Formation of a protocluster: A virialized structure from gravoturbulent collapse

Abstract

Context. Stars are often observed to form in clusters and it is therefore important to understand how such a region of concentrated mass is assembled out of the diffuse medium. The properties of such a region eventually prescribe the important physical mechanisms and determine the characteristics of the stellar cluster. Aims. We study the formation of a gaseous protocluster inside a molecular cloud and associate its internal properties with those of the parent cloud by varying the level of the initial turbulence of the cloud with a view to better characterize the subsequent stellar cluster formation. Methods. We performed high resolution magnetohydrodynamic (MHD) simulations of gaseous protoclusters forming in molecular clouds collapsing under self-gravity. We determined ellipsoidal cluster regions via gas kinematics and sink particle distribution, permitting us to determine the mass, size, and aspect ratio of the cluster. We studied the cluster properties, such as kinetic and gravitational energy, and made links to the parent cloud. Results. The gaseous protocluster is formed out of global collapse of a molecular cloud and has non-negligible rotation owing to angular momentum conservation during the collapse of the object. Most of the star formation occurs in this region, which occupies only a small volume fraction of the whole cloud. This dense entity is a result of the interplay between turbulence and gravity. We identify such regions in simulations and compare the gas and sink particles to observed star-forming clumps and embedded clusters, respectively. The gaseous protocluster inferred from simulation results presents a mass-size relation that is compatible with observations. We stress that the stellar cluster radius, although clearly correlated with the gas cluster radius, depends sensitively on its definition. Energy analysis is performed to confirm that the gaseous protocluster is a product of gravoturbulent reprocessing and that the support of turbulent and rotational energy against self-gravity yields a state of global virial equilibrium, although collapse is occurring at a smaller scale and the cluster is actively forming stars. This object then serves as the antecedent of the stellar cluster, to which the energy properties are passed on. Conclusions. The gaseous protocluster properties are determined by the parent cloud out of which it forms, while the gas is indeed reprocessed and constitutes a star-forming environment that is different from that of the parent cloud.

Cloud Structure of Three Galactic Infrared Dark Star-forming Regions from Combining Ground- and Space-based Bolometric Observations

“…Such an N-PDF is similar to thatobserved in G28.34+0.06, and those of some active OB-cluster-forming regions (e.g., G10.6−0.4, W49A) reported in Lin et al (2016). We note that an intermediate density bump present at volume density probability function (ρ-PDF) in the cloud's early evolved phase is also seen in the formation of protoclusters simulated by Lee & Hennebelle (2016).…”

Episodic accretion constrained by a rich cluster of outflows

“…This would suggest that cloud and subsequent star formation would have started in the south-western part of W43-MM1 first. This fits in the framework of recent cloud formation models (e.g., Lee & Hennebelle 2016;Vázquez-Semadeni et al 2019), where several cloud and star formation events sum up to form the final population of stars. That said, the anti-correlation observed in Fig.…”

Spin alignment of stars in old open clusters

“…Despite the initial size, the cluster expands after gas expulsion due to the shallowing of the gravitational potential. To study the stars formed inside the molecular cloud, sink particle algorithms are used 24 . Once the density reaches the threshold of 10 10 H 2 cm −3 , we detect the mass concentration and check whether the mass is virially unstable and the flow is locally converging.…”