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notes intersteller medium

A 25 log10 mag 0 f f graphite 25 log10 e

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Unformatted text preview: th gas (roughly) •  Composition: C, Si, O, Mg, Fe •  Size: sub-micron, down to nm sized PAHs (polycyclic aromatic hydrocarbon) •  Shape: mostly amorphous rather than crystalline, can be very irregular Dust grains: ex>nc>on •  Absorb and sca\er op>cal and UV photons very efficiently, re ­emit in longer wavelength. Aλ = 2.5 log10 mag ￿ 0 Fλ Fλ ￿ graphite = 2.5 log10 (e ) = 1.086τλ τ AV AV RV ≡ ≡ AB − A V E (B − V ) 1 = Color Excess τB /τV − 1 silicate (Reddening) RV depends on dust proper>es, and is deter ­ mined by observa>ons, which is in turn used to constrain dust models. RV =3.1 in the solar neighborhood. (Cardelli et al. 1989) Dust grains: ex>nc>on •  Absorb and sca\er op>cal and UV photons very efficiently, re ­emit in longer wavelength. ex>nc>on = absorp>on + sca\ering (albedo = sca\ering / ex>nc>on) graphite Ex>nc>on cross sec>on for a single grain (Mie theory): σ ≈ π a2 for λ ￿ a σ ∝ 1/λ for λ > a Ex>nc>on curve: silicate •  Contains rich informa>on about grain proper>es (composi>on, size distribu>on, etc.) •  Parameterized by RV •  Of crucial importance for galac>c and extragalac>c observa>ons (Cardelli et al. 1989) Dust grains: emission spectrum Dust grains: chemistry •  H2 forma>on: Direct forma>on of H2 by collisions of two H atoms is very unlikely (unable to get rid of the excess energy efficiently). In the present day universe most of the H2 form by combining two H atoms afer their adsorp>on by the surface of a dust par>cle: H2 H H H H grain grain grain •  Recombina>on and grain charging Grains can be charged by colliding with electrons and ions, with equilibrium net charge ￿ ￿￿ ￿ Zgr ≈ −150...
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