Sr and Ba yields of the First Generation(s) of stars: Constraints from metal-poor stars
We present our chemical abundance analysis of ten new extremely metal-poor stars with $-4.05\leq\mbox{[Fe/H]}\leq-2.33$, based on high-resolution (R $\sim28,000$) Magellan/MIKE spectra. Eight of our stars have low heavy-element abundances of $\mbox{[Sr/H]}<-4.5$ and $\mbox{[Ba/H]}<-4.0$, making them Small Accreted Stellar System (SASS) stars. Four are hyper neutron-capture-element poor with $\mbox{[Sr/H]}<-5.0$, including Gaia DR3 5729400267359655680, which sets a new record for the lowest detected Sr abundance of $\mbox{[Sr/H]} =-6.4$. We identify four distinct [Sr/Ba] groups within the wider SASS star population which span a large range from $\mbox{[Sr/Ba]} =-2.0$ to +1.6, pointing to multiple types of progenitor events and different nucleosynthesis processes/sites. To explore the origins of this large [Sr/Ba] range, we adopt site-agnostic Sr yields of $\mbox{[Sr/H]}=-6$, $-5.75$, $-5.42$, and $-4.93$ for the four groups. Applying those yields suggests that the majority of SASS stars formed from gas enriched by $\sim$1-10 progenitor events, consistent with expectations from their extremely metal-poor nature. We thus attribute the [Sr/H] abundance scatter to intrinsic variations in the Sr yield per nucleosynthesis site/event. Our proposed Sr yields for each [Sr/Ba] group and associated nucleosynthesis origin are a reasonable and representative approximation, good to within a factor of a few, and can constrain future theoretical heavy element nucleosynthesis calculations in early core-collapse supernovae.