Statistics – Computation
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992phdt........70b&link_type=abstract
PhD Dissertation, Montreal Univ. Quebec Canada
Statistics
Computation
Fragmentation, Gravitational Collapse, Star Formation, Rotating Matter, Binary Stars, Stellar Systems, Protostars, Molecular Clouds, Rotation, Accretion Disks, Stellar Gravitation, Stellar Envelopes, Computational Astrophysics
Scientific paper
The formation of binary and multiple systems through the fragmentation of elongated clouds is investigated. We present three-dimensional hydrodynamic calculations, using a smooth particle hydrodynamics (SPH) code, of the gravitational collapse and fragmentation of elongated clouds including rotation either about an axis perpendicular to the cloud's major axis or about an arbitrary axis. The initial cloud parameters are J0, the ratio of the absolute value of the gravitational to thermal energies; L/D, the ratio of length to diameter of the cloud; and betaparallel and betaperpendicular, the values of beta, which is the ratio of the absolute value of rotational to gravitational energies, for the component of rotation parallel and perpendicular to the cloud's major axis, respectively. The collapse forms two fragments, one on either side of the equatorial plane. The rotation about an axis perpendicular to the major axis ensures that the two fragments do not collide at the center of the cloud. A binary system is thus formed. The binary systems have initially high eccentricity and unequal mass systems are possible with slight initial density gradients. For low values of J0, the fragments are single and each one is surrounded by a disk. When rotation about an arbitrary axis is included, these disks are parallel but noncoplanar. In this case, interactions at closest approach can be important for dissipating orbital energy. For higher values of J0 and betaparallel, the fragment plus disk system can sub-fragment according to one of three modes: disk fragmentation involves the fragmentation of a circumfragmentary disk due to tidal forces with the other fragment; bar fragmentation is the direct sub-fragmentation of one of the fragments through an intermediate bar stage; and bar-arm fragmentation involves the fragmentation of a bar plus spiral arm. The multiple systems formed through these processes are noncoplanar systems, agreeing with observations that show that at least 35% of multiple systems are noncoplanar. Gravitational torques and tidal forces play a significant role in the star formation process. Tidal forces in an eccentric system can also be important for enhanced accretion at closest approach. This demonstrates the importance of treating star formation, not as an isolated phenomenon, but in a global treatment incorporating the initial cloud structure and interactions between groups of fragments and stars. Unequal mass components in a binary system are found to have differing amounts of circumstellar material. This is primarily due to the more massive component being situated closer to the cloud's center of mass. Observations of pre-main sequence (PMS) stars find that the more massive component is often redder due to circumstellar matter. Many star forming regions are found to contain structures similar to those formed in the numerical simulations presented here. Cloud dynamics can be used as a diagnostic where collapse is occurring and whether rotation is involved. The gravitational collapse causes large infall velocities while conservation of angular momentum increases the rotation.
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