University of Hertfordshire

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Steady-state nucleosynthesis throughout the Galaxy

Research output: Contribution to journalArticlepeer-review

  • Roland Diehl
  • Martin G. H. Krause
  • Karsten Kretschmer
  • Michael Lang
  • Moritz M. M. Pleintinger
  • Thomas Siegert
  • Wei Wang
  • Laurent Bouchet
  • Pierrick Martin
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Original languageEnglish
Article number101608
Number of pages14
JournalNew Astronomy Reviews
Early online date24 Dec 2020
Publication statusPublished - 1 Jun 2021


The measurement and astrophysical interpretation of characteristic γ-ray lines from nucleosynthesis was one of the prominent science goals of the INTEGRAL mission and in particular its spectrometer SPI. Emission from 26Al and from 60Fe decay lines, due to their My decay times, originates from accumulated ejecta of nucleosynthesis sources, and appears diffuse in nature. 26Al and 60Fe are believed to originate mostly from massive star clusters. The radioactive decay γ-ray observations open an interesting window to trace the fate and flow of nucleosynthesis ejecta, after they have left the immediate sources and their birth sites, and on their path to mix with ambient interstellar gas. The 26Al emission image obtained with INTEGRAL confirms earlier findings of clumpiness and an extent along the entire plane of the Galaxy, supporting its origin from massive-star groups. INTEGRAL spectroscopy resolved the line and found Doppler broadenings and systematic shifts with longitude, originating from large-scale galactic rotation. But an excess velocity of 200 km s −1 suggests that 26Al decays preferentially within large superbubbles that extend in forward directions between spiral arms. The detection of 26Al line emission from the nearby Orion clusters in the Eridanus superbubble supports this interpretation. Positrons from β + decays of 26Al and other nucleosynthesis ejecta have been found to not explain the morphology of positron annihilation γ-rays at 511 keV that have been measured by INTEGRAL. The 60Fe signal measured by INTEGRAL is diffuse but too weak for an imaging interpretation, an origin from point-like/concentrated sources is excluded. The 26Al / 60Fe ratio is constrained to a range 0.2–0.4. Beyond improving precision of these results, diffuse nucleosynthesis contributions from novae (through 22Na radioactivity) and from past neutron star mergers in our Galaxy (from r-process radioactivity) are exciting new prospects for the remaining mission extensions.


Funding Information: We are grateful for constructive comments of the referee, which helped the quality of this review paper. The INTEGRAL/SPI project has been completed under the responsibility and leadership of CNES; we are grateful to ASI, CEA, CNES, DLR (No. 50 OG 1101, 1601, and 1901, and No. 50 OR 0901, 1206, 1611, and 1316), ESA, INTA, NASA and OSTC for support of this ESA space science mission. This research was also supported by the DFG cluster of excellence Origin and Structure of the Universe, Germany , and by the ChETEC Action (CA16117) of COST (European Cooperation in Science and Technology) . Publisher Copyright: © 2020 Elsevier B.V. Copyright: Copyright 2020 Elsevier B.V., All rights reserved.

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