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Massive stars as major factories of Galactic cosmic rays

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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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).

Additional information

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.

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.

References

  1. 1.

    Drury, L. O. Origin of cosmic rays. Astropart. Phys. 39-40, 52–60 (2012).

  2. 2.

    Blasi, P. The origin of Galactic cosmic rays. Astron. Astrophys. Rev. 21, 70 (2013).

  3. 3.

    Bell, A., Schure, K., Reville, B. & Giacinti, G. Cosmic ray acceleration and escape from supernova remnants. Mon. Not. R. Astron. Soc. 431, 415 (2013).

  4. 4.

    Cardillo, M., Amato, E. & Blasi, P. On the cosmic ray spectrum from type II supernovae expanding in their red giant presupernova wind. Astropart. Phys. 69, 1 (2015).

  5. 5.

    Zirakashvili, V. N. & Ptuskin, V. S. Type IIn supernovae as sources of high energy astrophysical neutrinos. Astropart. Phys. 78, 28 (2016).

  6. 6.

    Borkowski, K. J. et al. Radioactive scandium in the youngest galactic supernova remnant G1.9 + 0.3. Astrophys. J. Lett. 724, L161 (2010).

  7. 7.

    Aharonian, F., Sun, X.-n. & Yang, R.-z. Energy distribution of relativistic electrons in the young supernova remnant G1.9 + 0.3. Astron. Astrophys. 603, A7 (2017).

  8. 8.

    Cristofari, P., Gabici, S., Terrier, R. & Humensky, T. B. On the search for Galactic supernova remnant PeVatrons with current TeV instruments. Mon. Not. R. Astron. Soc. 479, 3415 (2018).

  9. 9.

    H.E.S.S. Collaboration TeV γ-ray observations of the young synchrotron-dominated SNRs G1.9 + 0.3 and G330.2 + 1.0 with H.E.S.S. Mon. Not. R. Astron. Soc. 441, 790 (2014).

  10. 10.

    Kafexhiu, E., Aharonian, F., Taylor, A. M. & Vila, G. S. Parametrization of gamma-ray production cross-sections for pp interactions in a broad proton energy range from the kinematic threshold to PeV energies. Phys. Rev. D 90, 123014 (2014).

  11. 11.

    Casse, M. & Paul, J. A. Local gamma rays and cosmic-ray acceleration by supersonic stellar winds. Astrophys. J. 237, 236–243 (1980).

  12. 12.

    Cesarsky, C. J. & Montmerle, T. Gamma rays from active regions in the galaxy—the possible contribution of stellar winds. Space Sci. Rev. 36, 173–193 (1983).

  13. 13.

    Bykov, A. M. Nonthermal particles and photons in starburst regions and superbubbles. Astron. Astrophys. Rev. 22, 77 (2013).

  14. 14.

    Parizot, E., Marcowith, A., van der Swaluw, E., Bykov, A. M. & Tatischeff, V. Superbubbles and energetic particles in the Galaxy. I. Collective effects of particle acceleration. Astron. Astrophys. 424, 747–760 (2004).

  15. 15.

    Bykov, A. M. & Toptygin, I. N. Interstellar turbulence and the kinetics of cosmic rays. Akad. Nauk SSSR Izvestiia Seriia Fizicheskaia 46, 1659–1662 (1982).

  16. 16.

    Klepach, E. G., Ptuskin, V. S. & Zirakashvili, V. N. Cosmic ray acceleration by multiple spherical shocks. Astropart. Phys. 13, 161–172 (2000).

  17. 17.

    Ackermann, M. et al. A cocoon of freshly accelerated cosmic rays detected by Fermi in the Cygnus superbubble. Science 334, 1103–1107 (2011).

  18. 18.

    Yang, R.-z. & Aharonian, F. Diffuse γ-ray emission near the young massive cluster NGC 3603. Astron. Astrophys. 600, A107 (2017).

  19. 19.

    Yang, R.-z., de Oña Wilhelmi, E. & Aharonian, F. Diffuse gamma-ray emission in the vicinity of young star cluster Westerlund 2. Astron. Astrophys. 611, A77 (2018).

  20. 20.

    Aharonian, F., Buckley, J., Kifune, T. & Sinnis, G. High energy astrophysics with ground-based gamma ray detectors. Rep. Prog. Phys. 71, 096901 (2008).

  21. 21.

