Implications of 1.8 MeV gamma-ray observations for the origin of Al-26

Recent analysis of COMPTEL data has revealed an extremely close correlation between 53 GHz microwave free-free and 1.8 MeV gamma-ray line emission. Wh ile microwave free-free emission arises from the ionized interstellar mediu m, 1.8 MeV; gamma rays are emitted during the radioactive decay of Al-26. W e argue that the close correlation can only be understood if massive stars (M greater than or similar to 20 M-.) are at the origin of Galactic Al-26. Based on the measured proportionality factor, we estimate the Al-26 yield o f an "equivalent O7 V star" to be (1.0 +/- 0.5) x 10(-4) M,. Using an estim ate for the total Galactic Lyman continuum luminosity of Q = 3.5 x 10(53) p hotons s(-1), we derive the Galactic Al-26 mass to be 3.1 +/- 0.9 M,. The m ass estimate is compared to theoretical nucleosynthesis predictions for Al- 26 from core-collapse supernovae and Wolf-Rayet stars. We circumvent the pr oblem of using a weakly constrained star formation rate for this comparison by determining the star formation rate self-consistently from our models, using the Galactic Lyman continuum luminosity. The effects of mass loss and metallicity are considered, and the uncertainties of predicted Al-26 produ ction rates due to poorly known initial mass limits for the candidate sourc es are discussed. Assuming solar metallicity throughout the entire Galaxy, we predict a Galactic Al-26 mass Of 1.6 +/- 0.3 M-., Of which similar to 60 % is produced by core-collapse supernovae, while similar to 40% originates from Wolf-Rayet stars. Taking the Galactic metallicity gradient into accoun t increases the Galactic Al-26 mass to 2.2 +/- 0.4 M-., consistent with the observed value. The increase mainly arises from enhanced production by Wol f-Rayet stars in the metal-rich inner Galaxy; these contribute similar to 6 0% of the Galactic Al-26 budget. We predict that the metallicity gradient s hould produce an inner-to-outer Galaxy intensity contrast of similar to 30% between 1.8 MeV and Galactic free-free emission, which should be observabl e by the future gamma-ray spectrometer SPI on the International Gamma-Ray A strophysics Laboratory (INTEGRAL).