1. Max-Planck-Institut für extraterrestrische Physik, D-85748 Garching, Germany
2. Centre d'Etude Spatiale des Rayonnements and Université Paul Sabatier, 31028 Toulouse, France
3. DSM/DAPNIA/Service d'Astrophysique, CEA Saclay, 91191 Gif-Sur-Yvette, France
4. Clemson University, Clemson, South Carolina 29634-0978, USA
5. ESA/ESTEC, SCI-SD 2201 AZ Noordwijk, The Netherlands
6. Space Sciences Laboratory, Berkeley, California 94720, USA

Correspondence to: Roland Diehl¹ Correspondence and requests for materials should be addressed to R.D. (Email: rod@mpe.mpg.de).

Abstract

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 -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 disk^{3, 4, 5}. Furthermore, prominent -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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