Gamma rays from active regions in the galaxy: The possible contribution of stellar winds

Abstract

Massive stars (> 20 3,/o) release a considerable amount of mechanical energy in the form of strong stellar winds. A fraction of this energy may be transferred to relativistic cosmic rays by diffusive shock acceleration at the wind boundary, and/or in the expanding, turbulent wind itself. Massive stars are most frequently found in OB associations, surrounded by H II regions lying at the edge of dense molecular clouds. The interaction of the freshly accelerated particles with matter gives rise to ~/-ray emission. In this paper, we first briefly review the current knowledge on the energetics of strong stellar winds from O and Wolf-Rayet stars, as well as from T Tauri stars. Taking into account the finite lifetime of these stars, we then proceed to show that stellar winds dominate the energetics of OB associations during the first 4 to 6 million years, after which supernovae take over. In the solar neighborhood, the star formation rate is constant, and a steady-state situation prevails, in which the supernova contribution is found to be dominant. A small, but meaningful fraction of the COS-B 7-ray sources may be fueled by WR and O stellar winds in OB associations, while the power released by T Tauri stars alone is perhaps insufficient to account for the y-ray emission of nearby dark clouds. Finally, we discuss some controversial aspects of the physics of particle acceleration by stellar winds.

Detection of extended very-high-energy γ-ray emission towards the young stellar cluster Westerlund 2

“…The wind-blown bubble around WR 20a, and the blister to the west of it are seen as depressions in the radio continuum map. The blister is indicated by white dots as in Whiteoak & Uchida (1997), and appears to be compatible in direction and location with the center of gravity of HESS J1023-575. to diffusive shock acceleration from supersonic winds in a windblown bubble around WR 20a, or the ensemble of hot and massive OB stars from a superbubble in Westerlund 2, breaking out beyond the edge of a molecular cloud (Tenario-Tagle 1979;Völk & Forman 1982;Cesarsky & Montmerle 1983;Bykov 2001). Accordingly, one has to consider that such acceleration sites will also contribute to the observed flux of cosmic rays in our Galaxy (Cassé & Paul 1980).…”

Observations of the Large Magellanic Cloud withFermi

“…The Large Magellanic Cloud (LMC) is thus an excellent target for studying the link between cosmic-ray acceleration and gamma-ray emission since the galaxy is nearby (D ≈ 50 kpc; Matsunaga et al 2009;Pietrzynski et al 2009), has a large angular extent of ∼8 • , and is seen at a small inclination angle of i ≈ 20 • −35 • (Kim et al 1998; Van der Marel 2006), which avoids source confusion. In addition, the LMC is relatively active, housing many supernova remnants, bubbles and superbubbles, and massive star-forming regions that are all potential sites of cosmicray acceleration (Cesarsky & Montmerle 1983;Biermann 2004;Binns et al 2007). …”

The answer is blowing in the wind

“…But the acceleration mechanism itself can be quite different, depending on whether the remnant is isolated (where Fermi-I acceleration dominates) or is part of an interacting system of several remnants embedded in a turbulent medium such as a stellar association (mainly Fermi-II acceleration) 4,7 . Because the massive stars that lead to supernovae are born together in stellar clusters, roughly 80% of supernova explosions are expected to take place near others within a relatively short period of time 8,9 -much like a closing fireworks display -creating a large 'superbubble' filled with hot, tenuous, turbulent plasma [10][11][12][13] .…”

“…Because the massive stars that lead to supernovae are born together in stellar clusters, roughly 80% of supernova explosions are expected to take place near others within a relatively short period of time 8,9 -much like a closing fireworks display -creating a large 'superbubble' filled with hot, tenuous, turbulent plasma [10][11][12][13] . Cosmic rays, it has been argued 4,[7][8][9] , are much more likely to be accelerated in such superbubbles predominantly via the Fermi-II process, rather than -or, possibly, in addition toin isolated supernova remnants where the Fermi-I mechanism holds sway. The HESS findings 1 , which seem to confirm the viability of turbulent, Fermi-II acceleration, whether by massive stars alone or by supernova remnants in concert with massive stars, will be welcome news to the proponents of the superbubble mechanism of cosmic-ray acceleration.…”