UC Merced

Open star clusters are the essential building blocks of the Galactic disk; “strong chemical tagging” – the premise that all star clusters can be reconstructed given chemistry information alone – is a driving force behind many current and upcoming large Galactic spectroscopic surveys. In this work, we characterize abundance patterns for 9 elements (C, N, O, Ne, Mg, Si, S, Ca, and Fe) in open clusters (OCs) in three galaxies (m12i, m12f, and m12m) from the \textit{Latte} suite of FIRE-2 simulations to investigate if strong chemical tagging is possible in these simulations. We select young massive ($\ge$$10^{4.6} M_{\odot}$) OCs formed in the last $\sim$100 Myr and calculate the intra- and inter-cluster abundance scatter for these clusters.

We compare these results with analogous calculations drawn from observations of OCs in the Milky Way and find the intra-cluster scatter of the observations and simulations to be comparable. While the abundance scatter withineach cluster is minimal ($\lesssim$$0.020$ dex), the mean abundance patterns of different clusters are not unique. We also calculate the chemical difference in intra- and inter-cluster star pairs and find it, in general, to be so small that it is difficult to distinguish between stars drawn from the same OC or from different OCs. Despite tracing three distinct nucleosynthetic families (core-collapse supernovae, white dwarf supernovae, and stellar winds), we conclude that these elemental abundances do not provide enough discriminating information to use strong chemical tagging for reliable OC membership.

Open clusters are relatively bright, easy to measure objects that are thought to be reliable tracers of the overall galactic radial metallicity gradient. Studies have shown that open clusters decrease in metallicity with increasing distance from the Galactic center; this distribution has often been modeled with a two-component linear fit, with a transition (knee), occurring between 10 and 16 kpc. Recent work from the APOGEE collaboration has significantly increased ($>$ 150) the sample size of open clusters with robust measurements of chemistry across the Galactic disk.

However, the susceptibility of this fit to the effects of sampling bias has been yet to be explored and could have large implications for the inferences drawn from the radial metallicity gradient. We selected young massive clusters less than 3 Myr old in our simulations and fit the radial metallicity gradient using the MCMC technique and find that sampling of open clusters significantly alters the observed trend of the fit parameters inner slope (m1), intercept (b), outer slope (m2) and knee (k). We performed bootstrapping of the fit to find that sampling can have an impact on the outer slope but significantly impacts the inner slope and the location of the knee. Using both bootstrapping and fixed sized random sub-samples of open clusters, we find that for reliable recovery of the fit parameters, we require at least 200-250 clusters spread across a wide range of galactocentric radii. With smaller or spatially biased samples, the fit parameters become highly sensitive to the sampling bias.

It has been shown that more than 50\% of the disk galaxies, including our Milky Way galaxy, has a central bar with billions of stars clustered together. Theory suggests that galactic bars spin down throughout their evolution due to an angular momentum exchange with the inner parts of their dark matter halos. As opposed to a bar with a fixed pattern speed, Chiba et al. (2019) proposed that ‘resonance sweeping’ due to a decelerating galactic bar can explain local kinematic substructure in the solar neighborhood, like the Hercules stream. To date, resonance sweeping - a process of trapping and dragging the orbits of stars - has been explored both analytically and with test particle simulations that lack self-gravity. Here, we take such analyses a step further and examine resonance sweeping with a high resolution ($\sim$ $10^9$ particles) self-consistent N-body simulation. We identify stars in Corotation Resonance and Outer Lindblad Resonance and find a significant number of stars remain in resonance later in time, suggesting resonant sweeping of orbits due to the decelerating bar. This result in a more realistic, self-gravitating disk indicates that the method of resonance sweeping can indeed be applied to Gaia data.