Not all stars form in clusters – Gaia-DR2 uncovers the origin of OB associations

Abstract

Historically, it has often been asserted that most stars form in compact clusters. In this scenario, present-day gravitationally-unbound OB associations are the result of the expansion of initially gravitationally-bound star clusters. However, this paradigm is inconsistent with recent results, both theoretical and observational, that instead favour a hierarchical picture of star formation in which stars are formed across a continuous distribution of gas densities and most OB associations never were bound clusters. Instead they are formed in-situ as the lowdensity side of this distribution, rather than as the remnants of expanding clusters. We utilise the second Gaia data release to quantify the degree to which OB associations are undergoing expansion and, therefore, whether OB associations are the product of expanding clusters, or whether they were born in-situ, as the large-scale, globally-unbound associations that we see today. We find that the observed kinematic properties of associations are consistent with highly substructured velocity fields and additionally require some degree of localised expansion from sub-clusters within the association. While most present-day OB associations do exhibit low levels of expansion, there is no significant correlation between radial velocity and radius. Therefore, the large-scale structure of associations is not set by the expansion of clusters, rather it is a relic of the molecular gas cloud from which the association was formed. This finding is inconsistent with a monolithic model of association formation and instead favours a hierarchical model, in which OB associations form in-situ, following the fractal structure of the gas from which they form.

Impact of Low-Energy Cosmic Rays on Star Formation

“…Clustering of young massive stars in star-forming regions has a profound effect on the structure and evolution of the ISM on 100 pc scales with a possibility of the break out of the thin HI Galactic disc in the form of chimneys (Mac Low and McCray, 1988;Tomisaka and Ikeuchi, 1986;Kim et al, 2017;El-Badry et al, 2019). The massive star-forming regions are structured as they are observationally represented by compact clusters of young massive stars as well as by highly substructured OB associations with lower stellar density than in the compact clusters (see, e.g., Krumholz et al, 2019;Ward et al, 2019). The winds of massive stars and their subsequent SNe create large-scale superbubbles and supershells in the ISM through their great momentum and energy release (see, e.g., Heiles, 1979;Mac Low and McCray, 1988;El-Badry et al, 2019).…”

Extended stellar systems in the solar neighborhood

“…Over the past decade, this view has been challenged on multiple occasions (e.g., Elmegreen 2008;Bressert et al 2010;Kruijssen 2012;cf. Parker & Meyer 2012), where the Gaia mission (Gaia Collaboration et al 2016) and particularly the second data release (Gaia DR2;Gaia Collaboration et al 2018) have recently initiated a renaissance in our understanding of the predominant conditions for star formation (e.g., Wright & Mamajek 2018;Cantat-Gaudin et al 2019a;Kuhn et al 2019;Wright et al 2019;Ward et al 2020). These insights have been promoted mostly by investigating the large-scale structure and kinematics of massive OB associations with ages up to a few million years.…”

OB stars and YSO populations in the region of NGC 6334–NGC 6357 as seen withGaiaDR2

“…In this picture supersonic turbulence introduces fluctuations in the density distribution of the giant molecular cloud, with localized high-density regions producing bound clusters, while low-density regions drive the formation of dispersed unbound stellar populations. In this way, most OB associations were never bound clusters, but instead, they were formed in situ in low-density environments following the fractal and velocity structure of the gas from which they form (e.g., Ward et al 2020;Gouliermis 2018), as was already observed by Wright et al (2014), who find for Cyg OB2 that massive stars can form in relatively low-density environments. In this frame, because OB stars in NGC 6357 are clustered while they distribute more as an association in NGC 6334, we can speculate that NGC 6357 OB stars originate from denser conditions than the ones in NGC 6334.…”