Astronomy and Astrophysics – Astronomy
Scientific paper
Sep 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999dda....31.0801b&link_type=abstract
American Astronomical Society, DDA meeting #31, #08.01
Astronomy and Astrophysics
Astronomy
Scientific paper
Fragmentation during gravitational collapse has been reasonably successful at explaining the formation of binary and multiple stars. However, nearly all three dimensional collapse calculations have ignored the effects of magnetic fields, whereas magnetic fields are generally regarded as a dominant force in molecular clouds. A few three dimensional collapse models have assumed the presence of a frozen-in magnetic field, but these models did not lead to fragmentation. Recently, three dimensional models that allow for magnetic field loss by ambipolar diffusion have shown that fragmentation is possible for initially prolate, rotating, magnetically-supported cloud cores. The models simulate the effects of magnetic fields by adding the magnetic field pressure term to the gas pressure, an approximation that is valid for clouds with high electrical conductivity and straight magnetic field lines. The magnitude of the magnetic field is assumed to depend on the density raised to a fractional power, as found by previous work on magnetic collapse. Ambipolar diffusion is simulated by reducing the field strength over a time period on the order of 10 cloud free fall times, again in agreement with previous work. The main effect of the magnetic field is to delay the collapse phase. Once collapse begins, a rotating cloud can fragment into a binary protostar, provided that its initial ratio of rotational to gravitational energy (beta ) exceeds about 0.01. Because the critical value of beta ~ 0.01 falls roughly at the median of the distribution of rotational energies for pre-collapse dense cloud cores, these models provide a plausible explanation for why about half of all primary stars have a binary companion: the initial amount of rotation may be the key quantity.
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