Radioactive 26Al from massive stars in the Galaxy

Gamma-rays from radioactive Al-26 ( half-life similar to 7.2 x 10(5) years) provide a 'snapshot' view of continuing nucleosynthesis in the Galaxy(1). The Galaxy is relatively transparent to such gamma-rays, and emission has been found concentrated along its plane(2). This led to the conclusion(1) that massive stars throughout the Galaxy dominate the production of Al-26. On the other hand, meteoritic data show evidence for locally produced Al-26, perhaps from spallation reactions in the protosolar disk(3-5). Furthermore, prominent gamma-ray emission from the Cygnus region suggests that a substantial fraction of Galactic Al-26 could originate in localized star-forming regions. Here we report high spectral resolution measurements of Al-26 emission at 1808.65 keV, which demonstrate that the Al-26 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 Al-26. 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.