An In-depth Investigation of the Primordial Cluster Pair ASCC 19 and ASCC 21

TL;DR

This study identifies and characterizes a young primordial cluster pair, ASCC 19 and ASCC 21, near Orion, revealing their shared origin, physical properties, and future dynamical evolution as an unbound double cluster.

Contribution

The paper provides the first detailed analysis of the primordial cluster pair ASCC 19 and ASCC 21, including their origin, properties, and dynamical state, based on Gaia and spectroscopic data.

Findings

Clusters are young (~8.9 Myr) and physically close (~27 pc apart).

They share similar ages, velocities, and metallicities, indicating a common origin.

The pair is unbound and will not merge into a single cluster.

Abstract

Utilizing \texttt{Gaia} data from the literature, we report a new young ($\sim$8.9~Myr) cluster pair, ASCC~19 and ASCC~21, located near the Orion star-forming complex. The clusters are separated by a 3D distance of ~27.00~$\pm$~7.51~pc. Both clusters share a common age (Log(age)~=~6.95~$\pm$~0.05), similar radial velocities ($R_{v}$~=~21.34~$\pm$~4.47~km s$^{-1}$ for ASCC~19 and $R_{v}$~=~20.05~$\pm$~3.86~km s$^{-1}$ for ASCC~21), and comparable metallicities ([Fe/H]~=~$-$0.14~$\pm$~0.25~dex for ASCC~19 and [Fe/H]~=~$-$0.12~$\pm$~0.04~dex for ASCC~21, from LAMOST-DR11). These similarities suggest that the clusters likely originated from the fragmentation of the same molecular cloud, forming a primordial cluster pair. Furthermore, the formation of the two clusters is attributed to the coalescence of multiple subclusters, as inferred from the distribution analysis between metal abundances and distances to clusters' centers. Neither cluster shows significant mass segregation. Their members with radial velocities exceeding 100~km s$^{-1}$ are young variables. Additionally, a tidal interaction between the clusters is observed. Comparisons of the Roche radius with tidal radii, and velocity difference with orbital velocity, suggest that the pair is an unbound system, that is, a double cluster. Finally, orbital motion simulations show that the clusters will not merge into a single system.