Unusual neutron-capture nucleosynthesis in a carbon-rich Galactic bulge star

Metal-poor stars in the Galactic halo often show strong enhancements in carbon and/or neutron-capture elements. However, the Galactic bulge is notable for its paucity of carbon-enhanced metal-poor (CEMP) and/or CH-stars, with only two such objects known to date. This begs the question whether the processes that produced their abundance distribution were governed by a comparable nucleosynthesis in similar stellar sites as for their more numerous counterparts in the halo. Recently, two contenders of such stars were discovered in the bulge, at [Fe/H] = $-1.5$ and $-$2.5 dex, both of which show enhancements in [C/Fe] of 0.4 and 1.4 dex, [Ba/Fe] in excess of 1.3 dex, and also elevated nitrogen. The more metal-poor of the stars is matched by standard $s$-process nucleosynthesis in low-mass Asymptotic Giant Branch (AGB) polluters. The other star shows an abnormally high [Rb/Fe] ratio. Here, we investigate the origin of the abundance peculiarities in the Rb-rich star by new, detailed measurements of heavy element abundances and by comparing the chemical element ratios of 36 species to models of neutron-capture nucleosynthesis. The $i$-process with intermediate neutron densities between those of the $s$- and $r$-processes has been previously found to provide good matches of CEMP stars with enhancements in both $r$- and $s$-process elements, rather than invoking a superposition of yields from the respective individual processes. However, the peculiar bulge star is incompatible with a pure $i$-process from a single ingestion event. Instead, it can, statistically, be better reproduced by models accounting for two proton ingestion events, or by an $i$-process component in combination with $s$-process nucleosynthesis in low-to-intermediate mass AGB stars, indicating multiple polluters. [abridged]