Tracing the Milky Way spiral arms with ²⁶Al

The role of nova systems in the 2D distribution of ²⁶Al

¹ Dipartimento di Fisica, Sezione di Astronomia, Università di Trieste, Via G. B. Tiepolo 11, 34143 Trieste, Italy
² I.N.A.F. Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, 34131 Trieste, Italy
³ I.N.F.N. Sezione di Trieste, via Valerio 2, 34134 Trieste, Italy
⁴ INAF – Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
⁵ ICRANet, Piazza della Repubblica 10, 65122 Pescara, Italy

^★ Corresponding author; arianna.vasini@inaf.it

Received: 23 July 2024
Accepted: 11 November 2024

Abstract

Context. Massive stars are one of the most important and investigated astrophysical production sites of ²⁶Al, a short-lived radioisotope with an ~1 Myr half-life. Its short lifetime prevents us from observing its complete chemical history, and only the ²⁶Al 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 ²⁶Al come from nova systems that pollute the interstellar medium with a large delay, thus partly erasing the correlation between ²⁶Al and SFR.

Aims. In this work, we aim to describe the 2D distribution of the mass of ²⁶Al as well as that of massive stars and nova systems in the Milky Way (MW), to investigate their relative contributions to the production of ²⁶Al.

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 ²⁶Al comes from massive stars and novae, and one with massive stars only. We then compared the predictions to the ~2 M_⊙ of ²⁶Al 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 ²⁶Al mass consists in damping the spiral pattern by a factor of five. Regarding the nucleosynthesis, we find that ~75% of the ²⁶Al is produced by novae and the ~25% by massive stars.

Conclusions. We conclude that novae cannot be neglected as ²⁶Al 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 ²⁶Al.