Unbound Young Stellar Systems: Star Formation on the Loose

Abstract

Unbound young stellar systems, the loose ensembles of physically related young bright stars, trace the typical regions of recent star formation in galaxies. Their morphologies vary from small few pc-size associations of newly formed stars to enormous few kpc-size complexes composed of stars few 100 Myr old. These stellar conglomerations are located within the disks and along the spiral arms and rings of star-forming disk galaxies, and they are the active star-forming centers of dwarf and starburst galaxies. Being associated with star-forming regions of various sizes, these stellar structures trace the regions where stars form at various length-and timescales, from compact clusters to whole galactic disks. Stellar associations, the prototypical unbound young systems, and their larger counterparts, stellar aggregates, and stellar complexes, have been the focus of several studies for quite a few decades, with special interest on their demographics, classification, and structural morphology. The compiled surveys of these loose young stellar systems demonstrate that the clear distinction of these systems into welldefined classes is not as straightforward as for stellar clusters, due to their low densities, asymmetric shapes and variety in structural parameters. These surveys also illustrate that unbound stellar structures follow a clear hierarchical pattern in the clustering of their stars across various scales. Stellar associations are characterized by significant sub-structure with bound stellar clusters being their most compact parts, while associations themselves are the brighter denser parts of larger stellar aggregates and stellar complexes, which are members of larger superstructures up to the scale of a whole star-forming galaxy. This structural pattern, which is usually characterized as self-similar or fractal, appears to be identical to that of star-forming giant molecular clouds and interstellar gas, driven mainly by turbulence cascade. In this short review, I make a concise compilation of our understanding of unbound young stellar systems across various environments in the local universe, as it is developed during the last 60 years. I present a factual assessment of the clustering behavior of star formation, as revealed from the assembling pattern of stars across loose stellar structures and its relation to the interstellar medium and the environmental conditions. I also provide a consistent account of the processes that possibly play important role in the formation of unbound stellar systems, compiled from both theoretical and observational investigations on the field.

Kinematics in Young Star Clusters and Associations with Gaia DR2

“…While star formation in the Galaxy occurs in large star-forming complexes, most of the stars formed in these complexes quickly disperse into the field without remaining bound to one another as members of open clusters (Adams 2010;Gouliermis 2018). Star formation takes place in turbulent, clumpy giant molecular clouds, and yet the star clusters that do remain bound tend to have smooth stellar density distributions.…”

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

“…Vela OB2 is therefore a clear example of an OB association that was formed globally unbound but still exhibits signs of expansion. For a review of unbound young stellar systems see Gouliermis (2018).…”

From the top down and back up again: star cluster structure from hierarchical star formation

“…Most stars in the Universe are field stars, gravitationally bound only to their host galaxies and not to any discernible smaller element of structure. However, when the locations of initial star formation are considered, there is strong evidence that most stars are born in a statistically clustered, correlated configuration (Lada & Lada 2003;McKee & Ostriker 2007;Bressert et al 2010;Gouliermis et al 2015;Grasha et al 2017;Gouliermis 2018). The star formation efficiency M M g a s of typical giant molecular clouds is only of order 1 − 10% (Myers et al 1986; Mooney & Solomon 1988;Williams & McKee 1997;Evans et al 2009;Lada et al 2010;Heiderman et al 2010;Murray 2011;Kennicutt & Evans 2012;Lee et al 2016), possibly due to stellar feed-Contact e-mail: mgrudich@caltech.edu back disrupting the molecular cloud once a certain stellar mass has formed (Murray et al 2010;Hopkins et al 2012Hopkins et al , 2014Grudić et al 2018).…”