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ii mm Emiissiion from Dust i li iin Gallaxiies
i Michael A. Dopiita, ii ll li i l i RSAA, Australliian Natiionall Universiity ii
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i I Why iis Dust IImportant?
Thermal emission may extend into mm region, if grains are cool enough, BUT: Draine & Lazarian have shown that grains may produce emission due to: Electric Dipole radiation from aspherical

charged rotating grains OR Magnetic Dipole radiation from rotating

magnetically-active grains.

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I IISO Data on PNe:
An amazing variety of grain species have been detected: crystalline silicates (enstatite MgSiO3, forsterite Mg2SiO4) amorphous silicates (either fayalite Fe2SiO4 or amorphous olivine (Fe,Mg)2SiO4) organic carbon grains - aliphatic - aromatic - anthracite (c.f. 2200е feature) - surface hydrogenated nano-diamonds water ice carbides (SiC, MgC, TiC) sulfides (triolite FeS, MgS, SiS2, ) carbonates (!!) (CaCO3, CaMg(CO3)2)
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NASA Samples of Cosmic Grains
(collected by Aerogel on U2 Aircraft)

Iron Sulfide Amorphous Silicate

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ii Thermal Emiissiion ll
Usually, the slope of the thermal component at low frequency (lattice vibration) is assumed to be: j= A



1.7

which is determined by the slope of the

absorption coefficient,

=

0.7:

However, Menella et al. (1998) showed that strongly dependent on grain temperature!



is

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1mm 3mm

295K (0.8) 24K (1.2) 24K (2.3)

295K (1.3)

Amorphous Carbon Grains

Fayalite

from Mennella et al. (1998) ApJ, 496, 1058
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3mm

1mm

100µm

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i The Implliicatiions: II li
The lattice vibrational spectral slope in the mm region should be flatter where there is hot dust: i.e. in regions with AGN or in violent (warm) starbursts.

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ii Dipole Emiissiion from Grains ii ll ii
The grain has a intrinsic dipole moment µ = N1/2 and a dipole moment due to charge µ = Zea ...so a rotating grain produces dipole emission with power: P = (2/3c2)4µ2sin2
+ +

+ ++ + +
a
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Rotational Rate of Grains: ii ll ii
Determined by "plasma drag" & "electrostatic focussing" "Plasma drag" - is the interaction of the dipole moment with the electric field produced by passing ions. "Electrostatic Focussing" - or rather defocussing, deflects +vely charged ions with high angular momentum away, reducing rotational energy deposition. As a consequence, peak dipole emission is at ~ 30GHz.

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Draine & Lazarian 1998 ApJ 494, L19

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il i ii Magnetic Diipolle Emiission ii µ
Magnetically active rotating grains produce electric dipole emission. Theory is rather more complex than for electric dipole emission (Draine & Lazarian, 1999 ApJ, 512, 740)

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Research School of Astronomy & Astrophysics

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Research School of Astronomy & Astrophysics

mm Workshop 2001