the near,mid and far IR emission, we understand that there have to exist a distributionof grains sizes, ranging from a few angstroms to a couple of micrometers.- The interstellar extinct curve includes importantinformation about the serial sizes because generally speaking, a serial absorbs and also scatters light most effectively at wavelengths similar to the size2a.The extinct curve rises native the near-IR to the near-UV, through a wide absorption function at about -1 4.6µm-1 ( 2175Å),followed by a steep rise right into the far-UV-1 10µm-1.">There should exist in the ISM a populace of huge grains through a > /2 0.1 µmto account because that the extinct at visible wavelengths,and a populace of ultrasmall grains through a µm come account because that the far-UV extinction in ~ =0.1µm (seeLi 2004a because that details).- The interstellar polarization curve rises indigenous IR, has a maximum somewhere in the optical and then decreases toward the UV. "> There should exist a population of aliginvernessgangshow.net, nonspherical seed with typical sizes that a /2 0.1µm responsible for the height polarization at 0.55µm.- Interstellar grains absorb starlight in theUV/visible and also re-radiate in the IR. The IR emissions spectrum the the MilkyWay diffuse ISM, approximated using the IRAS 12, 25, 60 and also 100 µmbroadband photometry, the DIRBE-COBE 2.2, 3.5, 4.9, 12, 25, 60, 100, 140and 240 µm broadband photometry, and also the FIRAS-COBE 110µm µmspectrophotometry, is characterized by a modification black-body of -1.7B(T = 19.5 K)peaking at ~ 130 µm in the wavelength rangeof 80 µm µm,and a substantial amount of emissions at µmwhich far exceeds what would certainly be expected from dust in ~ T 20 K. In addition, spectrometers board the IRTS(Onaka et al. 1996;Tanaka et al. 1996) and ISO(Mattila et al. 1996) have displayed that the diffuse ISM radiates strongly in emission features at 3.3, 6.2, 7.7, 8.6, and also 11.3 µm. "> There should exist apopulation the "cold dust" in the size range ofa > 250Å, cook by starlight to equilibrium temperatures of 15 K T K and also cooled by far-IR emissions to producethe emission at > 60µm i beg your pardon accounts because that ~ 65% that the complete emitted power(see Li & Draine 2001b);there must likewise exist a population of "warm dust" in the size variety ofa Å, stochastically cook by singlestarlight photons to temperatures T >> 20 Kand cooled through near- and also mid-IR emission to create the emissions at µm i m sorry accounts because that ~ 35% the the total emitted power(see Li & Draine 2001b;Li 2004a).- The scattering nature of dust grains (albedo and also phase function) administer a means of constraining the optical properties of the grainsand are thus indicators of their size and also composition.The albedo in the near-IR and optical is quite high (~ 0.6), v a clear dip to ~ 0.4 about the 2175Å hump,a increase to ~ 0.8 about -1 6.6µm-1, and also a drop come ~ 0.3 through -1 10µm-1;the scattering asymmetry factor practically monotonically rises from~ 0.6 come ~ 0.8 from -1 1µm-1 come -1 10µm-1 (seeGordon 2004)."> An appreciablefraction of the extinction in the near-IR and also optical have to arise fromscattering; the 2175Å hump is an absorption featurewith no scattered component; and ultrasmall grainsare predominantly absorptive. - The "anomalous" Galactic foreground microwaveemission in the 10-100GHz region(Draine & Lazarian 1998a,b), the photoelectric heater of the diffuse ISM(Bakes & Tielens 1994,Weingartner & Draine2001b), and (probably) the ERE(Witt & Vijh 2004)also administer direct or indirect proof for the existenceof nanometer-sized grains in the ISM (see section 2 inLi 2004afor details).- Both micrometer-sized presolar grains (such together graphite, SiC, corundum Al2O3, and also silicon nitride Si3N4) and nanometer-sized presolar grains (such together nanodiamonds and titanium carbide nanocrystals)18of interstellar beginning as shown by their anomalous isotopic composition have been figured out in primitive meteorites (seeClayton & Nittler 2004for a recent review). Presolar silicate grains have actually recently beenidentified in IDPs(Messenger et al. 2003).Submicron-sized gems (Glass with embedded Metals and also Sulfides) the presolar origin have likewise been figured out in IDPs and their 8-13µm absorb spectrum were comparable to those it was observed in interstellar molecule clouds and young mainly objects (seeBradley 2003for a recent review).- Very huge interstellar grains (with radiia > 1µm) beginning the solar system have actually been detect by the interplanetary spacecraft Ulysses and Galileo(Grün et al. 1993,1994). Vast grains the radii of a ~ 10 µm whose interstellarorigin was indicated by their hyperbolic velocitieshave to be detected through radar methods(Taylor et al. 1996).But Frisch et al. (1999)and Weingartner & Draine(2001a)argued the the lot of very huge grains inferred indigenous thesedetections were complicated to reconcile through the interstellar extinction and interstellar element abundances. (2) grain Shape. The detection of interstellar polarization plainly indicates that some fraction of the interstellar grains must be nonspherical and also aliginvernessgangshow.net. The fact that the wavelength dependency of the interstellar polarization exhibits a steep decrease towards the UV argues that the ultrasmall grain component responsible for the far-UV extinction rise is either spherical or no aliginvernessgangshow.net.