Magnetically Regulated Star Formation in Turbulent Clouds

1. MagneticallyRegulatedStarFormationinTurbulentClouds Zhi-YunLi (University of Virginia) FumitakaNakamura (Niigata University) OUTLINE • Motivations • Numerical Simulations • Conclusion

2. ControlofStarFormation 1. Supersonic Turbulence? (e.g. Larson 1981; Mac Low & Klessen 2004) • Strengths: (a) observed on large-scales; (b) create dense cores through shocks • Potentialproblems: (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 • Potentialproblem: 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

3. Turbulence-AcceleratedMagneticallyRegulatedStarFormation • Supersonicturbulence 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 • StrongMagneticfields • prevent turbulence from converting a large fraction of mass into stars in a crossing time • ensure quiescent cores out of turbulent cloud wedemonstratethehybridscenariobynumericalexperiments

4. The Setup of NumericalSimulations (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 Av=1andB=9G magnetically subcritical (by 20%) • supersonic turbulence at time=0 rms Mach number=10 (decaying)

5. time unit • tg=1.9Myrs • sound speed • Cs=0.2km/s • redplus=star • 0.5Meach • total mass • 302M 3.7pc star formation efficiency (SFE) = mass of stars/total mass of cloud e.g., SFE at t=2.0 tg or 3.8 Myrs: 15 x 0.5/302 = 2.5%

6. EvolutionofStarFormationEfficiency • rate of star formation • per unit mass • R = 7x10-9 year-1 • cloud depletion time due to star formation R-1=1.4x108 years time in units of collapse time (1.9 Myrs) efficiency of a few percent over cloud lifetime of several million years Whyinefficient?

7. MagneticallySupercriticalFilaments • strong B fields prevent prompt collapse • forced flux reduction in shocks through AD • magnetically supercritical filaments produced “fertileislands” in a “barrensea” tg • depletion time of filaments about40 Myrs or 20 tg long-lived supercritical filaments • only the densest parts of filaments directly involved in star formation - densecores

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

9. Quiescent Cores predominantly quiescent (subsonic) cores

10. TurbulenceAcceleratedStarFormation time in units of average collapse time 1.9 Myrs

11. MagneticallyRegulatedStarFormation non-magnetic weakeroutflows tooefficient? (Lada & Lada 2003) Clusters? moderately supercritical Dispersed? time in units of collapse time1.9 Myrs moderately subcritical

12. 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

14. 3D Magnetically Supercritical Clouds (M=10, =0.8) x B field z 1 tg

15. y x B field

16. y z