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OB Associates & The Supernova Remnant G65.2+5.7 In The Cygnus Superbubble

28/11/2011

The Cygnus superbubble is an area of space which presides over one of the most intense ISM enrichment regions in the sky. A number of interesting objects, such as OB associates (e.g. VI Cyg 8A, VI Cyg 9, and VI Cyg 12; Abbott et al. 1981) and huge Wolf-Rayet stars constantly pumping the local medium from the stripping of their outer layers via enormous stellar winds to supernovae explosions and remnants (e.g. G65.2+5.7, Cygnus Loop and HB21; Uyaniker et al. 2001) streaming huge shock waves into the ISM and the X-ray injections from binary transients (e.g. Cygnus X-6 and Cygnus X-7; Cash et al. 1980), all make it a tumultuous and exciting area of the sky to observe.

It would come as no surprise that the radiation and stellar winds from massive stars and supernova (SN) explosions determine the structure and energy content of the interstellar medium (ISM) (McKee & Ostriker 1977; Ferriere 2001). Hence, from a purely topological view, the ISM is a highly inhomogeneous on the small scale (Dyson 1997), due to the wealth of objects that constantly eject material and energy in a rather chaotic manner. But how would such a super structure form from this perceived chaos?

The most massive stars, with masses ranging from eight to roughly one hundred solar masses, are designated as spectral types O and B. These massive stars have been found to form in close proximity to each other called OB associations (Ambartsumian 1947); or, bigger still in the Wolf-Rayet (WR) category (Wolf & Rayet 1867). These massive stars have strong stellar winds (Castor,  Abbott & Klein 1975), and all of these stars are expected explode as supernovae at the ends of their lives, ejecting their outer hydrogen envelopes into the surrounding regions and thus, enriching the ISM. The strongest stellar winds from massive stars can release kinetic energy of the order of 1051 ergs (~1044 J) over their lifetimes (Howarth & Prinja 1989;  Mokiem et al. 2007; Fehon-Gagné et al. 2011), which is equivalent to a supernova explosion. These winds can form stellar wind bubbles dozens of light years across (Castor et al. 1975).

Fig. 1: Left ~ Finder chart for the Cygnus region in Galactic coordinates. The thick dashed-dotted ellipse shows the location of the CSB and solid ellipses indicate the approximate position and extent of the OB associations. Thick dashed lines show the boundaries of the radio loops and plus signs mark the positions of the stars from the list of Garmany & Stencel (1992) and of candidate stars showing expansion (Comerón et al. 1998). The dotted circles denote the prominent H II regions. Right ~ The Cygnus superbubble as seen in the ROSAT 1.5 keV data. (Both Credit: Uyaniker, B. et al. (2001) The Cygnus Superbubble Revisited. A&A, 371 pp.675-697.)

Stars in OB associations are not gravitationally bound, but they drift apart at small speeds (of around 20 km s-1), and they exhaust their fuel rapidly (after a few millions of years). As a result, most of their supernova explosions occur within the cavity formed by the stellar wind bubbles (Uyanıker et al. 2001). These explosions never form a visible supernova remnant, but instead expend their energy in the hot interior as sound waves. Both stellar winds and stellar explosions thus power the expansion of the superbubble in the interstellar medium. Inside OB associations and massive WR groups, such as the one found at the Cygnus nebula site (Abbott, Bieging & Churchwell 1981), the stars are close enough together that their wind bubbles merge together; forming a giant bubble. At this stage the bubble is starting to inflate as it pushes out into the ISM. What it needs is an extra kick.

The kick is provided via the deaths of our massive stars in the Cygnus OB group. Massive stars die in hugh supernovae core-collapse explosions, ejecting matter out at tremendous energies. This blast wave can cause the bubble reach even larger sizes, with expansion velocities up to several hundred km s-1 (McKee & Cowie 1975; Mac Low & McCray 1988). This theory founded itself on the discovery of the supernovae remnant SNR G65.2+5.7 in the near vicinity.

What is special about this SNR is that it never became part of the Cygnus superbubble, yet providing interesting clues as to the nature of the super-inflation of such structures: they had to come via supernovae explosions. These explosions never formed a visible supernova remnant, other than SNR G65.2+5.7. Instead they expended their energy in the hot interior as sound waves (Bochkarev & Sitnik 1985). Thus, both stellar winds and stellar explosions power the expansion of the superbubble in the ISM from humbler beginnings.

So what is the eventual fate of such super-inflated cosmic structures? Large enough superbubbles can blow through the entire galactic disk (Tomisaka &Ikeuchi 1986; Mac Low & McCray 1988), releasing their energy into the surrounding galactic halo or even into the intergalactic medium (IGM). It is with this that the sheer scale, power and strength of these huge objects becomes apparent.

Journal References:

  • Cash, W. et al. (1980) The X-ray Superbubble In Cygnus. The Astrophysical Journal: Letters, 238 (2): pp. L71-L76.
  • Abbott, D. C.; Bieging, J. H. & Churchwell, E. (1981) Mass Loss From Very Luminous OB Stars & The Cygnus Superbubble. The Astrophysical Journal, 250 (1): pp. 645-659.
  • Bochkarev, N. G. & Sitnik, T. G. (1985) On The Structure & Origin Of The Cygnus SuperbubbleAstrophysics & Space Science, 108 (2): pp. 237-302.
  • Uyanıker, B. et al. (2001) The Cygnus Superbubble Revisited. Astronomy and Astrophysics, 371 pp.675-697.
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