Superbubble

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A superbubble is an astronomical term used to describe a cavity hundreds of light years across filled with 106 K gas blown into the interstellar medium by multiple supernovae and stellar winds. The Sun lies near the center of an old superbubble, whose boundaries can be traced by a sudden rise in dust extinction of stars at distances greater than a few hundred light years.

The most massive stars, with masses ranging from eight to roughly 100 solar masses and spectral types of O and early B are usually found in groups called OB associations. Massive O stars have strong stellar winds, and all of these stars explode as supernovae at the ends of their lives.

The strongest stellar winds release kinetic energy of 1051 ergs (1044 J), equivalent to a supernova explosion. These winds can blow stellar wind bubbles dozens of light years across.[1] Supernova explosions, similarly, drive blast waves that can reach even larger sizes, with expansion velocities of as much as several hundred km s-1.

Stars in OB associations are not gravitationally bound, but only drift apart at speeds of around 20 km s-1. As a result, most of their supernova explosions occur within the cavity carved by the strongest stellar wind bubbles.[2][3] They never form a visible supernova remnant, but instead efficiently deposit their energy into the hot interior as sound waves. Large enough superbubbles can blow entirely through the galactic disk, releasing their energy into the surrounding galactic halo or even into the intergalactic medium.[4][5]

The interstellar gas swept up by superbubbles generally cools, forming a dense shell around the cavity. These shells were first observed in line emission at 21 cm from hydrogen,[6] leading to the formulation of the theory of superbubble formation. They are also observed in X-ray emission from their hot interiors, in optical line emission from their ionized shells, and in infrared continuum emission from dust swept up in their shells. X-ray and optical emission are typically observed from younger superbubbles, while older, larger objects seen in 21 cm may even result from multiple superbubbles combining, and so are sometimes distinguished by calling them supershells.

[edit] References

  1. ^ Castor, McCray, & Weaver 1975 Castor, J., McCray, R., & Weaver, R. (1975). "Interstellar Bubbles". Astrophys. J. (Letters) 200: L107-L110.
  2. ^ Tomisaka, K., Habe, A., & Ikeuchi, S. (1981). "Sequential explosions of supernovae in an OB association and formation of a superbubble". Astrophys. Space Sci. 78: 273-285.
  3. ^ McCray, R., Kafatos, M. (1987). "Supershells and Propagating Star Formation". Astrophys. J. 317: 190-196.
  4. ^ Tomisaka, K., Ikeuchi, S. (1986). "Evolution of superbubble driven by sequential supernova explosions in a plane-stratified gas distribution". Publ. Astron. Soc. Japan 38: 697-715.
  5. ^ Mac Low, M.-M., McCray, R. (1988). "Superbubbles in Disk Galaxies". Astrophys. J. 324: 776-785.
  6. ^ Heiles, C. (1979). "H I shells and supershells". Astrophys. J. 229: 533-544.
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