Snapshots of r-process production in the Milky Way disk

Stars enhanced in heavy neutron capture elements like europium (Eu) provide a window into the rarest and most exotic nucleosynthetic events. Most Eu-enhanced stars discovered thus far are found among metal-poor populations, with $[\mathrm{F}\mathrm{e}\mathrm{/}\mathrm{H}]<-1.0$. Here, we report the discovery of $17$ vetted Eu-enhanced stars ($[\mathrm{E}\mathrm{u}\mathrm{/}\mathrm{F}\mathrm{e}]>0.7$) in the Milky Way disk with metallicities in the range $-0.87<[\mathrm{F}\mathrm{e}\mathrm{/}\mathrm{H}]<-0.21$ identified from among $74717$ stars in the GALAH survey. 14 of these stars additionally have $[\mathrm{B}\mathrm{a}\mathrm{/}\mathrm{E}\mathrm{u}]<0$, marking enhancement from the rapid neutron capture ($r$-) process and making these r-II stars. Included in their number is a star with a remarkably high $[\mathrm{E}\mathrm{u}\mathrm{/}\mathrm{H}]=1.01\pm 0.09$ at $[\mathrm{F}\mathrm{e}\mathrm{/}\mathrm{H}]=-0.21$. In addition to Eu, all 17 stars show enhancements in other neutron capture elements, especially in Nd, while otherwise appearing chemically consistent with typical disk stars, particularly in their $\alpha$-element abundances. Two of the Eu-enhanced stars exhibit indications of binarity with strong enrichment in Ba and Y, marking potential signatures of the slow or intermediate neutron capture processes. This sample of 17 Eu-enhanced stars, which span both the high- and low-$\alpha$ disks, are valuable tracers of neutron capture processes, including the $r$-process, at high metallicities. Although we use criteria for Eu-enhanced stars typically applied to the halo to enable comparison to literature studies of $r$-process enhanced stars, we propose a new, metallicity-dependent selection for Eu enhancement that is better-suited to the regime of the Galactic disk.