The origin of cosmic rays is a long-standing mystery in Astrophysics especially concerning the highest energy particles in the spectrum. In recent years high energy particles have been identified from very-high-energy γ-ray observations of young massive star clusters such as for example Westerlund 1 & 2, Cygnus OB2, and 30 Dor in the LMC. The underlying origin of this emission is still poorly constrained, especially for what concerns the relation between the high-energy emission and the physical properties of the stars. This is mostly due to the difficulties to estimate the wind luminosity, as in most of the stellar systems the stellar population is not well known. Meanwhile, this is a fundamental ingredient to quantify the acceleration power of this source class and make predictions about their γ-ray visibility, and to understand whether they can significantly contribute to the Galactic cosmic rays. At the same time, this is an important piece of information to recognize the origin of the emission within these stellar environments.

The strong winds of hot massive stars may act individually and collectively to drive termination shocks at the edges of their parental star clusters, which can (re-)accelerate particles. The majority of these stars are expected to be in binary systems, such as colliding wind binaries, X-ray/γ-ray binaries and microquasars, all of which can produce γ-ray emission. As massive star clusters age, the stellar evolution of their population can also affect the high-energy output through powerful Wolf-Rayet winds and supernova explosions. It is clear that our interpretation of the observed emission heavily depends on the assumptions made about the stellar population and how it changes with time.

Programme

In this special session, we propose to bring the massive star and high energy communities together to address the key stellar physics uncertainties in understanding the origin of ?-ray emission from stellar clusters. This full-day session will consist of three blocks on different themes relating to massive star clusters and their high energy emission:

• Connecting individual and collective stellar winds
• Binary systems as cosmic ray accelerators
• Connecting stellar populations to gamma ray emission from clusters

Invited speakers

• Lucia Härer
• Jonathan Mackey
• Stefano Menchiari
• Samar Safi-Harb
• Andreas Sander
• Stefanie Walch-Gassner

Scientific organisers

Listed in alphabetical order:
Valenti Bosch-Ramon, Universitat de Barcelona
Paul Crowther, University of Sheffield
Cormac Larkin, MPIK & ARI (co-chair, contact person)
Jonathan Mackey, DIAS
Emma de Oña Wilhelmi, DESY-Zeuthen (co-chair)
Lida Oskinova, Potsdam
Giada Peron, Florence (co-chair)
Lieke van Son, Flatiron Institute
Jorick Vink, Armagh (co-chair)

Contact

cormac.larkin @ mpi-hd.mpg.de