Nonaxisymmetric Evolution of Magnetically Subcritical Clouds: Bar Growth, Core Elongation, and Binary Formation

Abstract

We have begun a systematic numerical study of the nonlinear growth of nonaxisymmetric perturbations during the ambipolar diffusion-driven evolution of initially magnetically subcritical molecular clouds, with an eye on the formation of binaries, multiple stellar systems and small clusters. In this initial study, we focus on the $m=2$ (or bar) mode, which is shown to be unstable during the dynamic collapse phase of cloud evolution after the central region has become magnetically supercritical. We find that, despite the presence of a strong magnetic field, the bar can grow fast enough that for a modest initial perturbation (at 5% level) a large aspect ratio is obtained during the isothermal phase of cloud collapse. The highly elongated bar is expected to fragment into small pieces during the subsequent adiabatic phase. Our calculations suggest that the strong magnetic fields observed in some star-forming clouds and envisioned in the standard picture of single star formation do not necessarily suppress bar growth and fragmentation; on the contrary, they may actually promote these processes, by allowing the clouds to have more than one (thermal) Jeans mass to begin with without collapsing promptly. Nonlinear growth of the bar mode in a direction perpendicular to the magnetic field, coupled with flattening along field lines, leads to the formation of supercritical cores that are triaxial in general. It removes a longstanding objection to the standard scenario of isolated star formation involving subcritical magnetic field and ambipolar diffusion based on the likely prolate shape inferred for dense cores. Continuted growth of the bar mode in already elongated starless cores, such as L1544, may lead to future binary and multiple star formation.

5 pages, 2 figures, accepted by ApJL

References in corpus (5)

• Tracing the Mass during Low-Mass Star Formation. II. Modelling the Submillimeter Emission from Pre-Protostellar Cores
• A Survey for Infall Motions toward Starless Cores. II. $CS(2-1)$ and $N_2{H}^{+}(1-0)$ Mapping Observations
• Singular Isothermal Disks: II. Nonaxisymmetric Bifurcations and Equilibria
• The Structure and Evolution of Magnetized Cloud Cores in a Zero--Density Background
• Ring Formation in Magnetically Subcritical Clouds and Multiple Star Formation

Cited by in corpus (12)

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• Collapse and Fragmentation of Rotating Magnetized Clouds. I. Magnetic Flux - Spin Relation
• Quiescent Cores and the Efficiency of Turbulence-Accelerated, Magnetically Regulated Star Formation
• Magnetically Regulated Star Formation in Turbulent Clouds
• Formation and Collapse of Nonaxisymmetric Protostellar Cores in Planar Magnetic Molecular Clouds
• FRagmentation and Evolution of dense cores Judged by ALMA (FREJA). I (Overview). Inner $\sim$1000 au structures of prestellar/protostellar cores in Taurus
• Constraining the structure of the non-spherical preprotostellar core L1544
• Binary and Multiple Star Formation in Magnetic Clouds: Bar Growth and Fragmentation
• Fragmentation of Magnetically Subcritical Clouds into Multiple Supercritical Cores and the Formation of Small Stellar Groups
• High-Resolution Optical and Near-Infrared Images of the FS Tauri Circumbinary Disk
• Self-Gravitating Magnetically-Supported Protostellar Disks and the Formation of Substellar Companions
• The embedded ring-like feature and star formation activities in G35.673-00.847