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 TI supernova (S N II) progenitors, with, a lower mass limit of M-ms = 20 M., a unique featu re in the observed distribution of [Ba/Mg] versus [Mg/H] for extremely meta l-poor stars can be adequately reproduced. We associate this feature, a bif urcation of the observed elemental ratios into two branches in the Mg abund ance interval -3.7 less than or equal to [Mg/H] less than or equal to -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 sup ernovae. The derived mass of Ba synthesized in SNe II is 8.5 x 10(-6) M. fo r M-ms = 20 M. and 4.5 x 10(-8) M. for M-ms = 25 M.. We conclude that SNe L I with M-ms approximate to 20 M. are the dominant source of r-process nucle osynthesis in the early Galaxy. An SN-induced chemical evolution model with this M-ms-dependent Ba yield creates the y-branch, reflecting the differen t nucleosynthesis yields of [Ba/Mg] for each SN II with M-ms greater than o r similar to 20 M.. The second branch, which we call the i-branch, is assoc iated 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 the se 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 less than or equa l to 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, suc h as europium (Eu), to the extent that their production site is in common w ith Ba.