COLLAPSE AND FRAGMENTATION OF MAGNETIZED CYLINDRICAL CLOUDS

Gravitational collapse of the cylindrical elongated cloud is studied by numerical magnetohydrodynamical simulations. In the infinitely long cloud in hydrostatic configuration, small perturbation grow by the gravitational instability. The most unstable mode indicated by a linear perturbation theory grows selectively even from a white noise. The growth rate agrees with that calculated by the linear theory. First, the density-enhanced region has an elongated shape, i.e., prolate spheroidal shape. As the collapse proceeds, the high-density fragment begins to contrast mainly along the symmetry axis. Finally, a spherical core is formed in the nonmagnetized cloud. In contrast, an oblate spheroidal dense disk is formed in a cloud in which the magnetic pressure is nearly equal to the thermal one. The radial size of the disk becomes proportional to the initial characteristic density scale height in the r-direction. As the collapse proceeds, a slowly contracting dense part is formed (less-than-or-similar-to 10% in mass), which is separated from the other part of the disk in which the inflow velocity is accelerated as it reaches the slowly contracting dense part. On the basis of arguments on the Jeans mass and the magnetic critical mass, the evolution of the fragments formed in a cylindrical elongated cloud is classified. When the cloud reaches a cylindrical, magnetohydrostatic configuration, if the plasma beta (the ratio of thermal pressure to the magnetic pressure) in the center of the cloud is at least larger than greater-than-or-similar-to 0.02, the formed disks cannot be supported against the self-gravity, and it will eventually collapse. If the cylindrical cloud is supported mainly by the magnetic fields, beta less-than-or-similar-to 0.02, the cloud seems to be fragmented to stable disks.