What Drives Star Formation?

  • Richard M. Crutcher


Current theoretical models for what drives star formation (especially low-mass star formation) are: (1) magnetic support of self-gravitating clouds with ambipolar diffusion removing support in cores and triggering collapse and (2) compressible turbulence forming self-gravitating clumps that collapse as soon as the turbulent cascade produces insufficient turbulent support. Observations of magnetic fields can distinguish between these two models because of different predictions in three areas: (1) magnetic field morphology, (2) the scaling of field strength with density and non-thermal velocities, and (3) the mass to magnetic flux ratio, M/Φ. We first discuss the techniques and limitations of methods for observing magnetic fields in star formation regions, then describe results for the L1544 prestellar core as an exemplar of the observational results. Application of the three tests leads to the following conclusions. The observational data show that both magnetic fields and turbulence are important in molecular cloud physics. Field lines are generally regular rather than chaotic, implying strong field strengths. But fields are not aligned with the minor axes of oblate spheroidal clouds, suggesting that turbulence is important. Field strengths appear to scale with non-thermal velocity widths, suggesting a significant turbulent support of clouds. Giant Molecular Clouds (GMCs) require mass accumulation over sufficiently large volumes that they would likely have an approximately critical M/Φ. Yet H I clouds are observed to be highly subcritical. If self-gravitating (molecular) clouds form with the subcritical M/Φ of H I clouds, the molecular clouds will be subcritical. However, the observations of molecular cloud cores suggest that they are approximately critical, with no direct evidence for subcritical molecular clouds or cloud envelopes. Hence, the observations remain inconclusive in deciding between the two extreme-case models of what drives star formation. What is needed to further advance our understanding of the role of magnetic fields in the star formation process are additional high sensitivity surveys of magnetic field strengths and other cloud properties in order to further refine the assessment of the importance of magnetic fields in molecular cores and envelopes.


ISM: molecular clouds magnetic fields polarization stars: formation 


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  1. Ballesteros-Paredes, J. and MacLow, M.-M.: 2002, ApJ 570, 734.ADSCrossRefGoogle Scholar
  2. Balsara, D., Ward-Thompson, D. and Crutcher, R.M.: 2001, MNRAS 327, 715.ADSCrossRefGoogle Scholar
  3. Basu, S.: 2000, ApJ 540, L103.ADSCrossRefGoogle Scholar
  4. Bourke, T.L., Myers, P.C., Robinson, G. and Hyland, A.R.: 2001, ApJ 554, 916.ADSCrossRefGoogle Scholar
  5. Caselli, P., Benson, P.J., Myers, P.C. and Tafalla, M.: 2002, Api 572, 238.ADSGoogle Scholar
  6. Chandrasekhar, S. and Fermi, E.: 1953, ApJ 118, 113.ADSCrossRefMathSciNetGoogle Scholar
  7. Ciolek, G.E. and Basu, S.: 2000, Api 529, 925.ADSGoogle Scholar
  8. Crutcher, R.M., Troland, T.H., Goodman, A.A., Heiles, C., Kazès, I. and Myers, P.C.: 1993, ApJ 407, 175.ADSCrossRefGoogle Scholar
  9. Crutcher, R.M.: 1999, ApJ 520, 706.ADSCrossRefGoogle Scholar
  10. Crutcher, R.M. and Troland, T.H.: 2000, ApJ 537, L139.ADSCrossRefGoogle Scholar
  11. Crutcher, R.M., Nutter, D., Ward-Thompson, D. and Kirk, J.M.: 2004, ApJ 600, 279.ADSCrossRefGoogle Scholar
  12. Gammie, C.F., Lin, Y.-T., Stone, J.M. and Ostriker, E.C.: 2003, ApJ 592, 203.ADSCrossRefGoogle Scholar
  13. Girart, J.M., Crutcher, R.M. and Rao, R.: 1999, ApJ 525, L169.CrossRefGoogle Scholar
  14. Hartmann, L., Ballesteros-Paredes, J. and Bergin, E.A.: 2001, ApJ 562, 852.ADSCrossRefGoogle Scholar
  15. Heiles, C.: 2004, ApandSS 292, 77.Google Scholar
  16. Heitsch, F., Zweibel, E.G., MacLow, M.-M., Li, P.S. and Norman, M.L.: 2001, ApJ 561, 800ADSCrossRefGoogle Scholar
  17. Kylafis, N.D.: 1983, ApJ 275, 135.ADSCrossRefGoogle Scholar
  18. Lazarian, A.: 2003, JQSRT 79, 881.ADSCrossRefGoogle Scholar
  19. Matthews, B.C. and Wilson, C.D.: 2002, ApJ 531, 868.ADSCrossRefGoogle Scholar
  20. Mouschovias, T.Ch.: 1976, ApJ 207, 141.ADSCrossRefGoogle Scholar
  21. Mouschovias, T.Ch. and Spitzer, L.: 1976, ApJ 210, 326.ADSCrossRefGoogle Scholar
  22. Mouschovias, T.Ch. and Ciolek, G.E.: 1999, in: C.J. Lada and N.D. Kylafis (eds.), The Origin of Stars and Planetary Systems Kluwer Academic Publishers, Dordrecht, p. 305.Google Scholar
  23. Nakano, T. and Nakamura, T.: 1978, PASJ 30, 681.ADSGoogle Scholar
  24. Nakano, T.: 1984, Fund. Cosmic Phys. 9, 139.ADSGoogle Scholar
  25. Ostriker, E.C., Stone, J.M. and Gammie, C.F.: 2001, ApJ 546, 980.ADSCrossRefGoogle Scholar
  26. Padoan, P. and Nordlund, Â.: 1999, Api 526, 279.ADSGoogle Scholar
  27. Padoan, P., Goodman, A., Draine, B.T., Juvela, M., Nordlund, A. and Rögnvaldsson, Ö.E.: 2001, ApJ 559, 1005.ADSCrossRefGoogle Scholar
  28. Passot, T. and Vazquez-Semadeni, E.: 2003, AandA 398, 845.zbMATHGoogle Scholar
  29. Roberts, D., Crutcher, R.M., Troland, T.H. and Goss, W.M.: 1993, ApJ 412, 675.ADSCrossRefGoogle Scholar
  30. Schleuning, D.A.: 1998, ApJ 493, 811.ADSCrossRefGoogle Scholar
  31. Shu, F.H., Adams, F.C. and Lizano, S.: 1987, ARAandA 25, 23.ADSCrossRefGoogle Scholar
  32. Tafalla, M., Mardones, D., Myers, P.C., Caselli, P., Bachiller, R. and Benson, P.J.: 1998, ApJ 504, 900.ADSCrossRefGoogle Scholar
  33. Troland, T.H. and Heiles, C.: 1986, ApJ 301, 339.ADSCrossRefGoogle Scholar
  34. Troland, T.H. and Crutcher, R.M.: 2004, submitted.Google Scholar
  35. Ward-Thompson, D., Kirk, J.M., Crutcher, R.M., Greaves, J.S., Holland, W.S. and André, P.: 2000, ApJ 537, L135.ADSCrossRefGoogle Scholar
  36. Williams, J.P., Myers, P.C., Wilner, D.J. and Di Francesco, J.: 1999, ApJ 513, L61.ADSCrossRefGoogle Scholar

Copyright information

© Springer Science+Business Media Dordrecht 2004

Authors and Affiliations

  • Richard M. Crutcher
    • 1
  1. 1.Department of AstronomyUniversity of IllinoisUrbanaUSA

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