Influence of magnetic fieldson molecular cloud evolution

and turbulence

Hua-bai Li, Frank Otto, Ka Ho Yuen, Po Kin Leung

Department of PhysicsThe Chinese University of Hong Kong

10th East Asian Meeting on AstronomySep 30, 2016

Dynamically important B-fields IObservation: bimodal cloud orientations

● study of 13 filamentary molecular clouds in the Gould Belt

● clouds are aligned either parallel or perpendicular to the magnetic field of the local inter-cloud medium

● strong significance: probability for this correlation to occur from random orientations < 0.6%

Data Sources:● filamentary structures determined from dust

extinction maps (Dobashi 2011)● ICM B-field direction obtained from optical

stellar polarimetry data (Heiles 2000)

HB Li et al. MNRAS 436, 3707 (2013)

From large to small scales: NGC6334

Properties:● nearby:

distance ~ 1.7 kpc● young star-formation

sites:not yet strongly affected by stellar feedback

● forms massive stars (>8M☉):stronger thermal dust emission allows probing B-fields

photo credit: Sarah Willis (CfA)

Dynamically important B-fields II:Self-similar Fragmentation in NGC6334

SPAROHertz/CSO

SMA

optical

HB Li, Yuen, Otto, et alNature 520, 518 (2015)

● Self-similar density distribution & magnetic field configuration from >10pc to <0.1pc

● Cloud/clumps/cores elongated perpendicular to magnetic field

● Mean field direction does not change significantly over all scales

NGC6334: Hierarchical Configuration

region is likely sub-alfvenic

NGC6334: B-field vs. Density

Isotropic contraction:

Observed:

1st observation of B-n relation from single cloud!

Turbulence vs. B-field

● More kinetic energy in large scale motions than in small scales, but magnetic field strength ~ independent of scale.

● Turbulent eddies are elongatedalong the local magnetic field.More elongated atsmaller scales.

● Scale-dependentanisotropy inthe velocity field.

Detecting Velocity Anisotropy

● Tool: two-point 2nd order structure function of the velocity centroid

● fit angular dependence to sinusoid

⇒ anisotropy strength and orientation

Anisotropy strength depends on Ms and MA

What happens with self-gravity?

● low-density: constant anisotropy || B

● high-density: anisotropy disappears, then reappears ⟂ B

● no significant difference in Mach number!

Summary● Bimodal cloud orientation w.r.t. inter-cloud B-field:

B-field is important during formation of molecular cloud.

● Multi-scale study of NGC6334:- Self-similar density distribution & B-field configuration- Mean B-field direction preserved over scales- First B-n relation from single cloud ⇒ B∝n0.41

● Velocity Anisotropy can be used to detect B-field orientation,but sensitive to gravitational distortions in high-density regions.

Acknowledgements● Hong Kong Research Grants Council ($$$)● CUHK ($$$ and computational facilities)

Thanks for your attention!