Tracing the Milky Way spiral arms with ²⁶Al

Context. Massive stars are one of the most important and investigated astrophysical production sites of 26 Al, a short-lived radioisotope with an ~1 Myr half-life. Its short lifetime prevents us from observing its complete chemical history, and only the 26 Al that was recently produced by massive stars can be observed. Hence, it is considered a tracer of star formation rate (SFR). However, important contributions to 26 Al come from nova systems that pollute the interstellar medium with a large delay, thus partly erasing the correlation between 26 Al and SFR. Aims. In this work, we aim to describe the 2D distribution of the mass of 26 Al as well as that of massive stars and nova systems in the Milky Way (MW), to investigate their relative contributions to the production of 26 Al. Methods. We used a detailed 2D chemical evolution model where the SFR is azimuthally dependent and is required to reproduce the spiral arm pattern observed in the MW. We tested two different models, one where the 26 Al comes from massive stars and novae, and one with massive stars only. We then compared the predictions to the ~2 M ⊙ of 26 Al mass observed by the surveys of the Compton Telescope (COMPTEL) and International Gamma-Ray Laboratory (INTEGRAI). Results. The results show that novae do not trace SFR and, in the solar vicinity, they concentrate in its minima. The effect of novae on the map of the 26 Al mass consists in damping the spiral pattern by a factor of five. Regarding the nucleosynthesis, we find that ~75% of the 26 Al is produced by novae and the ~25% by massive stars. Conclusions. We conclude that novae cannot be neglected as 26 Al producers since the observations can only be reproduced by including their contribution. Moreover, we suggest that bulge novae should eject around six times more material than the disc ones to well reproduce the observed mass of 26 Al.