The dynamical state of stellar structure in star-forming regions

Abstract

The fraction of star formation that results in bound star clusters is influenced by the density spectrum in which stars are formed and by the response of the stellar structure to gas expulsion. We analyse hydrodynamical simulations of turbulent fragmentation in star-forming regions to assess the dynamical properties of the resulting population of stars and (sub)clusters. Stellar subclusters are identified using a minimum spanning tree algorithm. When considering only the gravitational potential of the stars and ignoring the gas, we find that the identified subclusters are close to virial equilibrium (the typical virial ratio Q_vir~0.59, where virial equilibrium would be Q_vir~0.5). This virial state is a consequence of the low gas fractions within the subclusters, caused by the accretion of gas onto the stars and the accretion-induced shrinkage of the subclusters. Because the subclusters are gas-poor, up to a length scale of 0.1-0.2 pc at the end of the simulation, they are only weakly affected by gas expulsion. The fraction of subclusters that reaches the high density required to evolve to a gas-poor state increases with the density of the star-forming region. We extend this argument to star cluster scales, and suggest that the absence of gas indicates that the early disruption of star clusters due to gas expulsion (infant mortality) plays a smaller role than anticipated, and is potentially restricted to star-forming regions with low ambient gas densities. We propose that in dense star-forming regions, the tidal shocking of young star clusters by the surrounding gas clouds could be responsible for the early disruption. This `cruel cradle effect' would work in addition to disruption by gas expulsion. We suggest possible methods to quantify the relative contributions of both mechanisms.Comment: 13 pages, 10 figures; Accepted for publication in MNRA

26Al kinematics: superbubbles following the spiral arms?

“…Recent observations did not find the expansion velocities that are expected if gas expulsion was important (e.g., Hénault-Brunet et al 2012). Hence, the dispersal is probably related to tidal effects (e.g., Kruijssen et al 2012a) that could take as long as 200 Myr (Kruijssen et al 2012b). It is therefore reasonable to assume that for the timescales of interest here, the great majority of massive stars are grouped (compare also Zinnecker & Yorke 2007).…”

How can young massive clusters reach their present-day sizes?

“…2.2 do not assume any primordial gas present initially in the cluster, i.e., the cluster is taken to be formed with 100% local SFE. A high local SFE has been claimed by several authors based on hydrodynamic calculations without or partially-implemented feedback processes (e.g., Klessen et al 1998;Bate & Bonnell 2004;Kruijssen et al 2012;Girichidis et al 2012;Dale et al 2015). Clearly, as the computations in Sec.…”

“…From such observations, it can be generally concluded that the individual filamentary overdensities and their junctions in molecular clouds are highly compact; of typical (projected) widths of 0.1-0.3 pc (André et al 2011. Both theoretical and observational studies indicate that groups of (proto-)stars preferentially form within the filaments and at filament junctions Schneider et al 2010Schneider et al , 2012Tafalla & Hacar 2014;Duarte-Cabral et al 2011), giving rise to 0.5 pc dense protostellar clumps (or pre-cluster cloud core; e.g., Lada & Lada 2003;Tapia et al 2011;Traficante et al 2015). Semi-analytic studies, e.g., Marks & Kroupa (2012), also indicate similar gas clump sizes that depend weakly on their total mass.…”

“…However, how they form in the first place still poses a major challenge in astronomy. It is crucial to address this fundamental question because most, if not all, stars appear to form in embedded clusters (Elmegreen 1983;Lada & Lada 2003). A key question in this regard is how the exposed clusters' parsec-scale, smooth, centrally pronounced, near spherical shape, observed at all ages 1 Myr, can be explained.…”

“…The rapidity of the gas expulsion is as well crucial. Both observations of embedded systems in the solar neighborhood (Lada & Lada 2003) and hydrodynamical simulations of fragmentation of gas clouds and gas accretion onto proto-stars (Bate 2009(Bate , 2012Bate et al 2014), that include radiative feedback and magnetic fields, indicate a maximum localized SFE of ≈ 30%. These conditions are discussed in more detail in Sec.…”