Superbubbles: Much ado about nearly nothing

1. Superbubbles: Much ado about nearly nothing By Adric Riedel

2. 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.

3. 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

4. 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

5. 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.

6. 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)

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

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

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

11. 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)

14. 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!

15. 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

16. 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

17. 4. Supergiant Shells • Formed from starbursts – larger than OB associations; 1054 ergs • Largest formation • Badly understood

18. 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.

19. Presentation Feature

20. 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)

21. 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

23. 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”

24. “Chimney Effect” 100s of parsecs

25. 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.

26. 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

27. How we can see Superbubbles • Charting the absorption components of ISM.

30. 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

32. 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)

35. 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.

36. 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

37. 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.

38. 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

39. 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.

40. 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).

41. 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.

43. 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?

44. 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” 2004PASJ 56, 1067