Massive Stars as Major Factories of Galactic Cosmic Rays

TL;DR

This paper suggests that young massive star clusters, especially in the Galactic Center, are major sources of galactic cosmic rays, accelerating protons up to PeV energies and significantly contributing to high-energy cosmic ray fluxes.

Contribution

It provides evidence that stellar clusters are continuous sources of cosmic rays with a $1/r$ density profile and identifies young massive stars as potential proton PeVatrons.

Findings

CR energy and radial distributions resemble those from stellar clusters.

CR density decreases as 1/r from clusters, indicating continuous injection.

Protons are accelerated up to ~1 PeV, with high efficiency of energy conversion.

Abstract

The identification of major contributors to the locally observed fluxes of Cosmic Rays (CRs) is a prime objective towards the resolution of the long-standing enigma of CRs. We report on a compelling similarity of the energy and radial distributions of multi-TeV CRs extracted from observations of very high energy (VHE) $\gamma$-rays towards the Galactic Center (GC) and two prominent clusters of young massive stars, Cyg~OB2 and Westerlund~1. This resemblance we interpret as a hint that CRs responsible for the diffuse VHE $\gamma$-ray emission from the GC are accelerated by the ultracompact stellar clusters located in the heart of GC. The derived $1/r$ decrement of the CR density with the distance from a star cluster is a distinct signature of continuous, over a few million years, CR injection into the interstellar medium. The lack of brightening of the $\gamma$-ray images toward the stellar clusters excludes the leptonic origin of $\gamma$-radiation. The hard, $\propto E^{-2.3}$ type power-law energy spectra of parent protons continues up to $\sim$ 1 PeV. The efficiency of conversion of kinetic energy of stellar winds to CRs can be as high as 10 percent implying that the young massive stars may operate as proton PeVatrons with a dominant contribution to the flux of highest energy galactic CRs.