Lecture 15 Ch 19 396409

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Lecture 15 Ch 19 396-409

• Exam 3 - March 25 review session 3/20 730pm in
  MH1005
• Interstellar Medium (ISM) - Dust Gas
• 1) What is it?
• Composition
• Temperature types of clouds
• 2) How can you detect IS gas dust?
• 3) Who cares?
• Extinction
• Reddening
• Relevance to star formation

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ISM - Composition

• 99 of 'stuff between stars' is in gas form
• 1 in dust form
• Hydrogen is by far the dominant ingredient
• He next in line - fractional amt of metals
• If the very early universe was composed of H, He,
  and a little Li ... where do/did metals come
  from?
• A lot of ISM is 'recycled' material
• Distribution of ISM is sparse
• Even 'dense clumps/clouds' are vacuum-like

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ISM Temperature Types of Clouds

• Gas can clump/collect into small/large clouds
• Cloud type determined by TEMPERATURE
• 1) Very hot HII regions (pretty rare)
• Energetic UV photons from 'hot' stars absorbed by
  neutral (regular) hydrogen
• Produces 'ionized hydrogren - HII' - bare proton
  free electron
• Every once in a while a proton electron will
  recombine, and electron will cascade to inner
  shells
• Produces hydrogen emission lines (emits photons)
• Absorbtion of UV photons happens at greater rate
  than recombination --gt get HII regions

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HII Region - 30 Dor (LMC) Black Hydrogren
emission (H alpha)
J. Wisniewski K. Bjorkman 2002 CTIO 0.9m
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ISM - Temperature Cloud Type

• 2) Cool HI gas (100 K)
• Usually diffuse (low density)
• 3) Cold Molecular Cloud (20 K)
• Molecular hydrogen most abundant species (H2)
• Very clumpy (but still vacuum-like)
• Nurseries for star formation
• Random useless trivia Other molecules detected
  include water ethyl alcohol (see text 401) - IS
  cloud 0.2 proof!
• No detection of 'ISM Coke' yet ... so no captain
  cokes

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Molecular Cloud - Orion Nebula
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Detecting ISM Clouds

• 1) HII regions
• Optical spectroscopy works (H alpha)
• 2) HI gas/clouds
• Observe '21 cm' radio line
• Not a 'normal' transition, has a long lifetime
• 3) Molecular (H2) clouds
• Extremely difficult to directly detect H2
• Use other species as 'tracers' i.e. CO

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In real life ...

• Nature doesn't necessarily order IS gas into 3
  categories
• Cores of molecular clouds surrounded by a
  'shield' of HI
• Might make intuitive sense ... since temperature
  is deciding factor

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'Real-life' Cloud
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IS Dust

• In addition to gas, there are a small amt of dust
  grains present in the ISM - distribution depends
  on temperature

ISM Dust Grain (collected from Florianus airplane)
aoss.engin.umich.edu/earth_space/star_tour5.html
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Who Cares?

• IS dust affects (nearly) all observations of
  astrophysical objects
• 1) Extinction (not dinosaurs or even endangered
  'screamapillers')
• Things appear dimmer than they really are due to
• a) absorbtion by dust grains
• b) scattering by dust grains
• 'reflection nebula', sun's red appearance near
  horizon
• 2) Reddening
• Extinction is wavelength dependent - so blue
  light is scattered out of the line of sight
  preferrentially
• Objects appear 'redder' than they really are

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NGC 891 - WIYN - Ex. of Extinction
kingpin.ucsd.edu/howk/EdgeOns/n891.html - J.C.
Howk
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HST WFPC2 - NGC4013 - Ex. of Extinction
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Scattering in action

• Blue light is preferrentially scattered more ...
• Assume you have a cloud of dust gas a strong
  source of background photons (a 'hot' star)
• Look in any direction other than the line of
  sight to the star you should see this 'excess
  scattered blue light'
• Called 'Reflection Nebula'

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Reflection Nebula - Pleiades
www.rdcrisp.darkhorizons.org/m45_reflection_nebula
.htm
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How do you study dust?

• Limited number of 'capture' experiments
• Spectroscopy
• Gives chemical fingerprint of dust - illuminated
  by background starlight
• Photometry
• Can give temperature distribution
• Polarimetry
• Grains will tend to partially align with magnetic
  fields (such as the galactic mag field) - learn
  about size, shape, composition of grains

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Open-Ended Questions

• 1) Dust affects most observations ... if dust in
  early universe was different than it is 'now'
  (i.e. if there was more/less dust present), how
  does this affect our abilities to study the early
  universe?
• 2) Metals in the ISM play in important role in
  the star formation process (to be discussed next
  lecture). In the very early universe, there were
  no metals. How did the first generation of stars
  form?

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Summary

• 1) ISM mainly gas, some dust (which is different
  than 'Earth dust')
• 2) ISM mainly H, some He, fractional amt of
  metals
• Much is recycled material
• 3) Temperature plays a critical role in determing
  whether a cloud is a molecular, HI, or HII region
• Also affects presence of dust (destroyed in hot
  regions)
• 4) Different observational techniques in
  different wavelength regimes are used to probe IS
  gas dust
• 5) Dust affects observations of most objects
• Extinction (things appear dimmer)
• Reddening (things appear 'redder')