The Formation and Evolution of Prestellar Cores

TL;DR

This paper reviews recent observational and theoretical insights into the formation and evolution of prestellar cores, emphasizing the roles of turbulence, magnetic fields, and future observational prospects.

Contribution

It synthesizes current understanding of core formation scenarios, highlighting the limitations of pure models and proposing a mixed turbulence-magnetic approach, while discussing magnetic and angular momentum evolution.

Findings

Pure ambipolar diffusion models have long growth times.

Hydrodynamic models struggle with core formation efficiency.

Turbulent magnetic models are promising for core formation.

Abstract

Improving our understanding of the initial conditions and earliest stages of star formation is crucial to gain insight into the origin of stellar masses, multiple systems, and protoplanetary disks. We review the properties of low-mass dense cores as derived from recent millimeter/submillimeter observations of nearby molecular clouds and discuss them in the context of various contemporary scenarios for cloud core formation and evolution. None of the extreme scenarios can explain all observations. Pure laminar ambipolar diffusion has relatively long growth times for typical ionization levels and has difficulty satisfying core lifetime constraints. Purely hydrodynamic pictures have trouble accounting for the inefficiency of core formation and the detailed velocity structure of individual cores. A possible favorable scenario is a mixed model involving gravitational fragmentation of turbulent molecular clouds close to magnetic criticality. The evolution of the magnetic field and angular momentum in individual cloud cores after the onset of gravitational collapse is also discussed. In particular, we stress the importance of radiation-magnetohydrodynamical processes and resistive MHD effects during the protostellar phase. We also emphasize the role of the formation of the short-lived first (protostellar) core in providing a chance for sub-fragmentation into binary systems and triggering MHD outflows. Future submillimeter facilities such as Herschel and ALMA will soon provide major new observational constraints in this field. On the theoretical side, an important challenge for the future will be to link the formation of molecular clouds and prestellar cores in a coherent picture.