Contributions of massive stars and classical novae to Galactic Al 26

The radioactive isotope is believed to be mainly produced by massive stars and ejected into the interstellar medium through their winds and supernovae. Due to its lifetime of 1.04,Myr, it traces recent nucleosynthesis and star formation. The isotope Al 26 Fe 60 is similarly long-lived, with a lifetime of 3.8,Myr, and originates only in core-collapse supernovae (ccSNe). Stellar evolution models in combination with Galactic chemical evolution (GCE) models tend to over-predict the nuc Fe 60 / ratio. Population synthesis models that attempt to explain both the mass in the entire Milky Way and its spatial distribution through comparisons with the 1.8,MeV decay γ-ray line match the observational constraints only with an enhanced star formation rate (SFR) of ≳ 5, . Classical novae may contribute significantly to the production, potentially lowering the SFR inferred from massive stars, although previous studies suggest that their contribution is negligible or łeq,10%. However, a large nova contribution would also alleviate some tension in the Al 26 Al 26 -1 Al 26 Fe 60 / ratio. Al 26 In this work we aim to determine the mass distribution of as a function of the Galactocentric radius and estimate the contribution of (very) massive stars and classical novae to the 1.8 ,MeV signal. Al 26 We used 20,yr of INTEGRAL/SPI observations of the γ-ray line at 1.8, and compare the resulting mass distribution with different GCE models. to test different assumptions about the radial structure of Al 26 In our baseline analysis, all tested source models favour substantial nova contributions, with best-fit fractions of 77--84%. Because the nova and massive-star amplitudes are anti-correlated, the full two-parameter 2σ confidence regions permit substantially smaller nova fractions. The preferred large nova contribution alleviates tension in the / ratio and lowers the SFR from γ-ray measurements to 1--2, . Fe 60 Al 26 Al 26 -1 Our baseline fits favour a substantial contribution from classical novae, but the quantitative lower bound depends strongly on the adopted source profile and on the treatment of the Solar neighbourhood. Under some tested assumptions, the data allow little or no nova contribution at the 2σ level. Although it is difficult to gauge both contributions directly, next generation MeV telescopes may be able to identify individual massive stars, such as γ^2-Velorum.