notes intersteller medium

Boom laboratory spectrum of coronene a type of pah

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Unformatted text preview: a 0.1µm T 104 K Dust grains: dynamics •  Transla>onal mo>on •  Gas drag  ­> coupled to the gas •  Lorentz force  ­> coupled to the magne>c field •  Radia>on pressure  ­> Poyn>ng ­Robertson effect (grains orbi>ng stars tend to spiral in) •  Rota>on Grains can be spun up by absorp>on/emission of photons, H2 forma>on, etc. •  Alignment of charged grains with magne>c field step 1: J step 2: J energe>cally more favored J B J B energe>cally more favored Forma>on of interstellar dust HD44179 (red rectangular nebular) Interstellar dust is likely produced in the outer atmospheres of red giants. Up: IR spectrum from the atmosphere of a red giant star. Bo\om: Laboratory spectrum of coronene, a type of PAH Atomic Hydrogen •  Atomic hydrogen comprises ~60% of the ma\er in the ISM •  Most diffuse H is in the form of HI in the disk with some in the halo •  HI gas is primarily heated by photoioniza>on, and is cooled by line emission •  HI gas can be detected either by UV absorp>on lines or through the 21cm line The 21cm line •  The 21 cm line is produced when the spin of the electron relative to the proton in the atom’s nucleus reverses (hyperfine structure splitting) emission p e- p + γ (5.87 × 10−6 eV) e- λ = 21 cm (radio) •  This transition is highly forbidden with a lifetime of 11 million years •  Still, the total number of hydrogen atom in a column of 1 cm area with 1 pc depth is >1018, there are enough transitions. Two phases of HI •  Cold neutral medium (CNM) T ∼ 100K nH ∼ 30 cm−3 fV ∼ 0.01 •  Warm neutral medium (WNM) T ∼ 5000K nH ∼ 0.6 cm−3 fV ∼ 0.4 Molecular Clouds •  •  •  •  Associated with star ­forming regions, concentrated in the galac>c plane ~20% of mass in the ISM (~2×109M), H2 being the dominant content Very op...
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This document was uploaded on 02/26/2014 for the course ASTRO 205 at Princeton.

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