Other
Scientific paper
Nov 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999phdt.........1j&link_type=abstract
Thesis (PhD). THE UNIVERSITY OF MICHIGAN, Source DAI-B 60/05, p. 2174, Nov 1999, 297 pages.
Other
2
Ammonia Cores, Young Stellar Objects, Infall Collapse
Scientific paper
This dissertation analyzes some essential aspects of the physics of star-forming dense cores. A large, comprehensive, database of mapped ammonia cores, serves as empirical motivation for the theoretical models analyzed. The database compiles the initial conditions for star-formation in dense cores across several star- forming regions. It also compiles the properties of embedded young stellar objects (YSOs). A thorough statistical analysis of this data reveals a significant dependence of core gas and YSO properties, on IRAS and cluster associations, as well as on the star-forming region to which the core belongs. It also reveals that the ratio of starless to stellar cores is too small to be consistent with ambipolar diffusion time scales. The strong correlation between the properties of embedded YSOs and the initial conditions of the cores that envelope them, motivates an examination of the collapse of these initial conditions to form stars. Existing infall collapse models are extended to include three new features: (i)the radiation pressure potential of the embedded YSO, (ii)a binary as well as star+disk potential and (iii)inflow due to turbulent dissipation. Each of these extensions provides valuable physical insight, and helps create a more realistic picture of the collapse process. Including the effects of radiation pressure in the infall problem, yields maximum stellar mass scales, that are functions of the initial conditions. These scales are less restrictive compared to those derived for spherical potentials. We also find that radiation pressure is less important in nonthermally dominated cores traversed by significantly large magnetic fields. Infall in a binary potential displays interesting features in its velocity and density fields. There is a bunching up of orbits and density fields about the more massive companion. For large viewing angles, the density peaks at radii comparable to the center of mass distance of the massive companion. For a subset of parameter space, the radial velocity fields can also display maxima, and the tangential fields can change direction. The relevance of nonthermal linewidths as essential initial conditions for star-formation, further motivates the study of a linearized MHD model to examine the relationship between nonthermal linewidths and profiles on the one hand, and the magnitude and geometry of the static and perturbed magnetic fields traversing star forming molecular cloud cores, on the other. Linewidths resulting from Alfvén or fast magnetosonic waves, are very sensitive to the geometry of the field as well as to the line of sight geometry. Predicted values for magnetic fields in the Taurus, Ophiuchus and Perseus cores fall in the 0-100 μG range. Line profiles computed in the optically thin limit display broadening, skewing, shifting and splitting features that can reveal important information about the magnetic field in a dense core.
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