HII regions slides

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Forbidden line coolingtransition rates very low: ~10 -3 100 per sec(compared to H recombination rates: 10 9 per sec)Therefore photons are v. likely to escape nebula before beingabsorbed can ignore absorptionThey can remove a lot of heat from the nebula, thus solving ourproblemCommon forbidden lines:Optical: [OIII] 4959,5007 Å, [NII] 6548,6584 Å, [SII] 6717,6731 ÅInfrared: [OIII] 52,88 m, [NIII] 57 m
Consequence of forbidden-line cooling:the higher the metallicity (i.e. heavy-element content) of anebula, the faster it cools to thermal equilibriumand the stronger the forbidden lines are
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Today:Recap on 1 st lecture:- Basic
Page 6 and 7: Distance = 450 pcDiameter ~ 1° (si
Page 8 and 9: Dark CloudsBok globules• relative
Page 12 and 13: ISM: average properties• Mean den
Page 14: A global view
Page 18 and 19: Cold, neutral medium (CNM)• domin
Page 20 and 21: www.milkywayproject.org
Page 22 and 23: Lagoon nebula
Page 25 and 26: PhotoionisationPhotoionisation: the
Page 27 and 28: Photoionisation: the removal of ele
Page 29 and 30: Balmer seriesLyman series(UV)Recomb
Page 31 and 32: Optical SpectrumPlanetary Nebula: N
Page 33 and 34: AtomsThe Maxwellian DistributionVel
Page 35: cross sectionse nucleuselectric fie
Page 39 and 40: For a typical O-star (e.g. the O6V
Page 42 and 43: M43Real examples1. no. of ionising
Page 44 and 45: However, Q per ionization is: (3/2)
Page 46 and 47: excitation/de-excitation of bound e
Page 48 and 49: Collisionally excited lines (CELs)B
Page 50 and 51: O +CELs (forbidden lines) are writt
Page 52 and 53: compare energy level diagrams1S 05.
Page 56 and 57: www.jodcast.netTopical podcast with
Page 59: DensitiesObservations n e ~ 100 cm
Page 62 and 63: Fraction of O2+ ions able to emit 4
Page 64 and 65: End of last lecture
Page 66 and 67: Today:HII regions: Practical method
Page 68 and 69: compare energy level diagrams1S 05.
Page 70 and 71: DensitiesObservations n e ~ 100 cm
Page 72 and 73: Practical methods and examples
Page 74 and 75: eal example
Page 76 and 77: • End of stellar existence for lo
Page 78 and 79: Optical Recombination Lines• ORLs
Page 80 and 81: Possible solutions?• Possibility
Page 82 and 83: Any other ideas?• Possibility 3:
Page 84 and 85: Knots: Abell 30• Results of empir
Page 86: Conclusions• Cold H-deficient kno
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HII regions slides

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