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

TL;DR

This study uses low-resolution spectroscopy to determine velocities and metallicities of five poorly known Galactic stellar clusters, revealing their possible origins and associations with known Milky Way structures.

Contribution

First spectroscopic metallicity and velocity measurements for Pfleiderer 2 and RLGC2, and orbital analysis linking these clusters to Galactic formation events.

Findings

Pfleiderer 2 likely formed within the Milky Way.

RLGC2 associated with Gaia-Sausage-Enceladus event.

Koposov 1 possibly stripped from Sagittarius dwarf spheroidal.

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, Mu\~noz 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 Mu\~noz 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.