Magnetically Regulated Star Formation in Turbulent Clouds Zhi-Yun Li (University of Virginia) Fumitaka Nakamura (Niigata University) OUTLINE  Motivations  Numerical Simulations  Conclusion

Control of Star Formation  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 1. Supersonic Turbulence?  Strengths: (a) inefficient; (b) subsonic cores  Potential problem: ambipolar diffusion (AD) timescale too long at low densities (McKee 1989; Myers & Khersonsky 1995) 2. Strong Magnetic Fields? roughly 10 x local free-fall times dense material needed for AD to be effective (e.g. Larson 1981; Mac Low & Klessen 2004) (e.g., Shu et al. 1999; Mouschovias & Ciolek 1999)

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

The Setup of Numerical Simulations  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 Av=1 and B=9  G magnetically subcritical (by 20%)  supersonic turbulence at time=0 rms Mach number=10 (decaying) (Li & Nakamura 2004; Nakamura & Li 2005)

time unit t g = 1.9 Myrs sound speed Cs=0.2 km/s red plus=star 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 t=2.0 t g or 3.8 Myrs: 15 x 0.5/302 = 2.5%

Evolution of Star Formation Efficiency efficiency of a few percent over cloud lifetime of several million years time in units of collapse time (1.9 Myrs) Why inefficient?  rate of star formation per unit mass R = 7x10 -9 year -1  cloud depletion time due to star formation R -1 =1.4x10 8 years

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”  depletion time of filaments about 40 Myrs or 20 t g long-lived supercritical filaments  only the densest parts of filaments directly involved in star formation - dense cores tgtg

 dense cores at the middle point of simulation (~4 Myrs)  peak column density more than 10 times average  10 cores in total Examples of Dense Cores

Quiescent Cores predominantly quiescent (subsonic) cores

Turbulence Accelerated Star Formation time in units of average collapse time 1.9 Myrs

Magnetically Regulated Star Formation time in units of collapse time1.9 Myrs non-magnetic moderately supercritical moderately subcritical too efficient? (Lada & Lada 2003) Clusters? Dispersed? weaker outflows

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 magnetic regulation for dispersed star formation perhaps for cluster formation as well

3D Magnetically Supercritical Clouds (M=10,  =0.8) z B field 1 t g x

y x B field

z y