- The 9.7 and also 18 µm silicate absorptionfeatures are polarized in part interstellar regions, most of whichare featureless. 19Polarization has additionally been detected in the 3.1 µm H2O, 4.67 µm CO and also 4.62 µmOCN- absorption functions (e.g. SeeChrysostomou et al. 1996).Hough et al. (1996)reported the detection that a weak 3.47 µm polarization attribute in the Becklin-Neugebauer object in the OMC-1 Orion thick molecular cloud, attributed come carbonaceous materials with diamond-like structure."> The detection ofpolarization in both silicate and also ice absorption features is consistentwith the presumption of a core-mantle serial morphology (e.g. SeeLee & Draine 1985).- So far only two lines that sight toward HD147933 and HD197770 have actually a weak 2175Å polarization feature detected(Clayton et al. 1992;Anderson et al. 1996; Wolff et al. 1997;Martin, Clayton, & Wolff1999). Even for these sightlines, the level of alignment and/or polarizing capability of the carrier need to be really small(see section inLi & Greenberg 2003for details)."> The 2175Å bumpcarrier is a really inefficient polarizer(i.e. It is either virtually spherical or poorly aliginvernessgangshow.net). - for this reason far, no polarization has actually been detected for theDIBs (seeSomerville 1996for a review), the 3.4 µm absorb feature(Adamson et al. 1999),20 and also the "UIR" emissions bands(Sellgren, Rouan, &Léger 1988). "> your carriers carry out notalign or absence optical anisotropy. (3) serial Composition. it isnow generally welcomed that interstellar grains consist that amorphous silicates and also some kind of carbonaceous materials; the previous is inferred indigenous the 9.7 µm Si-O extending mode and 18µm O-Si-O bending mode absorption attributes in interstellar regions as well as the truth that the cosmically abundant heavy elements such together Si, Fe, Mg are highly depleted; the last is mainly inferred from the 2175Å extinct hump (and the ubiquitous 3.4 µm C-H stretching vibrational band) and the reality that silicates alone room not able to provide enough extinct (see Footnote-14 ofLi 2004b). - The 9.7 µm and 18µm absorption functions are ubiquitously seen in a wide variety of astrophysical environments. These attributes are practically certainly due tosilicate minerals: they are respectively ascribed come the Si-O stretching and O-Si-O bending settings in some type of silicate product (e.g. OlivineMg2xFe2-2xSiO4).In the ISM, these attributes are vast and relatively featureless. "> Interstellarsilicates are mostly amorphous quite than crystalline.21- The toughness of the 9.7µm feature is approximately9.7 µm /AV 1/18.5 in the localdiffuse ISM. "> Almost all Siatoms have been locked increase in silicate dust, if presume solar abundance for the ISM (see Footnote-9 ofLi 2004b).22- The 3.4µm absorption feature is also ubiquitously seen in the diffuse ISM (but never in dense regions) of the Milky method and various other galaxies (e.g. Seyfert galaxies and ultraluminous infrared galaxies, seePendleton 2004for a recent review).This attribute is typically attributed come the C-H stretching mode in aliphatic hydrocarbon dust, although its precise nature continues to be uncertain. 23 - In principle, we could estimate the volume ratio of the silicate component to the aliphatic hydrocarbon component(1) if we know the band toughness of the transport of the 3.4 µm absorption attribute (seeLi 2004b),or (2) if we recognize the total abundances the interstellar aspects (seeLi 2005a). However, neither is exactly known. (4) distribution of Dust and its Association through Gas. Interstellar grains space unevenly distributed yet primarily confiinvernessgangshow.net to the galactic airplane with an effective thickness that ~ 200 pc. Top top average, the "rate that extinction" (the lot of visual extinction per unit distance) AV / L> is about ~ 1.8mag kpc-1 because that the sightlines close come the galactic plane and for ranges up come a few kiloparsecs from the Sun(Whittet 2003). Presume a typical grain dimension of ~ 0.1 µmand a common mass density of ~ 2.5 g cm-3for the interstellar serial material, we have the right to estimate the mean dust number density and also mass thickness in the solar neighbourhood ISM respectively to be ndust 1.1 × 10-12 cm-3 and dust 1.2 ×10-26 g cm-3from the "rate of extinction".24The association of interstellar dust and gas had actually been prove byBohlin, Savage, & Drake(1978)who found that the shade excess and the total hydrogen column density (determiinvernessgangshow.net indigenous the monitorings of HILyman-
and H2 absorption lines through the Copernicus satellite) were fine correlated:E(B - V) / NH 1.7 ×10-22 mag cm2for the diffuse ISM in the solar neighbourhood. This correlation has actually recently been evidenced by the monitorings with the much Ultraviolet Spectroscopic Explorer(FUSE) as much as E(B - V) 1.0 (Rachford et al. 2002),25 suggesting that the dust and gas are normally well combined in the ISM.From this proportion of E(B - V) to NHone can estimate the gas-to-dust mass ratio to it is in ~210 in the diffuse ISM if us take RV 3.1 (see Footnote-2in Li 2004b);together v the "rate the extinction"AV / L> 1.8 magkpc-1, one deserve to estimate the hydrogen number thickness to benH = RV AV /L> NH / E(B - V) 1.1 cm-3 and also a gas mass thickness of gas 2.6 ×10-24 g cm-3.

