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Dust in the ISM Extinction (UV to near-IR) A V (5500 A): 1 magnitude <=> N(H) ~ 2 x 10 21

Dust in the ISM Extinction (UV to near-IR) A V (5500 A): 1 magnitude <=> N(H) ~ 2 x 10 21 A V = R E(B - V) / [B 0 - V 0 ] - B, V are observed magnitudes in the B and V bands

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Dust in the ISM Extinction (UV to near-IR) A V (5500 A): 1 magnitude <=> N(H) ~ 2 x 10 21

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  1. Dust in the ISM Extinction (UV to near-IR) AV (5500 A): 1 magnitude <=> N(H) ~ 2 x 1021 AV = R E(B - V) / [B0 - V0] - B, V are observed magnitudes in the B and V bands - B0, V0 are the intrinsic magnitudes in the B and V bands - R ~ 3. Large grains R = 5 IR-emission from Dust Black-bodies, “grey- (or blue) bodies” & emissivity  Stochastically heated very small grains (Purcell Effect). Debye T3 Law: Heat-capacity shrinks as T3 below TDebye Debye temperature for most solids is 200 to 500 K.Tdebye C (heat capacity) scales as [T/Tdebye]3 Microwave emission form spinning dust: (Lazarian Effect)

  2. Absorption: Attenuation of starlight by dust (reddening”) AV ~ RV * E(B-V)RV~ 3 along most LOS E(B-V) = color-excess from B to V in mag. Visual => IR FUV Graphite: 0.22 m

  3. Ice features in NGC 7538: (Whittet et al. 1996)

  4. Cooling by IR radiation of dust • Absorption • Eabs = [Flux absorbed per unit area] x [effective grain area] • = Integral 4a2 Qa()  (cu/4) d • uenergy density of (absorbed) radiation field • Qa() = Qo () a/ao= efficiency with which grain • couples to radiation field. • Emission • Eem =[Planck function(for grain T)] x [effective grain area] • Eem = Integral 4a2 Qa()  B(Tgrain) d • At FIR & sub-mm, Bn(Tgrain) is in the R-J limit spectrum has slope • S+ Emissivity

  5. Small grains are stochastically heated by single photon (Purcell Effect) At low temperatures compared to the Debye temperature Grain heat capacity ~ Natoms * 3k for T > Tdebye a (T / Tdebye)3 for T < Tdebye ~ few x 100 K (Debye T3 law of low-temperature physics) Comparable to few eV for small grains: Natoms = < 103 Single photons raise grain to T ~ 103 K Rapid cooling by NIR radiation. Time-variable grain T: Spikes followed by exponential decay Responsible for NIR continua (above expected scattered flux) in reflection nebulae, etc.

  6. Emission from small, stochastically heated grains. (Draine & Anderson 1985, ApJ, 292, 494)

  7. Emission from small, stochastically heated grains. (Draine & Anderson 1985, ApJ, 292, 494

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