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Superbubbles: Much ado
about nearly nothing
By Adric Riedel
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.
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
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
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.
A Pointless Aside Slide
• Essentially, fractal
dimensions = fractional
dimensions.
• A line is 1D
• Now imagine the Koch
curve. It’s made of lines,
but it’s not all in 1D
• In the limit, it’s 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)
2. OB Associations
• Found in star-forming regions
• 50% of all O and B stars are in OB
Associations
• 1 supernova every million years
OB Associations
• The Orion Nebula is one of the most
prominent. Notice the other, non OB stars,
some still forming.
3. Supernovae
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Type 1a: White Dwarf overload
Older stars
Scale height is high (halo)
NOT the cause of Superbubbles
Supernovae
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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)
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!
Bubbles
• The Bubble Nebula is
one of three shells
around a massive star
• The star, BD+602522,
(note: not central) is
type O6.5IIIef, and
part of an O-B
association
Russell Croman Astrophotography
Bubbles
• Note the blue areasthis is gas ionized by
ultraviolet radiation.
• The central star is offcenter due to the
presence of the Giant
Molecular Cloud
(GMC) nearby
4. Supergiant Shells
• Formed from
starbursts – larger
than OB
associations; 1054
ergs
• Largest formation
• Badly understood
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.
Presentation Feature
Superbubbles
• Occur in OB associations from corecollapse supernovae- at least five or six SN
• Typical lifetimes on the order of 5×107 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 2×10-3cm-3 (Local Hot
Bubble) and 2-5×10-2cm-3 for Loop 1)
Evolution
• First defined in 1979 (Super Shells)
• Very large shell structures in the ISMdefined 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
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”
100s of
parsecs
“Chimney Effect”
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.
How we can see Superbubbles
• Holes in HI, shells of HII (fainter as you go
outward)
• Purple is Hα, Cyan is OIII. (N44, LMC)
250 ly
How we can see Superbubbles
• Charting the
absorption
components of ISM.
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
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)
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.
Implications
• Explains the Soft X-Ray background: We’re
inside a superbubble with its million-degree gas.
• May reconcile the massive-star origin of Gamma
Ray Bursters by providing an extremely lowdensity 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
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 bubblessix in the LB (~12 Myr ago)
• OB associations only exist
in Loop 1 now; the LB will
be squeezed out of
existence soon.
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 they’re believed
to be separate
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
SN1987a’s 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.
Galactic Cosmic Rays
• Supernovae don’t accelerate their own
ejecta into GCRs.
• Superbubbles carry these heavy elements
from Wolf-Rayet stars and other massive
SNe outward, mixing with solarcomposition material until “accelerated by
subsequent SN shocks within the
superbubble to provide the bulk of the
GCRs” (Binns et al 2005).
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 harderthey’re 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 won’t add up to
anything.
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?
Works Cited
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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, A&A, 436, 615
Walsh, B.Y. et al. “NaI and CaII absorption components observed towards the OrionEridanus Superbubble” 2005 A&A 440, 547
Zaninetti, L. “On the Shape of Superbubbles Evolving in the Galactic Plane” 2004
PASJ 56, 1067