Low-resolution spectroscopic characterisation of five poorly known Galactic stellar clusters

Abstract

Stellar clusters preserve crucial information on the formation and evolutionary processes that shaped the Milky Way (MW) as we see it today. Yet, several MW clusters still lack sufficient data to constrain their metallicity, ages, and, in some cases, even their basic kinematic properties. We present low-resolution MODS@LBT spectroscopy for five such systems (i.e. Koposov 1, Koposov 2, Muñoz 1, Pfleiderer 2, and RLGC2) from which we derive systemic line-of-sight velocities ($V_{\rm sys}$) with typical uncertainties of $10-20$ km/s per star, and metallicities based on the equivalent widths of the infrared Ca II triplet measured in red giant branch members. For Pfleiderer 2 and RLGC2, we provide the first spectroscopic determinations of their systemic velocities and metallicities, finding $V_{\rm sys}= 3 \pm 3$ km/s and $-316 \pm 4$ km/s, and $\mathrm{[Fe/H]}= -0.76 \pm 0.09$ dex and $-2.33 \pm 0.04$ dex, respectively. For the other three clusters we find results consistent with the existing literature. Thanks to our new spectroscopic measurements, we perform an orbital analysis to investigate their origin. We find that Pfleiderer 2 likely formed within the MW, RLGC2 is dynamically associated to the Gaia-Sausage-Enceladus accretion event, Koposov 1 was likely stripped from the Sagittarius dwarf spheroidal, while Muñoz 1 is only tentatively associated with the latter system. In the end, Koposov 2 at high orbital energy does not show a clear association with any known progenitor system.

Figures (8)

• Figure 1: Left panel: Gaia CMD for Pfleiderer 2 from vasilievbaumgardt2021. All stars with probability membership $> 80\%$ are shown in black, while our selected target stars are plotted in orange. A BaSTI isochrone hidalgo2018 is also plotted for reference, taking distance and reddening estimates from ortolani2009. Stars in our sample identified as outliers in $V_{\mathrm{los}}$ are marked with a coloured cross. Right panel: MODS spectra for the highest and lowest S/N stars among our sample, shown around the H$\alpha$ and CaT regions. Same plots for other clusters can be found in the Appendix \ref{['app:A']} (see Figs. \ref{['fig:data_2']} - \ref{['fig:data_3']}).
• Figure 2: Top row: vector-point diagram (left) for a synthetic Galactic field population (located within $1^\circ$ from Pfleiderer 2 and selected in the same CMD region populated by the RGB of the cluster) computed with a Besançon Galaxy Model robin2003. The proper motion of Pfleiderer 2 from vasilievbaumgardt2021 is indicated with a cross. On the top right panel we display the $V_{\mathrm{los}}$ distribution of cluster (orange) and field stars from the BGM (gray) as a function of the distance from the cluster center. In this panel, stars identified as outliers in $V_{\mathrm{los}}$ are marked with crosses. Bottom row: line-of-sight velocity (left) and metallicity (right) distributions for Pfleiderer 2 (orange) and for the BGM synthetic field population (gray).
• Figure 3: Distribution in the integral-of-motions ($E, L_{\mathrm{z}}, L_{\perp}$) space of the stellar clusters studied in this paper, compared to all Milky Way globular clusters from massari19. As reference, we plot the $1\sigma$ ellipses of the distributions of GCs dynamically associated with GSE and Sag dSph, while in situ GCs are plotted in black.
• Figure 4: Orbit of Pfleiderer 2 integrated backwards in time for 2.5 Gyr using OrbITdeleo2025 and accounting for the presence of the bar of the Milky Way. The current position of the cluster is shown with a orange point. The red circle shown in the X-Y projection of the orbit indicate the corotational radius, that is equal to 5.79 kpc for the assumed potenital in OrbIT.
• Figure 5: Same as Fig. \ref{['fig:data_Pfleiderer2']}, but for Koposov 1 (pink) and Koposov 2 (blue). Reference BaSTI isochrone are 7 and 13.5 Gyr old, $[\mathrm{Fe}/\mathrm{H}] = -1.2$ dex and $[\mathrm{Fe}/\mathrm{H}] = -2.9$ dex, respectively, and $\alpha$-enanched ($[\alpha/\mathrm{Fe}] = +0.4$ dex). We plot them assuming reddening from paust2014 for both clusters, and distances from paust2014 and cerny26 for Koposov 1 and Koposov 2.