A Stringent Limit on the Mass Production Rate of r-process Elements in the Milky Way

Abstract

We analyze data from several studies of metal-poor stars in the Milky Way, focusing individually on the main r -process elements (Eu) as well as the lighter neutron-capture element Sr, at the neutron-magic peak N  = 50. Because these elements were injected in an explosion, we calculate the mass swept up when the blast wave first becomes radiative, yielding a lower limit for the dilution of such elements and hence a lower limit on the ejecta mass that is incorporated into the next generation of stars. Our study demonstrates that in order to explain the largest enhancements in [Eu/Fe] observed in stars at low [Fe/H] metallicities, individual r -process production events must synthesize a minimum of roughly 10 −3 M ⊙ of r -process material. This provides a critical constraint on galactic chemical evolution models. We also show independently that if the site of Mg production is the same as that of Eu, individual injection events must synthesize up to ∼10 −3 M ⊙ of r -process material. On the other hand, demanding that Sr traces Mg production results in r -process masses per event of ∼10 −5 M ⊙ . This suggests that the astrophysical sites responsible for the genesis of the main r -process elements need to operate at a drastically reduced rate when compared to standard core-collapse supernovae.