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Radioactive 26Al from massive stars in the Galaxy


Gamma-rays from radioactive 26Al (half-life 7.2 × 105 years) provide a ‘snapshot’ view of continuing nucleosynthesis in the Galaxy1. The Galaxy is relatively transparent to such γ-rays, and emission has been found concentrated along its plane2. This led to the conclusion1 that massive stars throughout the Galaxy dominate the production of 26Al. On the other hand, meteoritic data show evidence for locally produced 26Al, perhaps from spallation reactions in the protosolar disk3,4,5. Furthermore, prominent γ-ray emission from the Cygnus region suggests that a substantial fraction of Galactic 26Al could originate in localized star-forming regions. Here we report high spectral resolution measurements of 26Al emission at 1808.65 keV, which demonstrate that the 26Al source regions corotate with the Galaxy, supporting its Galaxy-wide origin. We determine a present-day equilibrium mass of 2.8 (± 0.8) solar masses of 26Al. We use this to determine that the frequency of core collapse (that is, type Ib/c and type II) supernovae is 1.9 (± 1.1) events per century.

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Figure 1: Measurement of the 26 Al line from the inner Galaxy region with SPI/INTEGRAL.
Figure 2: Line position shifts with viewing directions along the inner Galaxy.

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This paper is based on observations with INTEGRAL, an ESA project with instruments and a science data center funded by ESA member states (especially the PI countries: Denmark, France, Germany, Italy, Switzerland, Spain), Czech Republic and Poland, and with the participation of Russia and the USA. The SPI project has been completed under the responsibility and leadership of CNES/France. The SPI anticoincidence system is supported by the German government. We are grateful to ASI, CEA, CNES, DLR, ESA, INTA, NASA and OSTC for support. We are grateful to Alessandro Chieffi, Nikos Prantzos, and Stan Woosley for discussions of theoretical nucleosynthesis yields.

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Correspondence to Roland Diehl.

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Supplementary information

Supplementary Methods

Observations and Data. The INTEGRAL Observatory and its spectrometer instrument have been launched in Oct 2002. Observations are composed of 7130 pointings along the plane of the Galaxy, and sum up to an exposure of 4 Ms at the Galactic Center, from the first two years of the mission. (PDF 158 kb)

Supplementary Methods

Data Analysis and Results. Spectra are determined from independent model fits in 0.5 keV bins. Splitting the sky model into longitude segments allows for spatially-resolved spectroscopy, and obtains Doppler shifts as expected from Galactic rotation. The variability of resulting spectra with different models for the spatial distribution of 26Al emission is modest to small. (PDF 211 kb)

Supplementary Discussion

Doppler Broadening. The width of the observed gamma-ray line depends on the state of the ISM. (PDF 33 kb)

Supplementary Discussion

Galactic Rotation. Different models for the spatial distribution of 26Al emission and rotation curves for the inner Galaxy lead to variations in expected line shifts. (PDF 65 kb)

Supplementary Discussion

Nucleosynthesis Yields. Different models for stellar evolution and supernovae predict somewhat different yields of 26Al. From current models, an assessment is made over the full range of massive stars. (PDF 81 kb)

Supplementary Methods

Deriving a Galactic Star Formation Rate from 26Al Gamma-rays. The determination of the supernova rate follows from the nucleosynthesis yield and its integration over the mass distribution of stars. The conversion to a star formation rate is described. (PDF 91 kb)

Supplementary Discussion

Star Formation Rate (SFR) and Supernova Rate (SNR) Estimates for the Galaxy. The different approaches determining supernova rates or star formation rates for the Galaxy are presented in a Table, with discussion of strengths and weaknesses. The 26Al-based approach is completely independent, and among the less-biased and more accurate methods. (PDF 58 kb)

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Diehl, R., Halloin, H., Kretschmer, K. et al. Radioactive 26Al from massive stars in the Galaxy. Nature 439, 45–47 (2006).

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