By Adric Riedel

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Superbubbles Much ado about nearly nothing

• By Adric Riedel

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1. The ISM

• For a long time, outer space was thought to be
  completely empty
• Dark clouds were discovered.
• Originally thought to be holes, around 1910
  several respected scientists started thinking
  they were in fact opaque clouds.

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History- the ISM

• Until the 1960s, the Interstellar Medium was
  believed to be cold clouds suspended in warm
  ionized gas. (only optical and radio were
  available)
• These clouds were in pressure equilibrium, thus
  stable- no heat transfers

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History- the ISM

• Early X-ray rockets and telescopes revealed a
  soft X-ray background (SXRB)
• This had to come from million degree gas, hence a
  third state (with a fourth- Galactic Molecular
  Clouds)
• The Hot Bubble model
• The million degree gas is hot enough to cool
  within a million years thus supernovae are
  needed to create more

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The ISM

• Now consists of hot (1 million K), warm
  (5000-10000 K), cold and very cold (Giant
  Molecular Cloud) gas
• Abundances of heavy elements vary depending on
  recent supernovae
• Complicated, chaotic system of knots and so on.
  Thermal phases are less distinct.
• Represented by a fractal dimension.

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A Pointless Aside Slide

• Essentially, fractal dimensions fractional
  dimensions.
• A line is 1D
• Now imagine the Koch curve. Its made of lines,
  but its not all in 1D
• In the limit, its infinitely bumpy, and has a
  fractional dimension of 1.26
• Somewhat easy way to adapt equations to non-ideal
  situations (replace r2 with r2.1)

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2. OB Associations

• Found in star-forming regions
• 50 of all O and B stars are in OB Associations
• 1 supernova every million years

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OB Associations

• The Orion Nebula is one of the most prominent.
  Notice the other, non OB stars, some still
  forming.

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3. Supernovae

• Type 1a White Dwarf overload
• Older stars
• Scale height is high (halo)
• NOT the cause of Superbubbles

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Supernovae

• Type 2 / 1b Core Collapse
• Young stars
• Disk-bound (low scale height)
• 90 of all core-collapse supernovae are believed
  to occur in OB associations (Binns et al. 2005)

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4. Bubbles

• Formed by the wind of a single massive star, or a
  single SNR
• Energy levels of 1051 ergs
• Limited by the energy of the SN and the
  surrounding gas density and temperature
• Identified as HII (ionized hydrogen) regions-
  ionized shockfronts.
• Visible!

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Bubbles

• The Bubble Nebula is one of three shells around a
  massive star
• The star, BD602522, (note not central) is type
  O6.5IIIef, and part of an O-B association

Russell Croman Astrophotography
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Bubbles

• Note the blue areas- this is gas ionized by
  ultraviolet radiation.
• The central star is off-center due to the
  presence of the Giant Molecular Cloud (GMC)
  nearby

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4. Supergiant Shells

• Formed from starbursts larger than OB
  associations 1054 ergs
• Largest formation
• Badly understood

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Supergiant Shells

• Oey (1999) Alternative mechanisms include
  impacts by high-velocity clouds and Gamma Ray
  Bursters.
• Only two known, both in the LMC
• Properties likely to be very different from
  superbubbles due to galactic size-scales.

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Presentation Feature
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Superbubbles

• Occur in OB associations from core-collapse
  supernovae- at least five or six SN
• Typical lifetimes on the order of 5107 yr
• Sizes from 100 pc to 1700 pc. Within 1 Myr,
  expands to 90 pc (105 Msun cluster) or even 150
  pc (106 Msun cluster)
• Internal densities of 210-3cm-3 (Local Hot
  Bubble) and 2-510-2cm-3 for Loop 1)

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Evolution

• First defined in 1979 (Super Shells)
• Very large shell structures in the ISM- defined
  largely by their edges.
• Start out as bubble-driven (wind)- 40 pc alone
• Quickly become dominated by SNR
• Combined force keeps the Superbubble in the
  Taylor-Sedov phase for years
• Eventually cool, become radiative

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Shape

• Not spherical
• Affected by
• Number of SNe
• Spatial distribution of SNe
• Temporal distribution of SNe
• Surrounding density
• How long it grew
• Current age
• Naturally hourglass or V-shaped depending on
  Z-position The Chimney Effect

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Chimney Effect
100s of parsecs
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Superbubble

• Were it not for the radiation of the O stars, the
  Orion Nebula would be invisible.
• Note that in this case, multiple O stars winds
  are involved.

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How we can see Superbubbles

• Holes in HI, shells of HII (fainter as you go
  outward)
• Purple is Ha, Cyan is OIII. (N44, LMC)

250 ly
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How we can see Superbubbles

• Charting the absorption components of ISM.

