Abstract
Gamma rays from the decay of 26Al offer a stringent constraint on the Galaxy's global star formation rate over the past million years, supplementing other methods for quantifying the recent Galactic star formation rate, such as equivalent widths of Hα emission. Advantages and disadvantages of using 26Al gamma-ray measurements as a tracer of the massive star formation rate are analyzed. Estimates of the Galactic 26Al mass derived from COMPTEL measurements are coupled with a simple, analytical model of the 26Al injection rate from massive stars and restrict the Galaxy's recent star formation rate to 5 ± 4 M☉ yr-1. In addition, we show that the derived 26Al mass implies a present-day Type II + Ib supernovae rate of 3.4 ± 2.8 per century, which seems consistent with other independent estimates of the Galactic core-collapse supernova rate. If some independent measure of the massive star initial mass function or star formation rate or Type II + Ib supernovae rate were to become available (perhaps through estimates of the Galactic 60Fe mass), then a convenient way to restrain, or possibly determine, the other parameters is presented.
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