Steady-state nucleosynthesis throughout the Galaxy

Abstract

The measurement and astrophysical interpretation of characteristic $\gamma$-ray lines from nucleosynthesis was one of the prominent science goals of the INTEGRAL mission and in particular its spectrometer SPI. Emission from ${}^{26}$Al and from ${}^{60}$Fe decay lines, due to their My decay times, originates from accumulated ejecta of nucleosynthesis sources, and appears diffuse in nature. ${}^{26}$Al and ${}^{60}$Fe are believed to originate mostly from massive star clusters. The radioactive decay $\gamma$-ray observations open an interesting window to trace the fate and flow of nucleosynthesis ejecta, after they have left the immediate sources and their birth sites, and on their path to mix with ambient interstellar gas. The ${}^{26}$Al emission image obtained with INTEGRAL confirms earlier findings of clumpiness and an extent along the entire plane of the Galaxy, supporting its origin from massive-star groups. INTEGRAL spectroscopy resolved the line and found Doppler broadenings and systematic shifts with longitude, originating from large-scale galactic rotation. But an excess velocity of  200 km s⁻¹ suggests that ${}^{26}$Al decays preferentially within large superbubbles that extend in forward directions between spiral arms. The detection of ${}^{26}$Al line emission from the nearby Orion clusters in the Eridanus superbubble supports this interpretation. Positrons from $\beta$⁺ decays of ${}^{26}$Al and other nucleosynthesis ejecta have been found to not explain the morphology of positron annihilation $\gamma$-rays at 511 keV that have been measured by INTEGRAL. The ${}^{60}$Fe signal measured by INTEGRAL is diffuse but too weak for an imaging interpretation, an origin from point-like/concentrated sources is excluded. The ${}^{26}$Al /${}^{60}$Fe ratio is constrained to a range 0.2–0.4. Beyond improving precision of these results, diffuse nucleosynthesis contributions from novae (through ²²Na radioactivity) and from past neutron star mergers in our Galaxy (from r-process radioactivity) are exciting new prospects for the remaining mission extensions.