Which young clusters/associations are we missing today?

TL;DR

This paper investigates how gas expulsion affects the detectability of young star clusters, revealing that many low-mass clusters are likely missed in current observations due to their expansion and decreased surface density over time.

Contribution

The study demonstrates that current detection methods bias the observed cluster population towards more massive clusters, and suggests GAIA can help identify the missing lower-mass clusters.

Findings

Only clusters with high initial surface densities are detectable after 5 Myr.

Most low-mass clusters (<3000 M_sun) are missed after 4 Myr due to expansion.

GAIA can potentially detect lower-mass, older clusters that current methods miss.

Abstract

Currently clusters/associations of stars are mainly detected as surface density enhancements relative to the background field. While clusters form, their surface density increases. It likely decreases again at the end of the star formation process when the system expands as a consequence of gas expulsion. Therefore the surface density of a single cluster can change considerably in young clusters/associations during the first 20 Myr of their development. We investigate the effect of the gas expulsion on the detectability of clusters/associations typical for the solar neighborhood, where the star formation efficiency is <35%. The main focus will be laid on the dependence on the initial cluster mass. Nbody methods are used to determine the cluster/association dynamics after gas expulsion. We find that, even for low background densities, only clusters/associations with initial central surface densities exceeding a few 5000 M(sun)/pc2 will be detected as clusters at ages ~5 Myr. Even the Orion Nebula cluster, one of the most massive nearby clusters, would only be categorized as a small co-moving group with current methods after 5 Myr of development. This means that cluster expansion leads to a selection effect - at ages of <1-2 Myr the full range of clusters/associations is observed whereas at ages > 4 Myr only the most massive clusters are identified, while systems with initially M_c < 3 000 M(sun) are missing. The temporal development of stellar properties is usually determined by observing clusters of different ages. The potentially strong inhomogeneity of the cluster sample makes this methods highly questionable. However, GAIA could provide the means to rectify this situation as it will be able to detect lower mass clusters.