Abstract
Extended gamma -ray emissions in the vicinity of young star clusters are believed to be produced by the interaction of CRs accelerated therein with the ambient gas. The detailed spatial analysis reveals 1/r type distribution of CRs, which indicates a continuous injection of CRs in these objects. The hard, propto E^{-2.3} type power-law energy spectra of parent protons continue up to sim 1 PeV. The efficiency of conversion of kinetic energy of powerful stellar winds to CRs can be as high as 10%. This implies that the young massive stars can operate as effective proton PeVatrons with a major or even dominant contribution to the flux of highest-energy galactic CRs.
Highlights
The origin of cosmic rays (CRs) remains as a mystery after these relativistic particles were discovered for more than one century
We argue that the compact stellar clusters, Arches, Quintuplet and Nuclear in Galactic Centre (GC), could be alternative sites for the CR acceleration
The power of a single O star is of the order 1036 − −1037 erg/s, depending on the mass; while, the power of WR star can be as high as 6 × 1037 erg/s (Cesarsky and Montmerle 1983; Parizot et al 2004)
Summary
The origin of cosmic rays (CRs) remains as a mystery after these relativistic particles were discovered for more than one century. The spatial distribution of CRs is a powerful tool to diagnose the injection history and acceleration site of CRs (Aharonian and Atoyan 1996; HESS Collaboration 2016). The extended GeV -ray source around the cluster NGC 3603 (Yang and Aharonian 2017) and the TeV -ray source associated with 30 Dor C (H.E.S.S. Collaboration 2015) are too weak for derivation of statistically significant radial distributions of CRs. On the other hand, the angular size of the diffuse GeV source associated with Westerlund 2 is too large to be detected with the current atmospheric Cherenkov telescopes. In the case of Cygnus Cocoon discovered by the Fermi LAT collaboration as a bright extended -ray source associated with Cyg OB2 (Ackermann et al 2011), the photon statistics is sufficient for derivation of spectral and spatial distributions of CRs. It is important that -ray emission of this source extends to TeV energies (ARGO-YBJ Collaboration 2014). Because of the increase of the 0-meson production cross-section with energy of incident protons and nuclei, the spectrum of secondary -rays appears slightly harder compared to the spectrum of parent protons, ≈ p − 0.1 (Kelner et al 2006), the power-law index of the proton distribution should be p ≈ 2.3
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