Particle acceleration in star clusters
● Young star clusters and associations are potential sites of cosmic
ray acceleration
● eg. Westerlund 2 (Yang+ 2017), NGC 3603 (Yang & Aharonian
2016), Cygnus OB2 (Ackermann+ 2011)
Yang & Aharonian (2016)
● Collective effect of
multiple shocks
from supernovae
and the winds of
massive stars
provide sufficient
energy for cosmic
ray acceleration.
● γ-ray production
due to accelerated
cosmic rays
interacting with
surrounding
interstellar medium
(hadronic
processes?)H.E.S.S. Collaboration (2018)
Emission processes
Why do we care?
● CRs are an important constituent of the ISM (energy density ~
1eV/cm³ – slightly larger than starlight, CMB, or Galactic B field)
● Crucial role for ISM physics and chemistry because they
dominate the ionisation of dense ISM clouds where the gas is
shieded from the UV radiation field
● Ions couple the gas to magnetic field → changes the dynamics
of the cloud, protoplanetary discs, complex molecule creation
● CRs detected indirectly through radiation processes
● Star clusters and associations are particle acceleration sites →
60Fe abundance in CRs, correlation with Galactic diffuse γ-ray
emission
Ackermann+ (2011)
Cygnus OB2 association