Astronomy and Astrophysics – Astronomy
Scientific paper
Feb 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996apj...458..714p&link_type=abstract
Astrophysical Journal v.458, p.714
Astronomy and Astrophysics
Astronomy
76
Hydrodynamics, Stars: Interiors, Stars: Pre-Main-Sequence, Stars: Formation, Stars: Rotation
Scientific paper
Modern studies of collapse and fragmentation of protostellar clouds suggest a wide variety of outcomes, depending on the assumed initial conditions. Individual equilibrium objects that result from collapse are likely to be in rapid rotation and can have a wide range of structures. We have undertaken a survey of parameter space in order to examine the role of dynamic instabilities in the subsequent evolution of these objects. Such instabilities can produce significant mass and angular momentum transport or, if violent enough, can lead to the breakup of the original object.
For the purposes of conducting a systematic study, we have so far considered only the n = 3/2 polytropic equilibrium states that might form from the collapse of uniformly rotating spherical clouds. We do not follow the collapses themselves, but use a simple procedure to connect presumed initial conditions to postcollapse equilibrium states. By varying the central concentration of the assumed initial cloud, we obtain equilibrium states distinguished primarily by their different specific angular momentum distributions. These equilibrium states range between starlike objects with angular momentum distributions analogous to the Maclaurin spheroids and objects that have moderately extended Keplerian-disk-like regions. Using a new self-consistent field code to generate the n = 3/2 axisymmetric equilibrium states and an improved three-dimensional hydrodynamics code, we have investigated the onset and nature of global dynamic instabilities in these objects.
The starlike objects are unstable to barlike instabilities at T/|W| ≥ 0.27, where T/|W| is the ratio of total rotational kinetic energy to gravitational potential energy. These instabilities are vigorous and lead to violent ejection of mass and angular momentum. As the angular momentum distribution shifts to the other extreme, one- and two-armed spiral instabilities begin to dominate at considerably lower T/|W|. These instabilities seem to be driven by mechanisms related to swing and SLING but operating under conditions that are very different from those that are usually considered. In flattened objects, one-armed spirals dominate all other disturbances. Although these spirals tend to saturate at nonlinear amplitude, they do transport significant amounts of mass and angular momentum. It is unclear at present whether or not they ultimately lead to breakup of the equilibrium object. We conclude that the nature of the global instabilities encountered during the process of star formation can be quite sensitive to the angular momentum distribution of the protostar.
Davis Glen A.
Durisen Richard H.
Pickett Brian K.
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