    Abramowski, A. et al. Discovery of extended VHE γ-ray emission from the vicinity of the young massive stellar cluster Westerlund 1. Astron. Astrophys. 537, A114 (2012).

  22. 22.

    Abramowski, A. et al. The exceptionally powerful TeV γ-ray emitters in the Large Magellanic Cloud. Science 347, 406–412 (2015).

  23. 23.

    Bartoli, B. et al. Identification of the TeV gamma-ray source ARGO J2031 + 4157 with the Cygnus Cocoon. Astrophys. J. 790, 152 (2014).

  24. 24.

    Aharonian, F. A. & Atoyan, A. M. On the emissivity of π0-decay gamma radiation in the vicinity of accelerators of Galactic cosmic rays. Astron. Astrophys. 309, 917–928 (1996).

  25. 25.

    Abramowski, A. et al. Acceleration of petaelectronvolt protons in the Galactic centre. Nature 531, 476–479 (2016).

  26. 26.

    Kelner, S., Aharonian, F. A. & Bugayov, V. Energy spectra of gamma-rays, electrons and neutrinos produced at proton-proton interactions in the very high energy regime. Phys. Rev. D 74, 034018 (2006).

  27. 27.

    Atoyan, A. M., Aharonian, F. A. & Völk, H. J. Electrons and positrons in the Galactic cosmic rays. Phys. Rev. D 52, 3265–3275 (1995).

  28. 28.

    Strong, A. W., Moskalenko, I. V. & Ptuskin, V. S. Cosmic-ray propagation and interactions in the Galaxy. Annu. Rev. Nuc. Part. Sci. 57, 285–327 (2007).

  29. 29.

    Actis, M. et al. Design concepts for the Cherenkov Telescope Array CTA: an advanced facility for ground-based high-energy gamma-ray astronomy. Exp. Astron. 32, 193 (2011).

  30. 30.

    Bykov, A. M., Ellison, D. C., Gladilin, P. E. & Osipov, S. M. Ultrahard spectra of PeV neutrinos from supernovae in compact star clusters. Mon. Not. R. Astron. Soc. 453, 113–121 (2015).

  31. 31.

    Aguilar, M. et al. Precision measurement of the proton flux in primary cosmic rays from rigidity 1 GV to 1.8 TV with the Alpha Magnetic Spectrometer on the International Space Station. Phys. Rev. Lett. 114, 171103 (2015).

  32. 32.

    Ellison, D. C., Drury, L. O. & Meyer, J.-P. Galactic cosmic rays from supernova remnants. II. Shock acceleration of gas and dust. Astrophys. J. 487, 197 (1997).

  33. 33.

    Binns, W. R. et al. Observation of the 60Fe nucleosynthesis-clock isotope in Galactic cosmic rays. Science 352, 677–680 (2016).

  34. 34.

    The Fermi-LAT Collaboration. Fermi Large Area Telescope third source catalog. Astrophys. J. Suppl. 218, 23 (2015).

  35. 35.

    Abdo, A. A. et al. Spectrum and morphology of the two brightest Milagro sources in the Cygnus region: MGRO J2019 + 37 and MGRO J2031 + 41. Astrophys. J. 753, 159 (2012).

  36. 36.

    Mirzoyan, R. & Mukherjee, R. TeV gamma-ray emission from PSR J2032 + 4127/ MT91 213 at periastron. Astronomer’s Telegram 10971 (2017).

  37. 37.

    Dermer, C. D. Secondary production of neutral pi-mesons and the diffuse Galactic gamma radiation. Astron. Astrophys. 157, 223–229 (1986).

  38. 38.

    Mori, M. Nuclear enhancement factor in calculation of Galactic diffuse gamma-rays: a new estimate with DPMJET-3. Astropart. Phys. 31, 341–343 (2009).

  39. 39.

    Figer, D. F. Young massive clusters. In Proc. IAU Symp.Massive Stars as Cosmic Engines (eds Bresolin, F. et al.) Vol. 250, 247–256 (2008).

  40. 40.

    Muno, M. P. et al. Diffuse, nonthermal X-ray emission from the Galactic star cluster Westerlund 1. Astrophys. J. 650, 203–211 (2006).

  41. 41.

    Hußmann, B. The Quintuplet Cluster—A Young Massive Cluster Study Based on Proper Motion Membership. PhD thesis, Universität Bonn (2014).

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Author information

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.

Competing interests

The authors declare no competing interests.

Correspondence to Ruizhi Yang.

Supplementary information

  1. Supplementary Information

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

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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.