Harvard Astronomy 201b

Top Ten List for HII Regions

In Uncategorized on March 3, 2011 at 6:36 pm
  1. Caused by ionizing radiation from stars, principally Type O and B. (Fluxes) [1a. Planetary nebulae, caused by young white dwarfs, are also HII regions. (Look.)]
  2. Associated with densest parts of GMCs, because O and B stars form there & don’t live very long (so they can’t move very far).   Lifetime of HII regions is ~ lifetime of the ionizing stars, ~millions of years. (Look at Ophiuchus, in WWTSL.)
  3. Grow outward from ionizing star.  First, ionization front, then “stability” is possible, see Strömgren…but…
  4. Frequently occur in very turbulent, chaotic, regions, where clusters of stars form.  This can lead to HII regions caused by >1 star at once, in a very non-smooth medium, leading to non-smooth “edges,” and highly irregular geometries. (Look.)
  5. Very young HII regions can have very compact (or “ultra-compact” (<0.1pc) or “hyper-compact”(<0.05 pc)) shapes. Mostly free-free emission. (Short review, recent paper.)
  6. Characteristic temperature ~10,000K.  (Temperature where the heating due to the ionizing radiation is balanced by radiative cooling.)  (Introductory Explanation.)
  7. Principal emission is from the ionized Hydrogen itself, as “recombination” lines.  In the optical, that’s H-alpha (see figure below).  In the radio, there are many additional recombination lines seen, from much higher “n” states. (Example.)
  8. Region around an HII region where gas is mostly neutral, but ionizing uv dominates the heating and chemistry is (called) a  “photon dominated” or “photo-dissociated” region (PDR).
  9. The PDRs caused by HII regions lead to most of the non-stellar IR emission and mm and sub-mm CO emission from galaxies.
  10. Make for better PR images (in our galaxy or others) than just about any other astronomical objects. (“Hubble” in Google Images does not give photos of Edwin!)

H-alpha in a “Bohr Atom”: Transition from n=3 to 2 at 6563 Angstroms (part of the “Balmer Series,” all to n=2)

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