2000ApJ...531L..33T

We suggest that if the astrophysical site for r-process nucleosynthesis in the early Galaxy is confined to a narrow mass range of Type II supernova (SN II) progenitors, with a lower mass limit of M_ms=20 M_☉, a unique feature in the observed distribution of [Ba/Mg] versus [Mg/H] for extremely metal-poor stars can be adequately reproduced. We associate this feature, a bifurcation of the observed elemental ratios into two branches in the Mg abundance interval -3.7≤[Mg/H]≤-2.3, with two distinct processes. The first branch, which we call the y-branch, is associated with the production of Ba and Mg from individual massive supernovae. The derived mass of Ba synthesized in SNe II is 8.5x10^–6 M_☉ for M_ms=20 M_☉ and 4.5x10^–8 M_☉ for M_ms=25 M_☉. We conclude that SNe II with M_ms~20 M_☉ are the dominant source of r-process nucleosynthesis in the early Galaxy. An SN-induced chemical evolution model with this M_ms-dependent Ba yield creates the y-branch, reflecting the different nucleosynthesis yields of [Ba/Mg] for each SN II with M_ms≳20 M_☉. The second branch, which we call the i-branch, is associated with the elemental abundance ratios of stars which were formed in the dense shells of the interstellar medium swept up by SNe II with M_ms<20 M_☉ that do not synthesize r-process elements, and it applies to stars with observed Mg abundances in the range [Mg/H]←2.7. The Ba abundances in these stars reflect those of the interstellar gas at the (later) time of their formation. The existence of a [Ba/Mg] i-branch strongly suggests that SNe II that are associated with stars of progenitor mass M_ms≤20 M_☉ are infertile sources for the production of r-process elements. We predict the existence of this i-branch for other r-process elements, such as europium (Eu), to the extent that their production site is in common with Ba.