Self-similar fragmentation regulated by magnetic fields in a region forming massive stars

Abstract

Most molecular clouds are filamentary or elongated1,2,3. For those forming low-mass stars (<8 solar masses), the competition between self-gravity and turbulent pressure along the dynamically dominant intercloud magnetic field (10 to 100 parsecs) shapes the clouds to be elongated either perpendicularly4 or parallel5 to the fields. A recent study6 also suggested that on the scales of 0.1 to 0.01 parsecs, such fields are dynamically important within cloud cores forming massive stars (>8 solar masses). But whether the core field morphologies are inherited from the intercloud medium or governed by cloud turbulence is unknown, as is the effect of magnetic fields on cloud fragmentation at scales of 10 to 0.1 parsecs7,8,9. Here we report magnetic-field maps inferred from polarimetric observations of NGC 6334, a region forming massive stars, on the 100 to 0.01 parsec scale. NGC 6334 hosts young star-forming sites10,11,12 where fields are not severely affected by stellar feedback, and their directions do not change much over the entire scale range. This means that the fields are dynamically important. The ordered fields lead to a self-similar gas fragmentation: at all scales, there exist elongated gas structures nearly perpendicular to the fields. Many gas elongations have density peaks near the ends, which symmetrically pinch the fields. The field strength is proportional to the 0.4th power of the density, which is an indication of anisotropic gas contractions along the field. We conclude that magnetic fields have a crucial role in the fragmentation of NGC 6334.

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Figure 1: The B-field directions of NGC 6334 and the local intercloud medium.
Figure 2: The B-field within the clumps/cores.
Figure 3: Self-similar fragmentation and field configurations at 100–0.01 pc.
Figure 4: Parameters used to estimate B-field strength.

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Acknowledgements

We thank R. Blundell for the SMA Director’s Discretionary Time that allowed the project to take off. We also thank the SPARO team led by G. Novak and the Hertz team led by R. Hildebrand. Discussions with Z.-Y. Li and the proofread by D. Wilmshurst and T.T. Wu improved the manuscript. The experiment was supported by the Hong Kong Research Grants Council, project ECS 24300314; by CUHK Direct Grant for Research 2013-2014, project ‘Modelling Star Formation with GPU Computing’; and by the Deutsche Forschungsgemeinschaft priority programme 1573, project number 46. The Submillimeter Array is a joint project between the Smithsonian Astrophysical Observatory and the Academia Sinica Institute of Astronomy and Astrophysics and is funded by the Smithsonian Institution and the Academia Sinica.

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Contributions

H.-b.L. designed and executed the experiment. K.H.Y. measured the field curvatures. F.O. performed the numerical simulations. The Chinese University of Hong Kong team was responsible for the manuscript. The CfA-ASIAA-Nanjing team helped with the SMA data acquisition and reduction.

Corresponding author

Correspondence to Hua-bai Li.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 A fitting of the observed B and n weighted by the signal-to-noise ratio.

B is from Extended Data Table 1 and n is approximated by (M1 + M2)/D3. The uncertainties of M, D, d, and R (Extended Data Table 1) are propagated to B and n; the error bars of 1σ are shown. The slopes of the two dashed lines are 0.37 and 0.44. The ‘Curve Fitting’ toolbox of Matlab is used to fit the data. While projection may affect measurements of field strengths (though not much in our case owing to the special LOS), the exponent is less affected, because the effect is the same for all the densities if the field directions are aligned (Fig. 3).

Extended Data Figure 2 Simulated Bn relations and cloud elongation with various magnetic criticality numbers.

Blue lines are the results from the simulations (see text) with various initial uniform field strengths B0. The initial uniform density of the spherical cloud is n0. B and n are the mean values within a cloud (regions with n > n0). The MFRs normalized by the critical value43 (criticality numbers) are shown near the ends of each blue line. The slope should never go beyond 2/3 (the red dashed line), the condition of isotropic contraction. A simulation with the criticality number of 600 was also performed and the slope is exactly 2/3. The observed slope of NGC 6334 with a 95% confidence bound is shown by the shaded zone. The short-to-long axis ratio after ten million years of each simulation is shown within a blue oval shaped with the same axis ratio. For simplicity, the contour of 20% of the peak value is used to define the short and long axes of a cloud. The ratios are measured in the same way for Fig. 2a–d, and their mean and standard deviation are also shown within the green oval.

Extended Data Table 1 Parameters used to estimate B-field strength

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Li, H., Yuen, K., Otto, F. et al. Self-similar fragmentation regulated by magnetic fields in a region forming massive stars. Nature 520, 518–521 (2015). https://doi.org/10.1038/nature14291

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