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Acknowledgments I thank the organizers F. Borghese and also R. Saijafor inviting me to this really exciting and also fruitful conference.I say thanks to F. Borghese, C. Cecchi-Pestellini, A. Giusto,M.A. Iatì, M.I. Mishchenko, and R. Saija for advantageous discussions.18von Helden et al. (2000)proposed that TiC nanocrystals might be responsible for the significant 21 µm emission attribute detected in end a dozen carbon-rich post-AGB starswhich continues to be unidentified due to the fact that its very first detection (Kwok, Volk, & Hrivnak1989). Based upon the Kramers-Kronig relations(Purcell 1969),Li (2003b)found the the TiC proposal is not feasible due to the fact that it needs at least50 times more Ti 보다 available.Back.19 The only exception is AFGL 2591, amolecular cloud surrounding a young mainly object, i beg your pardon displaysa narrow function at 11.2 µm superimposed top top the broad 9.7 µm polarization band, normally attributed to annealed silicates(Aitken et al. 1988).However, that is 3.1 µm ice absorption attribute is no polarized (Dyck & Lonsdale 1981,Kobayashi et al. 1980).Back.20 So much spectropolarimetricmeasurement that this function has been performed only for one sightline- the Galactic Center resource IRS7(Adamson et al. 1999).Unfortunately, no such measurements have actually been carried out because that the 9.7 µm silicate absorption featureof this sightline. Spectropolarimetric measurements for both these 2 bands of the exact same sightline would certainly allowa straight test of the silicate core-hydrocarbon mantle interstellar dust model(Li & Greenberg 1997), since this version predicts the the 3.4 µm feature would it is in polarized if the 9.7 µm attribute (for the very same sightline) is polarized(Li & Greenberg 2002).Back.21Li & Draine (2001a)estimated that the quantity of a µm crystalline silicate grains in the diffuse ISM is Kemper, Vriend & Tielens(2004)placed a much tighter upper limit that ~ 0.2% on the crystalline portion of the interstellar silicates along the sightline towards the Galactic Center.Back.22 The silicate absorption feature(relative come the visual extinction) along the route to the GalacticCenter is around twice that of the regional ISM: 9.7 µm /AV 1/9(Roche & Aitken 1985).It was originally thought that there to be very couple of carbonstars in the main regions that the Galaxy so the onewould intend a lot larger portion of the dust to besilicates than is the instance further out in the Galactic disk(Roche & Aitken 1985).However, this explanation was tested by the factthat the 3.4 µm aliphatic hydrocarbon dust absorptionfeature for the Galactic facility sources (relative to the visual extinction:3.4 µm /AV 1/150) is also about twice the of the neighborhood ISM(3.4 µm /AV1/250; Pendleton et al. 1994;Sandford, Allamandola, &Pendleton 1995).Back.23 end 20 various candidates havebeen suggest (see Pendleton & Allamandola2002for a summary). Therefore far, the experimental spectra the hydrogenatedamorphous carbon (HAC;Schnaiter, Henning & Mutschke 1999,Mennella et al. 1999)and the organic refractory residue,synthesized indigenous UV photoprocessing of interstellar ice cream mixtures(Greenberg et al. 1995),provide the best fit to both the as whole feature and also the positions and also relative toughness of the 3.42µm, 3.48 µm, and 3.51 µm subfeaturescorresponding to symmetric and also asymmetric stretches of C-H bonds inCH2 and also CH3 groups.Pendleton & Allamandola(2002)attributed this function tohydrocarbons through a blended aromatic and aliphatic character.Back.24 allow interstellar seed beapproximated by a solitary size the a (spherical radius) through a number densityof nd. The visual extinction led to by thesegrains through a pathlength of l isAV = 1.086 a2Qext(V) nd L,where Qext(V) is the dust extinction performance at V-band ( =5500Å). The dust number thickness can be derived from
The dust mass density is approximatelydust= ndust (4/3) a3d 1.2 × 10-16 g cm-3 if we take a 0.1µm, Qext(V) = 1.5, and d =2.5 g cm-3.Back.25 Dark clouds (e.g. The Oph cloud)seem to have actually lower E(B - V) / NH values, suggestinggrain development through coagulation(Jura 1980;Vrba, Coyne, & Tapia1993;Kim & boy name 1996).Back.