Evolution of radioactive elements in the LMC: predictions for future γ-ray surveys

Abstract

ABSTRACT Short-lived radionuclides, such as 26Al and 60Fe, are tracers of star formation. Therefore, their abundances can unravel the recent star formation history of the host galaxy. In view of future γ-ray surveys, we predict the masses and fluxes of these two elements in the Large Magellanic Cloud (LMC) using new chemical evolution models. Our best model reproduces the abundance patterns of [α/Fe] ratios, the gas mass, the average metallicity, the present time supernova and nova rates observed in LMC. We show three main results: (i) the best model for the LMC suggests a star formation rate very mild at the beginning with a recent burst, and a Salpeter-like initial mass function. (ii) The predicted mass of 26Al is 0.33 M⊙, 2/3 produced by massive stars and 1/3 by novae. (iii) The predicted mass of 60Fe is 0.44 M⊙, entirely produced by massive stars. This result suggests a larger fraction of 60Fe, at variance with the Milky Way. The explanation for this lies in the adopted initial mass function, that for the LMC contains more massive stars than for the Milky Way. These predictions can be useful for the COSI-SMEX mission planned for launch in 2027. The expected γ-ray line fluxes for the 1.809 MeV line of 26Al and the 1.173 and 1.332 MeV lines of 60Fe are in the range of (0.2–$2.7)\times 10^{-6}\, \mathrm{ph\, cm^{-2}\, s^{-1}}$ and (0.7–$2.8)\times 10^{-7}\, \mathrm{ph\, cm^{-2}\, s^{-1}}$, respectively. This new instrument could have the sensitivity to detect the upper end of the predicted 1.8 MeV flux within its nominal two-yr mission.