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

0s o

f pa

rsec

s

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