Probing the Site for r-Process Nucleosynthesis with Abundances of Barium and Magnesium in Extremely Metal-poor Stars

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 Mms = 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 8.5 × 10-6 M☉ for Mms = 20 M☉ and 4.5 × 10-8 M☉ for Mms = 25 M☉. We conclude that SNe II with Mms ≈ 20 M☉ are the dominant source of r-process nucleosynthesis in the early Galaxy. An SN-induced chemical evolution model with this Mms-dependent Ba yield creates the y-branch, reflecting the different nucleosynthesis yields of [Ba/Mg] for each SN II with Mms ≳ 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 Mms < 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 Mms ≤ 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.