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Massive stars as major factories of Galactic cosmic rays

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Abstract

The identification of the main contributors to the locally observed fluxes of cosmic rays is a prime objective in the resolution of the long-standing enigma of the source of cosmic rays. We report on a compelling similarity of the energy and radial distributions of multi-TeV cosmic rays extracted from observations of very-high-energy γ-rays towards the Galactic Centre and two prominent clusters of young massive stars, Cygnus OB2 and Westerlund 1. We interpret this resemblance as evidence that cosmic rays responsible for the diffuse very-high-energy γ-ray emission from the Galactic Centre are accelerated by the ultracompact stellar clusters located in the heart of the Galactic Centre. The derived 1/r decrement of the cosmic ray density with the distance from a star cluster is a distinct signature of continuous cosmic ray injection into the interstellar medium over a few million years. The lack of brightening of the γ-ray images towards the stellar clusters excludes the leptonic origin of γ-ray radiation. The hard, E−2.3-type, power-law energy spectra of parent protons continues up to ~1 PeV. The efficiency of conversion of the kinetic energy of stellar winds to cosmic rays can be as high as 10%, implying that young massive stars may operate as proton PeVatrons with a dominant contribution to the flux of the highest-energy Galactic cosmic rays.

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Fig. 1: γ-Ray luminosities and the radial distributions of CR protons in extended regions around the star clusters Cygnus OB2 (Cygnus Cocoon) and Westerlund 1 (Wd 1 Cocoon), as well as in the CMZ of the Galactic Centre assuming that the CMZ is powered by CRs accelerated in the Arches, Quintuplet and Nuclear clusters.

Data availability

This paper makes use of Fermi LAT data, which can be downloaded from the Fermi LAT data server (https://fermi.gsfc.nasa.gov/ssc/data/access/), the H.E.S.S results used in this paper can be obtained from https://www.mpi-hd.mpg.de/hfm/HESS/pages/publications/auxiliary/AA537_A114.html for Westerlund 1 and https://www.mpi-hd.mpg.de/hfm/HESS/pages/publications/auxiliary/auxinfo_GalacticCenter.html for CMZ. The CO data used can be downloaded from the Radio Telescope Data Center (https://www.cfa.harvard.edu/rtdc/CO/). The HI data can be downloaded from http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/440/775. The Planck dust opacity map used can be downloaded from the Planck Legacy Archive (https://pla.esac.esa.int/#maps).

Change history

  • 24 April 2019

    In the version of this Article originally published, the following ‘Journal peer review information’ was missing: “Nature Astronomy thanks Don Ellison, Giovanni Morlino and the other anonymous reviewer(s) for their contribution to the peer review of this work.” This statement has now been added.

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Authors

Contributions

R.Y. and E.d.O.W. performed the data analysis and helped with writing the manuscript. F.A. was responsible for the interpretation of the data and led the writing of the manuscript.

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Correspondence to Ruizhi Yang.

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The authors declare no competing interests.

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Journal peer review information: Nature Astronomy thanks Don Ellison, Giovanni Morlino and the other anonymous reviewer(s) for their contribution to the peer review of this work.

Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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Supplementary Information

Supplementary text, Supplementary Figures 1–11, Supplementary Tables 1–2, Supplementary references.

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Aharonian, F., Yang, R. & de Oña Wilhelmi, E. Massive stars as major factories of Galactic cosmic rays. Nat Astron 3, 561–567 (2019). https://doi.org/10.1038/s41550-019-0724-0

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