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Two theories of Superbubble Formation

• Coincident supernovae
• Supernova go off inside each other
• Most energy goes into re-plowing out material in
  the center, not expansion
• Expected in massive star-forming regions, like
  the spiral arms

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Two theories of Superbubble Formation

• Nearby Supernovae.
• The Supernova shells are outside each other, but
  merge into large superbubbles
• Expected in inter-arm regions (such as the Local
  Interstellar Medium)

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Effects of Superbubbles on the ISM

• Superbubbles stir up the Interstellar Medium.
• Superbubbles also supply the hot gas in the
  stellar halo via the chimney effect- the
  largest bubbles seem to have hourglass shapes in
  the z direction.
• Superbubbles and the winds of massive stars that
  make them, also enrich the Inter-Galactic Medium
  with heavy metals.

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Implications

• Explains the Soft X-Ray background Were inside
  a superbubble with its million-degree gas.
• May reconcile the massive-star origin of Gamma
  Ray Bursters by providing an extremely
  low-density environment for GRB/hypernovae to
  explode into (Scalo Wheeler 2001)
• Explain the turbulent ISM (may completely explain
  the gas topology of the SMC)
• Explains the hot gas in galactic haloes
• May be the cause of Cosmic Rays

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The Local Interstellar Medium

• A few cool clouds (5000 K) surrounding the solar
  system itself
• The Local Bubble (106.5 K gas) and Loop 1 (the
  same) were once the same bubble. (15 Myr ago)
• More SNe occured, separating the two bubbles- six
  in the LB (12 Myr ago)
• OB associations only exist in Loop 1 now the LB
  will be squeezed out of existence soon.

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Local Interstellar Medium

• Our view of the local bubble has changed a lot in
  recent years
• At one point, the Sun was thought to be inside
  the shell between the LB and Loop 1
• Now theyre believed to be separate

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Galactic Cosmic Rays

• The materials accelerated are condensed grains of
  heavy elements (Mayer Meynet 1993), formed from
  supernovae in OB associations
• The first dust evidence appeared in SN1987as
  spectrum after 450 days
• Isotope ratios of Ni measured by the ACE
  satellite suggest a 105 year lag time, then the
  force of another supernova.

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Galactic Cosmic Rays

• Supernovae dont accelerate their own ejecta into
  GCRs.
• Superbubbles carry these heavy elements from
  Wolf-Rayet stars and other massive SNe outward,
  mixing with solar-composition material until
  accelerated by subsequent SN shocks within the
  superbubble to provide the bulk of the GCRs
  (Binns et al 2005).

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Problem for Superbubbles

• Cold, dense ISM gas stops them
• Must be evaporated via conduction
• Once they cool, radiation takes over, interior
  brighter than shell
• Dense clouds make locating superbubbles harder-
  theyre not spherical
• Small magnetic fields resist expansion (more on
  this later)
• All supernovae have to go off at exactly the
  right time- too spread out, and they wont add up
  to anything.

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Problems for theory

• Current theories have superbubbles expanding
  faster than they apparently do. (Magnetic effects
  may help)
• What (Salpeter-type) stellar birth mass function
  is correct? (what percentage are massive?)
• Does the actual fractal dimension of the ISM
  match? (currently, superbubble models give 2.5 to
  2.8)
• Current models assume the interior density to be
  uniform- concentrating only on the shell
• Most models neglect rotational sheer
• Will Voyager 1 make it to the ISM before it fails?

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Works Cited

• Binns, W.R. et al. Cosmic-Ray Neon, Wolf-Rayet
  Stars, and the Superbubble Origin of Galactic
  Cosmic Rays 2005, ApJ, 634, 351
• Frisch, P. The Local Bubble, Local Fluff, and
  Heliosphere 1998, LNP, 506, 269F
• Garcia-Segura Oey, M.S. Superbubbles as Space
  Barometers 2004, JKAS, 34, 217
• Hasebe et al. Are Galactic Cosmic Rays
  Accelerated inside the Ejectae Expanding just
  after Supernova Explosions? 2005, NuPhyA, 758,
  292c
• Higdon, J.C. Lingenfelter, R.E. OB
  Associations, Supernova Generated Superbubbles,
  and the Source of Cosmic Rays 2005, ApJ, 268,
  738
• Ikeuchi, S. Evolution of Evolution of
  Superbubbles 1998, LNP, 506, 399
• Mac Low, M.M. McCray, R. Superbubbles in disk
  galaxies, 1988, ApJ, 324, 776
• Maiz-Apellaniz, J. The Origin of the Local
  Bubble 2001, ApJ, 560, L86
• Oey, M.S. Superbubbles in the Magellanic Clouds
  1999, IAUS, 190, 78O
• Scalo, J. Wheeler, J.C. Preexisting
  Superbubbles as the Sites of Gamma-Ray Bursts,
  2001, ApJ, 562, 664
• Walsh, B.Y. Lallement, R. Local Hot Gas,
  2005, AA, 436, 615
• Walsh, B.Y. et al. NaI and CaII absorption
  components observed towards the Orion-Eridanus
  Superbubble 2005 AA 440, 547
• Zaninetti, L. On the Shape of Superbubbles
  Evolving in the Galactic Plane 2004 PASJ 56,
  1067