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Séminaire MHD (SEMHD)

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Influence of turbulence and magnetic field on the star-formation process: An observational review
Philippe André (CEA Saclay)

3 avril 2009

While gravity is believed to be the main player in the star formation process, turbulence and magnetic fields have been invoked to play a major role in regulating the star formation rate in giant molecular clouds. I review the properties of low-mass dense cores as derived from observations and discuss them in the context of the current debate between two extreme paradigms for the star/core formation process: the quasi-static, magnetic picture vs. the dynamic, turbulent picture.
Neither of these extreme scenarios can explain all observations. Pure ambipolar diffusion has relatively long growth times for typical ionization levels and has difficulty satisfying core lifetime constraints. It is also inconsistent with the relative mass-to-magnetic-flux ratio between core and envelope recently measured in a few cases through Zeeman observations. Purely hydrodynamic pictures have trouble accounting for the observed spatial distribution of cores and young stellar objects within cloud complexes and for the low velocity dispersion measured within and around individual cores. Furthermore, there is strong evidence from, e.g., polarization studies that the envelopes of nearby clouds such as Taurus and the Pipe Nebula are magnetically dominated. A possible favorable scenario is a mixed model involving gravitational fragmentation and MHD turbulence in molecular clouds close to magnetic criticality.
Bibliographie :
– André P., Basu S., Inutsuka S. "The Formation and Evolution of Prestellar Cores", in Structure Formation in Astrophysics, éd. G. Chabrier, Cambridge University Press, 2008 (astro-ph/0801.4210).
– André P., Ward-Thompson D., Barsony M. "From Prestellar Cores to Protostars: the Initial Conditions of Star Formation", in Protostars and Planets IV, éd. V. Mannings, A. Boss, S. Russell (Tucson: Univ. Arizona Press, 2000), p. 59.
– Crutcher R., Hakobian N., Troland T. "Testing Magnetic Star Formation Theory", Astrophys. J., 692 (2009), 844.
– Heiles C., Crutcher R.M. "Magnetic Fields in Diffuse HI and Molecular Clouds", in Cosmic Magnetic Fields, éd. R. Wielebinski & R. Beck, LNP, 2005, 664, 137.
– Heyer M., Brunt C. "The Universality of Turbulence in Galactic Molecular Clouds", Astrophys. J., 615 (2004), L45.
– Heyer M. et al. "Magnetically Aligned Velocity Anisotropy in the Taurus Molecular Cloud", Astrophys. J., 680 (2008), 420.
– Larson R. "Turbulence and star formation in molecular cloud", MNRAS, 194 (1981), 809.
– Troland T., Crutcher R. "Magnetic Fields in Dark Cloud Cores: Arecibo OH Zeeman Observations", Astrophys. J., 680 (2008), 457.
– Ward-Thompson D., André P., Crutcher R. et al. "An Observational Perspective of Low-Mass Dense Cores II: Evolution Toward the Initial Mass Function", in Protostars and Planets V, éd. B. Reipurth, D. Jewitt, K. Keil (Tucson: University of Arizona Press, 2007), p. 33.

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Non disponible Philippe André (CEA Saclay)
Philippe André