notes intersteller medium

2010 molecular clouds are clumpy and very irregular

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Unformatted text preview: aque due to the dust Have a wide range of size and densi>es, from diffuse cloud (AV ~ 1) to giant molecular cloud (AV ~ 20 and up to 100) Molecular cloud: tracers Galac>c CO distribu>on (Dame, 2001) •  H2 can not be easily detected (lack of permanent electric dipole moment) •  Clouds are most ofen surveyed based on the CO molecule (rota>onal transi>on from J=1 to J=0, at wavelength of 2.6 mm) •  Other methods include observa>on of sub ­mm con>nuum emission from the dust Molecular cloud •  Because of high density, complex chemistry is possible: Other molecules include but not limited to: H2O, HCN (cyanide), OH (hydroxyl) and more complex molecules such as ethyl alcohol (CH3CH2OH) •  Giant molecular clouds are self ­gravita>ng Velocity dispersion increases with cloud size Velocity dispersion is supersonic => supersonic turbulence •  Presence of magne>c field (Larson 1981) Magne>c field of 5 ­1000 micro Gauss from Zeeman splirng measurement => magne>c energy comparable to the turbulent kine>c energy “PILLARS OF CREATION” IN A STAR ­FORMING REGION M16  ­ Eagle Nebula Gritschneder et al. 2010 Molecular clouds are clumpy and very irregular. HII regions •  They are formed near molecular clouds where newly formed massive stars ionize hydrogen and heavier elements like O, N. •  ~20% of the mass in the ISM •  Heated to high temperature (7000 ­15000 K) by photoioniza>on, cooled by op>cal and IR line emission, as well as free ­free (bremsstrahlung) emission •  Characteris>c spectrum of HII regions are strong emission lines: Recombina>on lines: radia>ve recombina>on to upper energy levels followed by cascade. Most prominent: H balmer series, in par>cular the Hα line. Collisionally excited lines: collisions with electrons excite meta ­stable levels resul>ng in forbidden line transi>ons such as [OIII]5008Å in op>cal, and fine ­ structure lines in mid ­far infrared •  Line fluxes and their ra>os used to diagnose temperature, density, abundances. The Orion nebula Visible The gas in Orion glows because it is ionized by 4 hot stars: HII region Infrared Strömgren sphere •  The size of the HII region can be es>mated by considering the requirement of ioniza>on ­recombina>on equilibrium H ￿ H + + e− RS Recombina>on rate per volume: O star nrecomb = αne np ˙ Primarily H+, e- In HII region, n e ≈ n p ≈ nH (fully ionized) Let Q be the rate of ionizing photons (E>13.6eV or λ<912Å) produced by the O and B stars. They will all be a...
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