By Adric Riedel

Superbubbles: Much ado about nearly nothing

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

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

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)

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 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

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 core-collapse 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 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

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”

“Chimney Effect”10

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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.

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 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

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.

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 solar-composition 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 harder-

they’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• 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 Orion-

Eridanus Superbubble” 2005 A&A 440, 547• Zaninetti, L. “On the Shape of Superbubbles Evolving in the Galactic Plane” 2004

PASJ 56, 1067