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tokens: the r- process event would overpower signatures left over from any neutron-capture processes that may occur before or after. But to make matters more complication, at least two stars in Ret II display rather low Sr and Ba values (not shown in the figure but see fig. 3 in Ji et al. 2016c ), and the UFDs without clear r -process enrichment do contain a lot of stars with significant amount of s -process material. Nevertheless, our results clearly show that for stars with measurable neutron-capture element abundances, AGB stars alone are not sufficient source to explain the origin of Ba and Sr for these stars. However, we also note that likely more stars exist in these systems that have lower Sr and Ba measurements than what is represented with the current observed sample. 2 Those stars (1) might not yet have such measurements due to being faint and corresponding poor data quality especially in the blue spectral regime preventing even meaningful upper limits, (2) or these stars simply have not even been observed yet at all. Such a putative low Sr-Ba-population may well be explained solely with an AGB origin scenario. Future observations can hopefully shed further light on this issue so that our predictions could be further put to the test. In fact, there are hints of this being the case. Metal-poor halo stars have [Ba/Fe] going down to about [Ba/Fe] ∼ −2.0 which is roughly at the level of the simulated stars formed last in our simulation (light green circles in the figure). Furthermore, all stars with red symbols with [Ba/Fe] > −2.0 form a branch not unlike that of MW stars albeit offset to lower [Ba/Fe] values. The lower of these stars overlap with the top end of the simulated later-time stars – clearly more overlap would be achieved with improved spectral line detections should additional data become available (e.g. with the Giant Magellan Telescope ). Figure 3. Observed abundances in UFDs overlaid with our RMS model results for [Ba/Fe] (top panels) and [Sr/Fe] (bottom panels). Symbols and colours are the same as in Fig. 2 . The simulated abundances agree well with the observed abundances, including the largely flat trend with [Fe/H]. The scatter of [Ba/Fe] and [Sr/Fe] in each galaxy is also an interesting piece of information. Observationally, Bootes I (Boo I), Carina II (Car II), and Coma Berenices (ComBer) show significant scatter while most other UFDs do not. We also note that the number of observations is limited due to observational difficulties such as stars being too faint to obtain higher signal-to-noise observations while also requiring high spectral resolution. Therefore, even for UFDs with little apparent scatter, significant scatter in [Ba/Fe] and [Sr/Fe] may still be present within. In our simulation, large scatter is observed both in the L- and M-UFDs. The Ba content of S-UFD is too little, and therefore we cannot discuss the scatter. There are two kinds of inhomogeneities to diversify the abundances of the neutron-capture