Magnetically Regulated Star Formation in Turbulent Clouds

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Magnetically Regulated Star Formation in
Turbulent Clouds

• Zhi-Yun Li (University of Virginia)
• Fumitaka Nakamura (Niigata University)

• OUTLINE
• Motivations
• Numerical Simulations
• Conclusion

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Control of Star Formation
1. Supersonic Turbulence?
(e.g. Larson 1981 Mac Low Klessen 2004)

• Strengths (a) observed on large-scales
• (b) create dense cores through
  shocks
• Potential problems (a) high efficiency of star
  formation
• (b) transonic or supersonic cores

2. Strong Magnetic Fields?
(e.g., Shu et al. 1999 Mouschovias Ciolek 1999)

• Strengths (a) inefficient (b) subsonic
  cores
• Potential problem ambipolar diffusion (AD)
  timescale
• too long at low densities (McKee 1989
  Myers Khersonsky 1995)

roughly 10 x local free-fall times
dense material needed for AD to be effective
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Turbulence-Accelerated Magnetically Regulated
Star Formation

• Supersonic turbulence creates dense regions where

• free-fall time is shorter and UV photons shielded

much shorter AD time scale

• larger gradient in field strength

faster magnetic diffusion

• Strong Magnetic fields

• prevent turbulence from converting a large
  fraction
• of mass into stars in a crossing time

• ensure quiescent cores out of turbulent cloud

we demonstrate the hybrid scenario by numerical
experiments
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The Setup of Numerical Simulations
(Li Nakamura 2004 Nakamura Li 2005)

• Idealizations

• sheet-like mass distribution
• square-box with periodic
• boundary conditions
• L(box)10 L(Jeans)
• Lagrangian particles for stars
• M(star)0.5 M?
• parameterized wind strength

• Initial Conditions

• column density Av1 and B9 ?G
• magnetically subcritical (by 20)
• supersonic turbulence at time0
• rms Mach number10 (decaying)

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• time unit
• tg1.9 Myrs
• sound speed
• Cs0.2 km/s
• red plusstar
• 0.5 M? each
• total mass
• 302 M??

3.7pc
star formation efficiency (SFE) mass of
stars/total mass of cloud
e.g., SFE at t2.0 tg or 3.8 Myrs 15 x 0.5/302
2.5
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Evolution of Star Formation Efficiency

• rate of star formation
• per unit mass
• R 7x10-9 year-1
• cloud depletion time
• due to star formation
• R-11.4x108 years

time in units of collapse time (1.9 Myrs)
efficiency of a few percent over cloud lifetime
of several million years
Why inefficient?
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Magnetically Supercritical Filaments

• strong B fields prevent prompt collapse
• forced flux reduction in shocks through AD
• magnetically supercritical filaments produced
• fertile islands in a barren sea

tg

• depletion time of filaments about 40 Myrs or 20
  tg
• long-lived supercritical filaments
• only the densest parts of filaments directly
• involved in star formation - dense cores

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Examples of Dense Cores

• dense cores at the middle point of simulation (4
  Myrs)
• peak column density more than 10 times average
• 10 cores in total

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Quiescent Cores
predominantly quiescent (subsonic) cores
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Turbulence Accelerated Star Formation
time in units of average collapse time 1.9 Myrs
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Magnetically Regulated Star Formation
non-magnetic
weaker outflows
too efficient? (Lada Lada 2003)
moderately supercritical
Clusters?
Dispersed?
time in units of collapse time1.9 Myrs
moderately subcritical
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Conclusions

• Inefficient star formation in moderately
  magnetically subcritical clouds with supersonic
  turbulence
• Dense cores formed out of turbulent magnetically
  subcritical clouds have predominantly subsonic
  internal motions
• Moderately magnetically supercritical clouds may
  form stars with SFEs comparable to embedded
  clusters

magnetic regulation for dispersed star formation
perhaps for cluster formation as well
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3D Magnetically Supercritical Clouds (M10, ?0.8)
B field
x
z
1 tg
15
y
x
B field
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y
z
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